mm m iglff . 9 LIGHT ITS NATURE, SOURCES, EFEECTS, AND APPLICATIONS. Illustrate bn a IJljotograpIj. PUBLISHED UNDER THE DIRECTION OF THE COMMITTEE OF GENERAL LITERATURE AND EDUCATION", APPOINTED BY THE SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE. The day is Thine — the night also is Thine ; Thou hast prepared the Light and the Sun.’* LONDON: Printed for the SOCIETY FOR PROMOTING- CHRISTIAN KNOWLEDGE; SOLD AT THE DEPOSITORY, GREAT QUEEN STREET, LINCOLN’S INN FIELDS; 4 , ROYAL EXCHANGE; 16 , HANOVER STREET, HANOVER SQUARE; AND BY ALL BOOKSELLERS. Note . — The Photograph which forms the Frontispiece to this work is perfectly pure and untouched by the pencil, both as regards the negative plate and the positive print, — it will, therefore, alford the reader an example of the merits or defects of the Photographic Art. It has been coated with a solution of gold, to which its colour, in its various shades, is due ; and it has been mounted with liquid caout- chouc. Its permanence is therefore secured, as far as is at present known. Great care has also been used in washing it, and in removing all deleterious substances from the paper. LONDON S PRINTED BT RICHARD CIA T, BREAD STREET HILL, PREFACE. The discoveries of Photographic science have communicated to the general subject of which this work treats a new interest and importance. And while the author has by no means intended in his present work to put forth a treatise on photography,— yet it cannot be questioned that this art, of which it treats in sufficient detail for ordinary purposes, gives a fresh value to what was already known about Light. In like manner it might be argued that the discoveries of James Watt, on steam and its applications, added an interest to the subject of Caloric ; and certainly, they directed both the attention of the people and that of philo- sophers to the science of heat. The writer of this little book thought it pro- bable that at the present time there might be VI PREFACE. many readers wlio would be glad to see a popular treatise on Light, and it has been his endeavour to prepare such a work for their use. He begs to acknowledge his many obliga- tions to the works of Professor Hunt, which constitute almost the only modern literature on Light in our language. From one of these works, the “ Manual of Photography,” the cuts on pages 232, 244, 249, 274, 296, and 297, with the descriptive letter-press, have been copied, by an arrangement with the publishers of that book — Messrs. Griffin & Co. of Glasgow. The writer begs to express his obligations to Messrs. Negretti & Zambra, for information given by them, especially on the albumen pro- cess in photography, and to Mr. Copeland for the permission to publish the photograph of his statuette of the Prince of Wales. January , 1857 . CONTENTS. PART I. NATURE OF LIGHT. PAGE. Chap. I. — Popular Errors about Light .... 1 II. — What is Light 15 III.— Constitution and Phenomena of Light . 30 PART II. SOURCES OF LIGHT. Chap. I. — Celestial Light 75 II. — Chemical Sources of Light 99 III. — Domestic Sources of Light 128 PART III. EFFECTS OF LIGHT. Chap. I. — Effects of Ltght on Man and Animals 161 II. — Effects of Light on the Vegetable Kingdom 182 PART IV, APPLICATIONS OF LIGHT. Chap. I. — History of the Art of Sun-painting . 201 II. — The Daguerreotype 224 III. — The Talbotype 268 PART I. NATURE OF LIGHT. CHAPTER, I. POPULAR ERRORS ABOUT LIGHT. Light is at once so necessary and so "beau- tiful a thing, that we cannot wonder that much curious inquiry has been made into its nature and effects. At one period philosophers were more intent upon making singular theories than upon verifying them by experiment; and then we may fairly conclude that upon this interesting subject many persons exhausted their imagi- native powers. Moonlight, for example, was supposed to have very peculiar nature and influence ; and popular superstitions were very rife with respect to it, as will be evident from the following notes gathered out of the history of a past era. It was supposed B 2 LIGHT. to affect the human intellect in the most extra- ordinary manner. It was thought to influence the growth of the hair. It was also thought to in- fluence the effect of medicines. Emetics were said to be best when taken while the moon was in the sign Taurus ; and catarrhs were then thought to be cured most quickly. It was said that the best time for bathing was when the moon was in Pisces. The moon was also to guide the husband- man and agriculturist. Her light and influence were thought necessary in order to make pros- perous the plough, the harrow, and the sickle. Timber was felled, peat was dug, and other agricultural operations performed according to the presumed favourable periods of our satellite’s influence. The operations of surgery and of medicine were thought to be favourably or unfa- vourably affected by her light. And it is said that up to the present hour it is a common belief in Angus, that if a child be weaned during the waning of the moon, it will waste away all the time that the moon continues to wane. In Spain there is a popular belief that moonlight spoils the complexion, and ladies hold up their fans so as to hide themselves from its blackening effects. We will proceed systematically in the pre- sent chapter to consider in a general way and in succession those errors and superstitions about LIGHT. 3 light which once obtained the most universal reception, but which are now, happily, fast dis- appearing under the influence of modern scien- tific teaching. From very early times, philosophers occu- pied themselves with investigations into the nature and effects of light, and it may be inter- esting to notice some of these speculations. Pythagoras conceived light to consist in the emission of innumerable minute particles, con- tinually taking place from the surface of all substances, and on entering into the eye, there producing the sensation of vision. Plato, on the contrary, and those who followed him, reversed this ingenious theory, and adopted a more extravagant and improbable one instead of it. They conceived that vision was the conse- quence of the emission of something from the eye, which met with certain other particles or emanations from external bodies. Aristotle contented himself with stating that light was the colour of fire, which does not contribute much to our knowledge. Many others suc- ceeded these philosophers, and perplexed them- selves and their disciples with vain and fruitless, though highly imaginative and fanciful, specula- tions. Until the time of Newton the nature and many of the properties of light remained obscure ; and even his powerful mind, though B 2 4 LIGHT. capable of forming an ingenious and probable theory on the nature of light, failed, as such efforts generally do, to demonstrate conclusions in a manner sufficiently incontestable to esta- blish their complete accuracy. If, therefore, the minds of philosophers, though powerfully exercised upon this subject, failed to seize its real features, and could not enlighten the world, it is not surprising that the people conceived and retained very fanciful ideas about the nature, properties, and effects of light. And those philosophers who lived in the seventeenth century, and who were characterised as much for their desire of astonishing and frightening, as by that of instructing the world, found in this beautiful creation a means for fully gratify- ing their own childish amusements, and for exciting supernatural fears in the minds of the ignorant among whom they lived. Like those who delight in deceiving, they were themselves deceived, and adopted as truths wild ideas, of which a child of our own age might discover the fallacy. There would seem to have been a constant tendency in the minds of the experimentalists of that, and of preceding and succeeding periods, to connect the nature of light and that of pre- cious metals, such as silver and gold. And it is certainly remarkable and deserving of atten- LIGHT. 5 tion, that preparations of silver and gold are among our present most reliable photographic agents, that daguerreotypes are impressions of light on plates of silver, and that gold can also be rendered sensitive to luminous influences. So long ago as 1556, the alchemists discovered that light had the property of blackening a preparation of silver, called “ horn silver,” and this discovery immediately gave birth to the most fantastic ideas as to the real nature of this subtle agent. They regarded light as the great primary cause which modified their salt, sulphur and mercury, and transmuted them into the precious metals. The theories of Homberg, in particular, embrace this idea and develop it. The sun’s rays, he observes, will insinuate them- selves into bodies so as greatly to increase their weight, and he adds, that four ounces of a sub- stance, called Regulus martis, in powder, were augmented by -jo th in weight, by being exposed for an hour at the distance of a foot and a half from the focus of the Duke of Orleans’ burning- glass, notwithstanding much of it was dissipated in smoke — a result the accuracy of which need not be doubted, but which was attributable to the absorption of oxygen by this substance, when heated in the air. A perfect metal, he says, is nothing more than very pure mercury, whose small particles are every way pierced and filled 0 LIGHT. with the sulphureous principle, or matter of light, which links and binds the whole mass together. “ Gold differs from silver in nothing but in having the globules of mercury whereof it consists penetrated through and through, and being more fully saturated with the sulphureous principle, or rays of light/’ So intense was the belief that light interpene- trated and influenced all material substances, and determined both their nature and form, that it was said that even inorganic substances, such as stones, were under its powerful effects. Of this kind were the moon-stones spoken of by several old authors. One writer positively affirms that he possessed a stone which followed in the alterations of its size and form all the changes of the moon. Another of these stones is said to have been brought to Europe from India, (the storehouse of marvels for that era,) which had the same character. This stone was of the size of a pigeon’s egg, and was quite black in all parts but one, where was a clear spot of a circular form. This spot enlarged as the moon enlarged, and waned in size as it waned, so that at the new moon it was only as large as a millet-seed, but at the full it had grown to the size of a pea. This mark is also said to have described a course on the black stone like that of our satellite in the sky, and was esteemed so LIGHT. 7 great a wonder that it was sent over to the King of England, for his inspection. Kircher tells us that he saw a crystal in a museum at Rome, which contained a fluid in its centre, which by its rise betokened that of the sun, and at sunset it sank back again. That gems and precious stones were influenced by light, absorbed it, and then gave it out in the dark, has long been a popular belief, and it is to a certain extent supported by fact. But in the early history of science, this fact, like many others, was regarded in a supernatural, or, to say the least, an extravagant light. In the Eastern fables no subject is more familiar to the reader than the wonderful gems which emitted light in the dark, and the subterranean abodes illuminated by precious stones alone. Carbuncles, in par- ticular, were invested with this singular power, and were thought to have a sympathy in the brilliancy of their light with the fortunes of the possessor, or of some person in whom he was interested. On the first discovery of this pro- perty in precious stones, it was scarcely credited even by those who were readily disposed to give belief to fables. But when a few authors had repeated the statement and confirmed it by appeal to certain facts, then popular belief went beyond the truth of the case. Cellini states that he has seen this precious stone glowing 8 LIGHT. like a coal with its own light, and similar statements are made by others. Other mineral substances were also found (together with bodies belonging to the animal and vegetable world) to emit light in the dark, and minute instructions are given by some authors for the preparation of these substances. The result of many of these investigations was to lead to the discovery of that remarkable elementary body, phosphorus, in a combined state, as the cause of most though not of all the instances of phosphorescence which were then known. That some of the old philosophers entertained clear ideas as to the cause of the emission of light in the dark by certain minerals, will be evident from their own statements. Thus Kircher says, that on exposing some of these substances to the light, they seem to absorb it into their interstices, and then when brought into the dark, the light thus imbibed is again emitted, and becomes visible. He states also, that if these substances are immediately shut up in a dark box after being exposed to bright light, their properties of giving out light in the dark will be retained longer than if ex- posed to the open air. This subject belongs to the interesting and remarkably perplexing phenomena included under the general term, phosphorescence. LIGHT. 9 Light was also thought to have a most re- markable influence over members of the animal and of the vegetable kingdom. Pliny states that ants are much influenced by the state of the moonlight in the arrangement of their labours. When there is little light from that satellite, they were said to work but little, and to be more and more diligent until the period of full moon. After this the insect labourers were said to rest and do little work until the new moon once more roused them to active employ- ment ! Others stated that the eyes of cats and other night-prowling animals were brilliant or dim in proportion as the moon shone with full or diminished effulgence. The spots on the panther’s skin, which bear some resemblance to the outline of the moon, were thought to be affected by its light, and exactly to imitate its various phases. Oysters and other shell-fish were said to grow fat or lean according to the phases of the moon, and this was a most widely- spread popular opinion. The well-known phenomenon of the sleep of plants, and the effect of sunshine and cloud upon the leaves and flowers of many plants, early attracted attention, and gave rise to a number of singular errors on the influence of light. Thus the common and singular plant found in every hedgerow, the arum , or, as it is 10 LIGHT. often popularly called, “ Lords and Ladies,” was said to have such an antipathy to the blaze of sunshine, that its singular envelope turned like a parasol and shaded the plant from the fierce heat of the sun all day. Even shadow was thought to have its influ- ences, as well as sunshine and moonlight. The shadow of a walnut-tree was thought to cause headaches, whilst that of the linden-tree imme- diately cured these ailments. Popular belief held too, that he who had been once bitten by a mad dog, and cured, would infallibly be again seized with the disease if exposed to the shadow of the cornel, or wild cherry-tree. It would seem almost incredible that such effects as these could be really attributed to the mere absence of light, or in other words, that shadow, which is merely the negation of light, should be deemed to have a positive nature and influence. Yet there is abundant evidence that such was really the belief entertained. Upon artificial light, as well as upon the great subject of natural light, absurd errors were en- tertained. One of the most enduring and wide- spread of these, was the fable about Perpetual Lamps, which it may be interesting to the reader to reproduce in these pages, since, not very long ago, at the meeting of one of the learned associations of our own age, this idea LIGHT. 11 was again brought forward, and supposed to have received fresh confirmation in the recent opening of a long-closed sepulchre. Camden believed, and has recorded, the tra- dition that the father of Constantine the Great was buried in York, and that there was found a lamp burning in the vault of the little chapel wherein he was thought to be buried. “ Lazius,” he says, “ tells us that the ancients had an art of dissolving gold into a fat liquor, and of pre- paring it so, that it would continue burning in the sepulchres for many ages.” Thus wrote Camden in 1607. A distinguished modern antiquary, at the meeting in question, stated that he was inclined to think there must be some foundation of fact for these ignited lamps, as in all cases the flame is said to have been extinguished on the admission of air into the tomb. A recent discovery of a perpetual lamp was made at Bacna, on the route from Granada to Cordova, and near the site of the ancient Castellum Priscum. A number of labourers were busy in harvest-time, in 1833, near the old Roman tower ; and a boy, idling in the vicinity, perceived a cleft amongst the ruins, which ap- peared to penetrate to a considerable distance. In the expectation of finding some treasure chambers, the farmer and some of his men effected a breach sufficiently large to admit of 12 LIGHT. the boy’s being lowered into the cavity by a rope. His feet soon touched the ground* and his eyes were dazzled by the yellow light cast upon the pavement, walls, and low stone bench which ran round the room. The labourers, in trying to loosen the lamp, broke it, and the liquid which fed the flame was lost. It is impossible to say what amount of cre- dibility attaches to this tale, but it appears to have been fully believed by many talented anti- quaries of our own time. In the “ Archseologia” of the Society of Antiquaries, will be found an account of the discovery of a Roman sepulchral lamp, in a “ barrow,” at Barblon in Essex. The tomb was opened by an archaeologist, fortunately, and the lamp was discovered in one corner of it, with all the appearance of having been long ex- tinguished. The lamp, with its contents, was sent to Mr. F araday , the eminent chemical philo- sopher. In it was contained a cake of a sub- stance, dry, brittle, and earthy in appearance. The upper surface of it was black, the lower green, from its contact with the bronze of the lamp. This substance was altogether combus- tible, and consisted simply of a fatty fuel, much changed by time. In the beak of the lamp was found a wick, evidently consisting of a fibrous vegetable material, about an inch in diameter, and half-consumed. Near the lamp stood what LIGHT. 13 has been believed to be a curule chair — indicative of the official authority, or of the noble rank, of the tomb-tenant. But in this instance the lamp was not said to have been alight, as it might have been, and doubtless would, had it been opened by some ignorant and superstitious person. There can really be little doubt that these per- petual lamps have had no existence beyond that which they have retained in the imagination of men. For it is well known that light, in ordinary circumstances, as in a lamp, is produced by the ignition of solid particles of matter ; the light of a lamp is due to the ignition of the carbon of its fuel. In burning, then, a certain amount of solid matter is consumed every minute. Dr. Ure calculates that a mould candle consumes rather more than a hundred grains of tallow per hour. If we allow, to make a rough estimate, sixty grains of solid carbon to such a light per hour, this would demand about seventy pounds of solid carbon for one year, or about three tons for a century, for the production of the light alone of such a flame. This is, however, only an approximative statement of the case, as we are still to account for that portion of the fuel which contributes to the non-luminous part of flame. Thus the whole of a tallow-chandler’s warehouse would only supply a mould candle with fuel for about a century. 14 LIGHT. The solution offered originally by the present writer in the periodical from which this ex- tract is taken, is this: — The gas phosphuretted hydrogen is the product, in certain circumstances, of the decay of animal substances, and instantly shines with a phosphorescent light on coming in contact with air. Is it not probable that the de- caying remains deposited in the grave may have, in the course of years, been slowly evolving small quantities of this gas? Let the tomb be sup- posed to contain some of this gas, and an extinct sepulchral lamp: some labourers break into it, the air falls upon the luminiferous gas, and the vault is filled with light, which the ignorant intruders refer to the lamp it enables them to distinguish ; they seize upon the lamp, and pre- sently the light disappears, the whole of the phosphuretted hydrogen has been consumed, and the vault is in darkness. The idea is perhaps worth entertaining, and appears to afford a simple and not improbable explanation of a long-lived archaeological chimera. We have, however, dwelt long enough in the region of popular errors about light, and shall now advance into the more interesting and in- structive domains of philosophic truth. CHAPTER II. WHAT IS LIGHT? Before proceeding to give an account of the phenomena of light, it appears desirable to offer a brief notice of the opinions entertained by modern theorists as to its real nature. For it greatly assists the mind in forming a clear conception of a subject, to have certain data presented to it, upon which opinions may be based, and from which ideas may be justly developed. Were light a ponderable substance, could it be seized by the chemist, and when entrapped, placed in the balance, submitted to the flame, or attacked by reagents, we might offer certain and conclusive information as to its composition, and, to an extent, as to its nature. But, like heat and the electric force, it eludes our grasp in all these directions. We cannot seize its subtle beams, and hold them whilst interrogating their nature. We cannot weigh the ray in which the mote dances, although we can seize the mote, and learn without error its nature and elements. We have no means of 16 LIGHT. really judging with accuracy as to the nature of light ; we can offer no distinct antago- nistic force to its wonderful beams. We are consequently left to conjecture in a great measure. But then conjecture may in this instance receive a certain degree of support from the phenomena exhibited by light, when we interfere with its passage by apparatus ; and though modern philosophy has ranged itself on the side of two distinct theories about light, these are not to be viewed as mere plausi- bilities, but derive a certain degree of relative probability from distinct and well-ascertained facts. No one can enjoy the delightful faculty of vision, which is to the mind a floodgate of sensations from the external world, to which without it we should be strangers, and not have occasionally asked himself what can be the nature of that force by which the world without thus penetrates into the world within. Let us contrast this faculty with the other senses, and the peculiar force of this inquiry will imme- diately be obvious. Touch is a communication of intelligence from physical objects through the nerves, which are physical structures, to the brain. Taste is the impression of certain chemical sub- stances upon similar nerves, communicated to the same centre of sensation. The sense of LIGHT. 17 smell is due to the rise of chemical vapour (for such even the fragrant odours of the violet and the rose can be proved to be) upon the air, and its application to nerves peculiarly disposed and arranged for the reception of odorous vapours. The faculty of hearing, again, is due to the pro- duction of waves in the air, which strike the nerves of hearing, and they convey the intelli- gence onwards. In each of these cases a physical cause produces a physical effect. Let us, however, caution the reader against an extreme deduction from these, which are well-ascertained facts. We know that it is the vapour of otto of roses, — which we can che- mically analyse, which strikes the nerve of smell, and gives us the perception of the rose odour ; but this is all we know. We know nothing as to the manner in which this remarkable intel- ligence is imperceptibly conveyed to the brain. We can trace its effects, but we know nothing as to the manner in which these effects are con- veyed along and through the delicate filaments of nervous matter. What we clearly perceive in these four senses is this, that certain physical causes which we can measure, weigh, decom- pose, and whose nature we can precisely state, produce the effects which we call feeling, taste, smelling, and hearing. But with regard to light, the case is widely C 18 LIGHT. different. We know not, with certain know- ledge, why the fields and flowers at night no longer become visible, and all nature loses its lovely garment of many colours ; nor why, when the clouds of morning are removed and the sun rises, the earth again is seen clad in green, the flowers in gold and purple, the birds in their respective hues, and the sky stretched as a blue vault above us. All we can know about this matter is, that it is so perceived by the eye, and that the apparent cause is light — in this instance, the light of the sun. But if we go to examine into the matter more minutely ; if we would know what light is, which produces these charming effects, just as we know what the medium is whereby strains of music come into our ears ; then we approach a region of theory and conjecture, where only a dim perception of what is probably true can be obtained even by the most penetrating mind. We shall endeavour to give the reader the most distinct views we can on this interesting but perplexing subject; and the attempt will not be altogether vain, if only he be thus led to think of the unperceived wonders which sur- round him on every side in this beautiful world in which he is placed. It would, indeed, be a mere presumption to suppose that what was left by Newton himself as a mere theory, can LIGHT. 19 be raised into the region of demonstration by any statements here to be brought forward : we must estimate the views of the nature of light held by modern philosophers at their proper value, as hypotheses; beyond this it is not safe or prudent to venture. When, to use the words of Sir John Herschel, “ two theories run parallel to each other, and each explains a great many facts in common with the other,” it becomes extremely difficult to say which is the correct one, and any means of judging by experiment between the two becomes important. In the case before us, the opposing theorists can each produce experiments which appear to decide the question on their respective sides, but the existence of such facts of apparently opposite tendency only affords us additional evidence of the impossibility, in the present state of our knowledge, of arriving at a satisfactory conclusion. The theory of Newton may be expressed in the following manner. Light is conceived to consist of excessively minute molecules of matter, projected from luminous bodies with immense velocity, and acted on by attractive and repul- sive forces residing in the bodies on which they impinge, which turn them aside from their course, and reflect or refract them in determinate proportions and directions. The velocity of the c 2 20 LIGHT progress of these minute particles was estimated at the enormous rate of, in round terms, 200,000 miles a second, and to be nearly the same what- ever the source of the light. These particles, penetrating the arrangement of lenses in the eye, and striking upon the sensitive surface, or retina, were thought to stimulate it, and thus excite the phenomena of vision. Thus, accord- ing to this view of the nature of light, a sun- ray is understood to mean a continued succession or stream of tiny particles, all moving with great velocity in a straight line, and following each other so close as to keep the sensitive surface of the eye continually excited, so that before the impression produced by one can have time to subside, another shall arrive, thus perpetuatiug the sensation. By means of a whirling stick with a lighted piece of charcoal at its point, it has been found that in order to give to the eye a continued sensation of light, it is sufficient to repeat the impression about eight or ten times in a second. In other words, if a flash of light were to be produced by an artificial arrangement, as often as ten times in a second, it would seem to be a continuous light, and not an interrupted one. From this it has been argued that it is not necessary to suppose the minute particles of light to follow each other very closely, since LIGHT. 21 they move at so high a velocity that if they were 1,000 miles apart from each other, still, at the rate of 200,000 miles a second, 200 of them would reach our retina per second in every ray. This consideration renders it easy to un- derstand that these minute atoms are not likely to disturb or jostle against each other in their passage through space. Their extreme minute- ness may be imagined from the fact, that although flying at such a vast velocity, per- ceiving the lenses of the eye, and striking upon the retina, a surface covered with the most tender and delicate nerves, still light produces no injurious effect upon our organs of sight — if they are in a healthy state ; but if diseased, then the eye can no longer endure the full impress of those particles, and a shade becomes necessary to protect it from their effects. If one of the tiny particles of light had but the weight of a single grain, the violence with which it would strike us, would equal that of a cannon- ball of 150 pounds’ weight moving at the rate of 1,000 feet a second! What, then, must be the wonderful minuteness of those particles which are thus continually pouring into our eyes, and communicating, instead of a sensation of violence and pain, a pure and gentle sense of gratification and pleasure ! The concentration of millions upon millions of these particles by 22 LIGHT. lenses and mirrors, was attempted in order to estimate their mechanical effect, but without success. In the newly-discovered art of photography, however, we have an evidence of a mechanical effect, or something very like it, being produced by solar light. Although it will be immedi- ately necessary for us to indicate the fact, that light is not to be regarded as a simple but com- pound creation, still, the evidence which we now adduce of the effect of certain parts of the solar ray appears to give some support to the Newtonian theory of the emission of particles of matter as constituting light. A silver plate highly polished and brought to the finest pos- sible state of surface, and then exposed to the vapours of iodine and bromine, will, if exposed to light and afterwards to the vapour of mercury, afford a most interesting evidence that something very like a mechanical effect has been exerted upon it. A picture appears on its surface, such as the outline and detail of a building, in all the perfection of the daguerreotype process. With a piece of cotton wool, and with an abrasive composition, consisting of powdered tripoli and spirits of wine, this impression can be easily removed, and the surface again may be po- lished, and glittering like a mirror, reveals no trace of the picture it formerly represented. LIGHT. 23 But if it be again exposed to the vapours of iodine and bromine, and then to that of mercury, the picture thought to be for ever lost, will reappear on the surface , provided the grinding down of the surface by the tripoli has not been carried too far, and this without any exposure to light again. It will, in fact, be found that a very prolonged scouring of the surface of the silver plate will be necessary in order to grind away that part of it into which the light has struck so deeply. A phenomenon such as this can be accounted for on no other supposition than that some mechanical action has taken place. Its effect is as obvious, as though some invisible hand had with a delicate graving tool marked the surface with the lines constituting the picture.* Again, if a plate of glass be coated with a solution of gun-cotton containing iodine, and then plunged into a watery solution of nitrate of silver, it is rendered wonderfully susceptible to the action of light. If it be then exposed in the camera obscura, and afterwards removed into a dark room, and there covered with a solution of a vegetable acid, the picture comes out in the most lovely and delicate manner. Let the plate be then washed and dried, and put aside for a * For this and the following examples the Author desires to be considered alone responsible. 24 LIGHT. time, it will exhibit on its surface the most unmistakeable evidence that some mechanical action has been concerned in producing the picture : for, in holding the plate in a hori- zontal position, and allowing the light to fall obliquely on its surface, the picture will be seen to be actually bitten into the surface of the gun-cotton. Delicate but distinctly visible tracery will reveal to us where the pencil of the sun has executed its surprising and rapid performance. In the latter case the picture requires to be of very first-rate excellency, and the plate in the best condition, otherwise the effect, so wonderfully mysterious in its nature, will be little if at all evident to the observer. Of its real existence, however, all those who are skilled in photographic art are fully sensible. Some persons think it impossible to regard such phe- nomena without the strongest disposition to give to the Newtonian theory the preference; others, again, may explain this appearance by the power there is in the solar ray to produce chemical action, and thus promote corrosion by the nitric acid in the nitrate of silver. The other view of the nature of light is that commonly called the Undulatory theory, and a most beautiful chain of consequences is deve- loped by this hypothesis. It is generally LIGHT. 25 called the theory of Huyghens, and has received the support and advocacy of Young, Fresnel, and other philosophers of the highest position. In this theory, light is supposed to he caused by the waves or vibrations of an infinitely elastic medium called ether, diffused through all space. This ether is supposed to pervade all material bodies, to occupy the intervals between molecules, and by its extreme rarity to offer no resistance to the motions of the earth, the planets, or comets in their orbits. It is thought that the particles of which this ether consists are capable of being set in motion or agitated by the particles of ponderable matter, and that the agitations thus produced spread in every direction with great rapidity, in the same way as the waves produced, by casting a stone upon the clear surface of a lake. Our sensation of light is thought to be due to the vibrations of this ether passing through the eye, and reaching and agitating the nerves of the retina, in the same way as the agitation of the air produces vibrations which affect our nerves of hearing with the sense of sound. By a wonderful process of calculation and experiment, it has been shown that the number of waves or undulations in a single second of time is, to produce violet light, 727,000,000,000,000, and for red light, 458,000,000,000,000; and this 26 LIGHT. calculation applies with equal correctness to either of the theories above mentioned. The essential difference in these hypotheses may be thus defined. In the Newtonian theory a luminous particle is supposed actually to come from the sun to the earth ; in the undulatory theory, the sun only occasions a disturbance of the ether, which is propagated with vast rapidity according to the same mechanical laws which regulate the propagation of undulations or waves in other elastic media, as air, water, or solids, according to their respective constitutions. In both these beautiful theories, an undulating or wave motion is admitted, and both theories explain most of the phenomena of light. A for- midable objection to the wave theory was once supposed to be the simple experiment of the production of colour by the prism. But amongst our own countrymen, Professor Baden Powell, of Oxford, has shown that the dispersion of light into different colours may be deduced very accurately from the undulatory theory of light, and he has calculated the proportions of the divisions of the spectrum with surprising accu- racy from theory. Newton suggested another apparently for- midable objection to this theory, which seems so simple, that no apology need be made for al- luding to it in an elementary work of this nature. LIGHT. 27 Sound, winch is by all allowed to consist of undu- lations in the air, comes freely round a corner ; why cannot light, if it be also merely a series of undulations in an elastic ether? Hence, why have we shadows? For the undulations of light ought apparently to be as capable of going out of a right line as those of sound in the air, and we should, therefore, have no shadows in nature. In reality, however, this objection has been completely overcome. It is the subject of a particular investigation by the Astronomer Royal ; and it has been shown that, on the ma- thematical theory of undulations, this objection becomes a triumphant proof of the fertility of that scheme to meet difficulties which at first seemed insurmountable. Newton himself was far too much of a seeker after truth to allow any theory, however interest- ing, to occupy too large a share of his attention. He so strongly disliked the fierce controversies which arose about his theory, that he said, “ I no further intend to be solicitous about matters of philosophy.” And subsequently, when speak- ing of further researches into the subject, he says, “ I find it yet against the grain to put pen to paper any more on that subject.” And to his illustrious contemporary, Leibnitz, he writes, “ I was so persecuted in the discussions arising from the publication of my theory of 28 LIGHT. light, that I blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow.” With a view to put the reader in possession of some information on this interesting subject, we have thus endeavoured to state generally the outlines of the Newtonian and undulatory theo- ries. And the adoption of either of these schemes of the constitution of light has this advantage, that it does not suffer the imagi- nation to take the place of reason ; for though neither can be proved to be true, yet both assist us in so remarkable a manner in our com- prehension of the varied phenomena of light, that it may be safely said “that either may represent the true state of the case.” And these theories have this value, “ that they offer an excellent means of classifying and grouping together various phenomena, and of referring facts to laws which, though possibly empirical, are yet, so far as they are so, correct represen- tations of nature, and as such, must be deducible from real primary laws whenever they shall be discovered.” * It is impossible to trace the path of a sun- beam through our atmosphere, without feeling some desire to know its nature, by what power it traverses the immensity of space, and the * Sir J. Herschel. LIGHT. 29 various modifications it undergoes at the surfaces and in the interior of terrestrial substances. We shall, therefore, in our next chapter, invite the reader’s attention to a general consideration of the constitution and phenomena of this inte- resting element, if such it may be called. CHAPTER III. CONSTITUTION AND PHENOMENA OF LIGHT. There is a very decided distinction to be drawn between the nature and the constitution of light. If we occupy ourselves with the nature of this wonderful force, we are led into a wide field of speculation, and have but little certainty of guidance from physical investigation in so doing. But if we examine the constitution and phenomena of light, we are upon a solid basis of investigation; for we can deal with it in a variety of ways by our apparatus, and inter- rogate its phenomena with some hope of defining their laws and mutual dependence. While the subject of our last chapter was one open to much doubt, that of the present is perfectly clear and certain in its character, and the laws of chemistry do not admit of a more satisfactory demonstra- tion than those of light. Light falling upon any substance is subject to three principal alterations. Some of it may be turned back into the medium, such as air, in which it was moving before it came to the sub- LIGHT. 31 stance ; then it is said to he reflected . Thus, a ray of light falling in the direction of A B on the surface CBD, is reflected in direction B E, so that the angle A B c is equal to the angle E B D. Some portion of it may proceed in the direction B F, if the substance is of the kind called transparent , mak- ing, with the sur- face, a different angle to what it made before . it is then said to be refracted , or its path broken at B, and some of the light of the ray A B at B is scattered , i. e. dashed in all directions from B. If we are looking at a piece of glass in the sunlight, the light by which we see its surface is scattered light ; if it reflects to us a bright dazzling spot, this is an image of the sun seen reflected light; and the light that passes through the glass on to the floor is refracted light . The law of refraction may be thus stated: — So long as a ray of light traverses a uniform medium, it continues its path in a right line, which it also preserves when it falls on a transparent substance in a direction perpendicular to its surface. But if the ray be 32 LIGHT. incident in an oblique direction, it becomes to a greater or less degree bent out of its original course, and this bending is called refraction. Its most simple example is the common experiment of placing a shilling at the bottom of an empty basin, and then filling it with water, when the coin becomes visible at a distance from which it could not previously have been seen. This result is due to the bending aside of the rays of light on their passing into the air from the water in the vessel. In some cases this refrac- tion is still more peculiar ; the light proceeds from B in two directions B F and B f', showing that it has been separated at B by the substance into two rays (see the figure of the preceding page) ; and this is called “ double refraction.” Humboldt has given an interesting outline of the gradual discovery of some phenomena of light, which will furnish us with a short notice of this part of our subject. While, he observes, Huyghens was occupied with the double refraction of light in crystals of Iceland spar, that is, with the separation of the pencils LIGHT. 33 of light into two parts, as they pass into the transparent crystal, he also discovered, in 1678, that kind of polarisation of light which hears his name. By polarisation is meant, that the ray of light said to he polarised is in a peculiar state different from common light, and subject to different laws; as if, in changing its condition, its constitution had suffered some radical change. The discovery of this isolated phenomenon, which was not pub- lished till 1690, and consequently only five years before the death of Huyghens, was fol- lowed, after the lapse of more than a century, by the great discoveries of Malus, Arago, Fresnel, Brewster, and Biot. Muller, in 1808, discovered polarisation by reflection from po- lished surfaces; and Arago, in 1811, made the discovery of coloured polarisation. A world of wonder, composed of manifold modified waves of light having new properties, was now revealed. A ray of light, which reaches our eyes after travelling millions of miles from the remotest regions of heaven, announces of itself in Arago’s polariscope, whether it is reflected or refracted, whether it emanates from a fluid, solid, or gaseous body, announcing also even the degree of its intensity. This discovery shed new and* certain light upon the condition of the sun’s burning envelopes, the light of comets, the zodiacal light, D 34 LIGHT. and tlie optical properties of our atmosphere. Thus, observes Humboldt, does man create new organs, which, when skilfully employed, reveal to him new views of the universe. Next to this great discovery, namely, the polarisation of light, the remarkable phenomenon, named the interference of light, deserves to be noticed. Faint traces of this phenomenon were observed, in 1665, by Grimaldi and by Hooke. Modern times owe the discovery of the real nature and existence of the interference of light to Thomas Young. By his investigations the re- markable fact was ascertained, that two rays of light emanating from one and the same source, but with a different length of path, destroy one another, and produce darkness ; a fact which is explained clearly on the undulatory theory, but with less satisfaction by the Newtonian theory. To go back, the velocity of light was measured by Bomer inl675; and at the same periodNewton was pursuing his laborious and splendid experi- mental researches into the phenomena of light. The publication of Newton’s great work on optics did not take place till 1704, but most of his important discoveries were made a con- siderable time before that event. Thus gra- dually, by labours tedious and oppressive to a degree of which we can form little concep- tion, the constitution and phenomena of light UGHT. 35 were revealed, and a system of accurate know- ledge, based on reliable experiment, was deve- loped. Let us now examine what we may call the structure of a ray of light. When a beam of sunlight enters a room, and falls on a white wall, it is in what we may call a combined state • — it appears like a streak of white light. In this state the ray left the great source of light, the sun, and now presents itself for our examination un- altered by its transmission through the millions of miles intervening between that luminary and ourselves ; from all that can be seen of it by the unassisted eye, it would not be thought to be otherwise than a simple ray, incapable of being resolved into any colour. Let us now exclude all other light from the apartment, and permit the ray to enter through a small hole in the shutter. We have now a beautiful object before us, but still not the least altered. A gleaming path ruled with the straightest lines shines through the air, filled with dancing motes, exhibiting to us the beam of light in its undecomposed character. A long piece of glass of triangular section, called a prism, may be purchased at any of the opticians ; or it may often be found in the ornaments of the girandoles, in our drawing-rooms. By the as- d 2 36 LIGHT. sistance of this simple instrument, we can inquire into the nature of the brilliant beam before us. Sir Isaac Newton adopted this simple apparatus when commencing his investigations on light, in discovering the composition of white light. In the account of his expenses at Cambridge occurs the item for the purchase of a prism, which must be regarded as the commencement of a series of investigations on light never since equalled, and, probably, never to be surpassed. Having made a circu- lar hole in a shutter, and darkened the chamber, he let in a small beam of sunlight, which, when intercepted by the prism, as in the figure, was so marvellously in- fluenced by it as to appear no longer an undivided beam of white light, but a streak of many charm- ing colours, called the Solar Spectrum . It was at first, to use his own words, “ a very pleasing divertisement to view the vivid and intense colours produced thereby;” but this pleasure 38 LIGHT. was immediately succeeded by surprise at various circumstances which he had not looked for. And it is certainly a remarkable circum- stance that the beam of light, which, shining on . the wall, appears only as a little round spot resembling the hole through which it has been admitted, is instantly resolved, when it has passed through the glass, into a streak of various colours, not less than five times larger than its breadth. The streak of colours into which the beam of light has thus resolved itself, by means of the simple instrument we have employed for this purpose, is called the prismatic or the solar spectrum. And inquiry into the consti- tution of this spectrum, or coloured streak, will inform us of many interesting qualities of the light of which it originally consisted. The dia- gram on the preceding page presents us with an analysis of the solar spectrum, and a standard of comparison for its different constituents. By this it will be seen that these distinct spec- tra coexist in the prismatic spectrum ; namely, light, heat, and actinism. Their distribu- tion over the surface covered by the spectral image is different in each case; the actinic spectrum is the longest, that of heat the next, and that of light the shortest. The intensities of the spectra also differ ; thus, as seen by the LIGHT. 39 curved lines, the intensity of the actinic force is greatest in the violet rays, that of heat in the red rays, and that of light in the yellow ; and conversely the actinic force is smallest in the yellow rays, that of heat in the violet rays, and that of light in the fluorescent rays. This description will, however, he better understood as the reader advances in this chapter. It might be questioned whether this beautiful streak of colours really did compose white light before passing through the prism, or whether they were due, not so much to the effect pro- duced upon the rays by the prism, as to some inherent colour, so to speak, in the prism itself. But a simple experiment answers this question in the most satisfactory manner, and proves un- questionably that the coloured streak is really decomposed light. By taking another prism similar to the first used, and placing it in con- tact with the former, so that the light passing through them both may be refracted in contrary directions, and thus made to assume the course they originally had in their way to the wall, this remarkable effect is produced : the brilliant streak wholly vanishes, and in its place the beam is seen to pass to the wall or screen, just as if no prisms interfered with its progress, a round white spot on the wall again indicating the termination of its course. 40 LIGHT. The prismatic spectrum affords us therefore evidence, that light is really a combination of rays, and not a single ray, and that these rays have different colours, and also different degrees of refrangibility ; that is to say, that some are more bent out of their course by their passage through a body like glass than others. Sir Isaac Newton, from his experiments on the spectrum, was led to believe that light really consisted of seven coloured rays, a union of which, in certain and definite proportions, con- stituted white light. Subsequent observers have sometimes thought that Newton’s conclusion is not in this instance correct. Sir David Brewster, amongst others, considers that light consists only of three coloured rays — blue, red, and yellow; ; and that a proper combination or series of com- binations of these three colours will produce white light, and all the gradations of colour now known to us. We shall, however, resume this subject again toward the close of this chapter. We have not yet completed our examina- tion of the spectrum. We shall find, on careful examination, that the colours exhibited by the streak are not all equally brilliant. The red is faintest at the one end, and brightens up as it approaches the orange, while the violet at the other end is also very faint, and deepens LIGHT. 41 as it approaches the middle colours. The brightest part of the Spectrum is the middle of the yellow, from which point it gradually declines on each side. The rays most bent aside, or, in the words of science, most refran- gible, are the violet ; and those least bent aside are the red. It may now be asked, what has become of the heat ; which we know to exist conjointly with the rays of light in the sunbeam ? A careful exami- nation of the prismatic spectrum will not fail to give us the reply to this question. It was for- merly thought that all the rays of light were attended by the same number of heat rays ; and that therefore, in the prismatic spectrum, the violet would be as hot a colour as the red or yellow. Dr. Herschel’s attention was early drawn to this subject by the following circum- stance: — “In a variety of experiments which,” he observes, “ I have occasionally made relating to the method of viewing the sun with large telescopes to the best advantage, I used various combinations of differently coloured darkening glasses. What appeared remarkable was, that when I used some of them, I felt a sensation of heat, though I had but little light, while others gave me much light, with scarce any sensation of heat. Now, as in these combinations the sun’s image was also differently coloured, it 42 LIGHT. seemed to me that the prismatic rays might have the power of heating bodies, very un- equally distributed among them.” In order to determine this question by ex- periment, the following method was adopted : — Each ray of the spectrum was made to pass through an opening in a piece of pasteboard, and to fall upon the blackened bulb of a highly sensitive thermometer. Thus it would soon be evident, by the rise of the fluid in the ther- mometer being greater when placed in rays of one colour than in those of another, whether the supposition entertained were correct or not. The result of accurate experiment completely con- firmed the suggestion ; and it was found that the red rays possessed a greater amount of heating power than any others in the whole streak. But another very remarkable discovery was the fruit of these investigations. By carrying the thermometer quite out of the streak, and a little beyond the termination of the red rays of the spectrum, it was found to rise to a higher degree than in any of the coloured rays. Although quite in the dark, yet it became evi- dent that rays of heat were falling upon its bulb, in a greater measure than when under the full play of any of the rays of the coloured streak. The evident conclusion to be drawn from this is, that rays of heat, in passing through LIGHT. 43 •the prism, became dissociated from the rays of light with which, up to that point, they had been blended, and that, being less bent aside in their passage through the glass than the rays of light, they fall chiefly upon another part of the screen. To express this fact in accurate terms, we may quote the words of the discoverer of this interesting phenomenon. “ A beam of radiant heat emanating from the sun,” says Dr.Herschel, “ consists of rays that are differently refrangible. The range of their extent, when dispersed by a prism, begins at the violet-coloured light, where they are most refracted, and have the least efficacy. We have traced these heat rays throughout the whole extent of the pris- matic spectrum, and found their power increas- ing, while their refrangibility was lessened as far as to the confines of red-coloured light. But their diminishing refrangibility and in- creasing power did not close here ; for we have followed them a considerable way beyond the prismatic spectrum into an invisible state, still exerting their increasing energy with a decrease of refrangibility up to the maximum of their power ; and have also traced them to that state where, though still less refracted, their energy — on account, we may suppose, of their now failing density — decreased pretty fast ; after which, the 44 LIGHT. invisible tliermometrical spectrum, if I may so call it, soon vanished.” It was found that the invisible heat rays exerted a considerable calorific power, at a point 1^ inch distant from the extreme red ray. Some interesting researches of more recent date have been made on this subject, and it has been found that the place of maximum heat is to a considerable extent influenced by the nature of the material of wdiich the prism is made, as also is the extent of the spectrum occu- pied by different colours. With a prism of water, the hottest part of the spectrum is in the yellow rays, and the same is true, also, of alcohol and turpentine. With a prism of sul- phuric acid, solution of muriate of ammonia, or of corrosive sublimate, it was in the orange rays ; while, in the case of glass, it varied from the middle of the red in crown and plate glass, to beyond the red in flint glass. These experiments reveal to us the very interesting fact, that the spectrum is itself a compound streak, and that other forces, though imperceptible to the sight, beside those of light, exist within and play beyond that bright tinted band of colours. In the experiments alluded to, the thermometer was alone employed for the purpose of detecting the existence of other rays besides those of light in the prismatic LIGHT. 45 spectrum ; and for the rays of heat, it forms a very sensitive medium of discovery. But in addition to the heat rays, it is now known that others exist independently of either the particles of light or of heat, though intimately blended with them in the undecomposed ray of light. The evidence of our sensations is sufficient to inform us, that in sunlight the rays of heat also exist ; and that these are not merely one and the same with light, is shown by the fact, that sensation of heat is felt behind substances which completely stop the light of the sun, as black cloth ; and that moonlight, however bril- liant, is attended only with the smallest percep- tible production of heat. We have, therefore, certain information in the simplest way, that light and heat rays both proceed from the sun, and that they are distinct from each other. But beyond this, neither the sensations of our body, nor the indications of mere mechanical apparatus, can give us' much in- formation ; and in the absence of such know- ledge, it might be concluded that no other force existed in the sunshine than its plea- sant light and heat. This, however, would be a very erroneous conclusion ; for it is now certainly ascertained, that another mysterious force exists in the sunbeam; a force whose wide-spread influence penetrates the three king- 46 LIGHT. doms of nature, and leaves effects wherever it comes. This force is called Actinism. Let us now see how we can conclude its existence from the spectrum, to which our attention is still directed. It may he done in the following simple manner : — Apiece of good letter paper is first to he washed over with a solution of sal ammoniac, of about 30 grains in each ounce of water, and dried. This may he done in open daylight, without injury, or such paper may be procured at the photographic stationers, where it is called salted paper. It is then to he washed over with a solution of nitrate of silver, of the strength of about 120 grains in each ounce of distilled water. This is to he done at night, or in a dark room ; and when the paper is dry, it is to he kept in a portfolio away from the light until wanted. Paper thus prepared, will give us a very remarkable and instructive view of the further constitution of the sunlight. Upon the white screen, or wall, on which we suppose the prismatic image to he formed, a piece of paper thus prepared is to he pinned with the prepared side towards the light. If the sunshine is very brilliant and the image very clear, the actinic rays will soon render their operations manifest ; and if the paper has been freshly and well prepared, and is exposed a sufficient LIGHT. 47 time to the light on the screen, a beautiful effect will become visible. A streak of colours, faint but clearly perceptible, is produced on the paper corresponding in a slight degree to the brilliant band of the spectrum. The red ray leaves a red impression on the paper, and this red tint passes into green, over the space on which the yellow rays fall. Above this a leaden hue is visible ; and about the blue ray, where the most decided effect is produced, the colour on the paper passes into a brownish black, and at the end of the spectrum the colour is a bluish brown, which tint is continued even beyond the visible edge of the spectrum ; showing, as in the case of the heat rays, that the rays which stain the paper are not mere light rays, but are of a separate and distinct kind. The part of the spectrum where the paper is most coloured is just the opposite to that which is most heated, — that is to say, the rays of heat tend towards one end of the streak, and those of this singular force, actinism, towards its opposite.* It was found that the thermometer indicated, beyond the edge of the visible streak of colours, that a very decided heating effect was produced. * A reference to the diagram on a preceding page will render this account quite intelligible. The curved lines show the relative distribution in intensity of light, heat, and acti- nism over the prismatic spectrum. 48 LIGHT. The paper prepared as described gives us also in the opposite direction the most positive infor- mation that actinic rays exist beyond the edge of colours at the other end of the spectrum. Now, it is a remarkable fact that the whole sur- face over which the actinic rays are diffused has been found to be more than double the total length of the ordinary luminous spectrum ; and consequently the spectrum, if we can so call it, formed by the invisible rays of actinism, is the largest of the number. In truth, it has been found that the luminous spectrum — that which is visible, and with the consideration of which, as a simple band of coloured rays, we commenced this subject — is the smallest of the results of the decomposition of light. The heat- spectrum is the next in size, and the actinic is the largest of them, being more than twice as long as the luminous. The actinic rays are much more dispersed than the rays of light. It is a remarkable fact, that there are two points in the luminous spectrum where the influence of the actinic rays appears to be in some degree neutralized, and we shall find our paper prepared as above described remain almost white. Yet one of these points is at the very brightest part of the luminous image where it might have been thought the greatest darkening of the paper would have been produced — this is LIGHT. 49 the seat of the yellow rays. The other point is where the greatest heat is perceived, namely, in the red rays. Hence, it would seem that both heat and light are distinct from actinic rays, and that they tend to counterbalance or render inoperative those rays. An ingenious experiment has been devised by Mr. Hunt, which in a very satisfactory way separates the actinic rays from the others. If we take a trough made of glass, and fill it with a solution of bichromate of potash, which is of a full yellow colour, and place it so that the prismatic rays must pass through it in order to reach the surface of our sensitive paper, it will be found that a very slight change has been made in the arrangement of the coloured bands of the spectrum. Yet, notwithstanding the amount of light upon the paper, no change whatever takes place in it, and it remains as white as before the experiment. The cause is that the actinic rays have all been stopped or drowned by the yellow trough through which the light has passed, and consequently merely light and heat now fall on the paper, and they have no power to darken its sensitive surface. If now a mirror be so arranged as to reflect an undecomposed ray on to the paper without passing through the yellow fluid, chemical or actinic change instantly begins, and it darkens E 50 LIGHT. rapidly. But, singular to notice, the paper does not darken over the space covered by the spectrum, although it does over every other part. It thus becomes obvious that the luminous influence protects the paper from change, or, to express the same fact in other terms, that light is antagonistic to the actinic force. ££ We are then,” to use Mr. Hunt’s words, “ driven to the hypothesis of the existence of a new agency, a new imponderable element, or a novel form of force which is broadly distinguished from the luminous and calorific principles or forces in its effects. To mark this, the term actinism has been proposed, and it is now very generally adopted. The word signifies nothing more than ray power, and therefore, as involving no theory, it is free from many of the objections which would apply to any other term adopted from preconceived ideas.” The spectrum still offers us matter for conside- ration. We have developed three great prin- ciples or forces coexistent in it, though differently dispersed over its area; modern research has shown that another curious phenomenon exists in this beautiful image besides those already noticed. If we take a piece of cobalt blue glass and look at the prismatic spectrum through it, we shall immediately perceive the existence of another band of rays in the spectrum, which LIGHT. 51 are quite invisible to the unassisted eye. These rays were first observed by Sir John Herschel, and are named by him the extreme red rays. They are of a crimson colour, and are placed below the ordinary red of the spectrum. We are also able to discover the existence of another set of rays by adopting the following simple expedient: — Instead of viewing the spectrum upon white paper, if we substitute for it paper coloured yellow by turmeric, a ray of high refran- gibility, and situated beyond the violet, becomes visible. This ray is of a peculiar neutral colour, and has been termed a grey or lavender ray. Thus, instead of the seven bands of Newton, we now discover the prismatic spectrum to con- sist of nine. The two last were unknown to him, having escaped his detection. The spectrum affords us another phenomenon of great interest at the present moment. It has long been observed that a solution of sulphate of quinine in water, rendered acid by addition of sul- phuric acid, presents a striking peculiarity when examined by daylight. The solution is perfectly transparent and colourless when we look through it ; but when looking at it with the light of day falling directly upon the surface observed, it assumes a beautiful silvery blue colour. Some of the varieties of fluor spar also exhibit this interesting phenomenon ; and a weak decoction e 2 52 LIGHT. of the inner bark of the horse-chestnut-tree, in water, exhibits it in the most distinct and beau- tiful manner. A peculiar yellow glass, coloured by oxide of uranium, possesses similar properties, but the light from it in this case is a pale sea- green. Sir John Herschel appears to have first given close attention to this subject; and he found that this remarkable play of colour was due to the effect on light of a small thickness of the fluid at the surface by which the light entered. It has also been found that this effect is pro- duced within the fluid. Mr. Stokes has given much attention to this phenomenon, which, from its having been noticed in fluor spar, is called fluorescence; and he has shown by a variety of beautiful experiments, that the rays which most strikingly exhibit this peculiarity are not visible in the ordinary spectrum, but may be made to become so by a little arrange- ment of apparatus. The prismatic spectrum is received on a lens, and the image thus pro- duced is then passed through a glass trough containing a solution of sulphate of quinine, and allowed to fall on a screen. The ordinary appearance of the spectrum is but little altered ; but if we look carefully in at the transparent sides of the trough, it will be found that beautiful small cones of light — of the peculiar colours of LIGHT. 53 the light from the surface of the quinine solution — pass into the solution to various depths where they seem to become lost. These beautiful rays commence in the violet streak and extend be- yond it to a considerable distance. We are unable to detect these rays, — which are thought by some to represent the actinic or chemical rays, by looking at the ordinary spectrum, but their existence is very distinctly shown in the manner just indicated. It is possible that these appearances may be due to incipient crystalline tendencies in the fluid, or tendencies to polar arrangements of its particles. It would seem as though the prismatic image were exhaustless in the variety of subjects for inquiry, which, on attentive consideration, it pre- sents ; and it may certainly be said that few natural phenomena have so much occupied the thoughts, labours, and even the lifetime of philosophers. The brilliant band of colours before us, upon the closest examination with the unassisted eye, appears only as a streak of light, without any dark parts : yet it has been found that no less than 2,000 dark lines, absolutely black, exist in it. These lines were first noticed by Dr. Wol- laston. They may be seen by looking at the spectrum through a good telescope. It is then seen that the coloured rays are crossed by a 54 LIGHT. great number of dark bands or lines, giving no light whatever; and these lines, having been very accurately examined and measured by Fraunhofer, an excellent optician of Munich, have since been called Fraunhofer's dark lines . These black lines always occupy the same posi- tion in the spectrum, and generally maintain the same relative positions, breadths, and intensities, though the extent of different colours may vary. The nature of these dark lines has been thus explained. They are supposed to represent rays of light which have been absorbed in their passage from the sun to the earth. In other words, these lines may be only the vacant places left in the stream of sunlight after being interrupted in its passage. Some of the rays seem to be absorbed in our own atmosphere, because the number of dark lines increases as the sun sets, i. e. as there is a greater depth of atmosphere for the light to traverse ; and recent observations have shown that the number of these dark lines is continually varying with the altera- tion of atmospheric conditions. Their general arrangement is, however, so fixed and certain that they are now employed as a sort of mea- sure for the spectrum, to determine the relative lengths of the bands of colour. With powerful instruments an immense number of them can be perceived. LIGHT. 55 Let us now return to the colours of the spec- trum. These, as we have already noticed, were thought by Sir Isaac Newton to be seven in number, and he deduced the law that the same colour always has the same angle of refrangi- bility. Sir David Brewster, on the contrary, con- siders that only three primary colours — blue, red, and yellow — exist in the spectrum, and that these three rays exist at every point of it, forming three primary spectra of equal lengths overlapping each other, and exhibiting the colours of the Newtonian spectrum by allowing one to be seen through that one which overlaps it. It would, however, seem that the Newtonian theory maintains its ground ; for M. Helmholz has thoroughly investigated the spectrum once more, and his results go to prove that as far as Newton was acquainted with the colours of the spectrum his views were correct as to the bond which exists between colour and refrangibility, and that to each colour belongs its peculiar angle of refrangibility. In addition to the seven colours of Newton, we are now able to mention two, if not three, others ; namely, the extreme red, the lavender rays, and the fluorescent rays. From the colours of the spectrum we may pass to the consideration of the colours of bodies in general. The colour of the most brilliant flower in the garden is not an inherent pro- 56 LIGHT. perty in it. This may appear a strange and incredible statement, yet it admits of an easy proof. Let us take the flower, and together with it a box of silk ribbons of every variety of colour, into a dark room. All other light being ex- cluded, let us now illuminate the apartment with a lamp capable of giving only an intense yellow light. The effect is very remarkable. The rose has lost its brilliancy, and has a livid yellow colour. The ribbons all look yel- low, and the most brilliant tints are entirely lost in the universal yellow hue of the apart- ment. The colours which do not look yellow, become changed into black. To what is this extraordinary phenomenon due? Simply to the fact that colour is not an inherent property of bodies, but their hues depend entirely upon the decomposition of light effected at their surfaces. The colours of natural objects depend, there- fore, upon some peculiar function of their sur- faces, by which they send back to the eye light of a peculiar colour: or if transparent, their colour, seen by transmitted light, depends upon the power of the medium to permit some rays to pass through while it stops or absorbs all the others. Thus a rose is red because its surface sends back only the red rays out of all those which fall upon it ; and an infusion of roses LIGHT. 57 is red because it permits only the red rays to pass through it, the others being arrested in their progress. Every surface has a peculiar constitution by which it gives rise to the di- versified hues of nature. “ The rich and lively green which so abundantly overspreads the surface of the earth, the varied colours of the flowers, and the numberless tints of animals, together with all those of the productions of the animal kingdom, and of the artificial combina- tions of chemical manufacture, result from powers by which the relations of matter to light are rendered permanent, until its phy- sical condition undergoes some change.” * On the undulatory hypothesis, if a body receiving white light effects no change in its vibrations, it appears white ; but if the sur- face has the property of altering the vibration to that degree which is calculated to produce redness, the result is a red colour, and other colours are similarly produced by an alte- ration in the vibrations. The annihilation of all the undulations or waves of light, produces blackness. In the Newtonian or corpuscular theory, the beam of white light is supposed to consist of certain coloured rays, each of which has physical properties peculiar to itself, and thus is capable of producing certain effects. * Professor Hunt : “ Poetry of Science.” 58 LIGHT. These rays falling upon a transparent or an opaque body, suffer more or less absorption, and, being thus dissevered, give the appearance of colour. A red body absorbs all the rays but red ; a blue surface, all but the blue ; a yellow, all but the yellow ; and a black surface absorbs the whole of the light which falls upon it. Chemistry informs us of a number of very interesting facts on the subject of colour, which show in the clearest manner that it is not an in- nate property of bodies, but is due to the arrange- ment of their particles. Thus, if we take a clear solution of iodide of potassium, which is as pel- lucid as water, and add it to a similarly clear and colourless solution of acetate of lead, or of nitrate of silver, or of bichloride of mercury, — colour, and that of the most vivid and brilliant kind, is instantly produced : in the first it is a rich golden yellow ; in the second, a beautiful primrose ; and in the third, a magnificent scarlet. But still more remarkable, a substance may be placed in our hands of a rich yellow colour, and yet, if touched with the point of the nail, it turns into a vivid crimson. This substance is the biniodide of mercury, which, if heated on a plate of glass, turns from scarlet to yellow, and remains of that colour until it is touched by some solid body, when it instantly undergoes the remarkable metamorphosis alluded to. Che- LIGHT. 59 mistry has also its chameleon. For if we melt peroxide of manganese with potash, so as to form a permanganate of potash, and then dis- solve this substance in a small quantity of water, it is at first of a fine green colour. If we then add more water to it, it becomes blue, and, still more, it then assumes a beautiful purple colour. The peculiar appearance of a solution of sulphate of quinine has already been noticed. It affords us an instance of a body reflecting a colour, while it is quite transparent and colour- less when light is seen through it. Some bodies reflect one colour while they transmit another. Thus, for example, smoke, which reflects a pale blue colour, transmits a red hue. Hence, the appearance of the smoke of a cottage-chimney against a background of foliage, is of a light blue colour, while, if we change our position and catch a glimpse of the sun through it, it will appear of a deep orange or red colour. The opal, so much valued as a gem, presents us with another instance of a body reflecting one colour, or rather a variety of colours, and transmitting another. That colour is due to certain physical con- ditions of the surfaces of bodies is also shown in a very interesting and beautiful way, by placing two pieces of plate glass together, and com- 60 LIGHT. pressing them, when a vivid play of colours, resembling the prismatic spectrum, is imme- diately seen, which must be due either to the thin lamina of air between the plates, or to some slight wave form given by compression to the glass. The beautiful colours of a soap-bubble are due to a similar cause. There is a simple arrangement called the iriscope, which affords us an interesting method of examining the hues of the soap-bubble at leisure. A solution of soap being made, is put into a phial while still boiling hot : the heat and escaping vapour expel much air ; the whole is then corked, and when cold there is an attenuated atmosphere produced in the vessel. By placing the bottle on its side, and giving it a gentle shake, a film is produced. After it has grown thin by standing, we shall see a great many concentric-coloured rays, and as the film grows thinner, the rays will dilate ; the centre will pass from white to blue and black, while the rays present a curious order of change. Newton has found that these rings are produced by films varying in thickness, from msWth to TTa^oooth °f an inch * This most interesting phenomenon has lately been made the subject of a patent in its appli- cation to the ornamenting of stationery. The iridescent paper of Messrs. De la Rue owes its * Hunt : “ Elementary Physics.” LIGHT. 61 beautiful colours entirely to the same cause as the soap-bubble ; they are produced simply by the agency of light upon a thin, transparent film of varnish. This paper is manufactured in the following way. The process adopted to render the film and its reflected colours permanent, together with the method of its ap- plication, being as follows : — The objects to be ornamented, whether insects, shells, birds, bronzes, paper-hangings, card-cases, &c., are immersed in a vessel of water. Upon the surface of the latter, when perfectly tranquil, is dropped a little oil or spirit varnish, which, spreading in all directions, becomes exceedingly attenuated, and reflects the most vivid colours of the spectrum. The varnish being fixed, the object, which is slowly raised in such manner that the film shall adhere to its surface, is then placed in a convenient situation, to permit the water draining off. When completely dry the film is found to be firmly attached, and perfectly iridescent, having lost nothing of its original brilliancy of colouring. This is a beautiful illustration of the production of colour on a thin transparent surface, by the agency of light. The refraction of light, its dispersion into the colours of which it consists, and the conse- quent reflection of these colours at different 62 LIGHT, angles of incidence, cause the interesting pheno- mena last noticed. An example on a large scale is afforded us by the rainbow. The sun- beam in this instance striking upon the little spheres or globes of water forming the rain- drops, becomes refracted on entering the first surface, and is reflected from the inner surface of the drop. As the different colours are refracted more or less, their emergence from the drop is at a different elevation. The least refran- gible rays, the red, form the outer portion of the lower bow ; the most refrangible, the violet, form its inner edge. The rainbow is, when LIGHT. 63 seen under very favourable circumstances, accompanied by another bow, which is called the supplementary or secondary bow. It is due to a double refraction and reflection of the sun’s rays. Inverted rainbows, with the concave of the arch turned to the sky, have been seen; these are due to the drops of rain suspended on spiders’ webs in the fields. The question has often been asked, “ What becomes of light?” If we direct a beam of sunshine on a piece of black velvet, it is lost. Very little of it is reflected. It seems to sink into the substance of the tissue, and there becomes annihilated. Day after day might see us occu- pied in the vain task of seeking to quench the absorbent power for light of a piece of velvet. At the end of a month’s sunshine, it would be still as capable as ever of absorbing more. Familiar though it seem, this is a very remark- able fact. If we adopt the Newtonian theory, which supposes the existence of material particles in light, what becomes of the millions of millions of these particles which we see enter the cloth, and leave it still inexhaustible in its capacity to receive them ? The most transparent bodies in nature absorb some light. The air and water absorb a large quantity of light. No body is, therefore, abso- lutely transparent, some light being lost in its 64 LIGHT. passage through it. From what we have already stated about the dark lines of the spectrum, it is obvious that our own atmosphere exercises a considerable absorptive influence over the rays of light which pass through it. It has indeed been calculated, that if our at- mosphere, in its present state, could be extended rather more than 700 miles from the earth’s surface, instead of nearly 40, as it is at present, the sun’s rays could not penetrate it, and our globe must roll on in darkness and silence, without a vestige of vegetable form or of animal life. It is, in the present state of our knowledge, impossible to give a satisfactory answer to the question, What becomes of light, on the New- tonian or corpuscular theory? It is thought that it is permanently retained within the sub- stances of bodies, and it has been supposed indeed to exist in them in what may be called a latent or hidden condition. It is known that heat exists in bodies in this condition, and can be developed and made sensible. Sir David Brewster says, we must believe, till we have evidence of the con- trary, that the light is actually stopped by the particles of the body, and remains within it in the form of imponderable matter; but this is rather an unsatisfactory recommendation. Perhaps one of the most singular and appa- LIGHT. 65 rently incredible statements in connexion with the general subject of the phenomena of light is, that which we have mentioned (page 34), and which informs us that two bright lights may be made to produce darkness . This is a most startling announcement, but it has been abundantly proved. If two rays of light radiate from two spots very close to each other, in such a manner that they cross each other at a point properly determined, any object placed at that line of interference will be illumi- nated with the beam of the two luminous rays. The length of the two rays is in this case ex- actly the same, the spot or object being at an equal distance from the two radiant points. But if now one of the radiant points be removed to a little distance, so as to make a minute difference in the length of the path pursued by the ray of light, while that of the other remains unaltered, then, strange to say, a black spot is produced. The two rays seem to destroy each other. This curious property is analogous to the beat- ing of two musical sounds nearly in unison with each other, the cessation of sound be- tween the beats when the two waves of sound | destroy each other, corresponding to destruction of light by the interference of the two rays. A more familiar example of this is the following. Standing at the confluence of two rivers, it will 66 LIGHT. be observed, that when the waves from each meet in the same state of vibration, the result- ing wave will be equal to the two combined ; if, however, one wave is half an undulation behind the other, the crest of the one will meet the hollow of the other, and comparatively smooth water will be the result. Thus it is sup- posed that the crest of one wave of the particles of light meets the hollow of the other wave, and that both thus become destroyed and pro- duce darkness. So the undulatory theory ex- plains this singular experiment. The velocity at which light travels has been the subject of accurate calculation, and appears to have been determined with con- siderable certainty. The most recent investi- gations are those of Sturm, and he has deter- mined that light is propagated at the rate of 166,072 geographical miles in a second, almost a million of times greater than the velocity of sound. According to the measurements of three eminent astronomers, rays of light of three fixed stars of very unequal magnitude, (a Cen- tauri, 16 Cygni, and a Lyrse,) require respec- tively 3, 9J, and 12 years to reach us from those three bodies. Yet in the course of a single hour, a ray of light traverses over a space of 592 millions of miles. The rays of light from one of the remotest nebulse reached LIGHT. 67 by the great telescope of the elder Herschel, required, it is calculated, not less than about two millions of years to reach this earth. We are consequently looking back over a space of two million years when we look at this wonderful object, for the rays of light which we see from it quitted it two million years ago. The light of the sun occupies eight minutes and thirteen seconds in coming from that luminary to the earth. A familiar experiment gives us a good con- ception of the immense velocity of light. A disc on the surface of which several colours or figures are painted, may be made to revolve at an enormous rate, and we lose the impression of its separate parts, all appearing blended into one. If now the room be darkened, and an elec- tric discharge from a Leyden jar be made in front of the disc, it will appear absolutely motionless and fixed for the instant. This shows us that the light is so rapid, that the disc, though flying at an immense velocity round, has not time to move on before the flash of light has caught the figure on its surface, and sent it to the eye. From , some recent experiments of M. Fizeau, it would appear that the velocity of artificial light very nearly approaches that of the light of the sun. It has been calculated that a locomotive, travelling at the rate of more than a mile a minute, would require 137 years to go from our T 2 68 LIGHT. globe to the sun. How far behind, therefore, are the greatest velocities attained by man, when contrasted with the amazing rapidity of a gleam of sunshine ! There is reason to believe that a considerable difference exists in light from dif- ferent sources. Electric light traverses 288,000 miles in a second. The singular phenomena of double refrac- tion, exhibited in so remarkable a manner by Iceland spar, must be briefly mentioned. The curious appearance presented on looking at an object through this mineral, was first noticed by a mathematician, of the name of Bartholin, who found in looking at the pages of a book through it, the letters appeared double. If we look at a spot on a card through this substance, two spots in every respect identical will be seen by the eye. This is due to the effect produced on the rays of light by the physical constitution of the mineral. One part of the rays is refracted according to the ordinary law of refraction, and is called the ordinary ray, and when seen by the eye, conveys the impression of one distinct image ; but the other part of the rays will be re- fracted according to an extraordinary law, and is called the extraordinary ray, and this also gives the impression of a distinct image to the sight. In this way, two distinct images of the single spot are produced and perceived by the eye, and LIGHT. 69 it is found that the light is in a different state in each ray, subject to peculiar laws of reflection. A young engineer of the name of Malus was amusing himself one afternoon by looking at the beams of the setting sun reflected from the windows of the Luxemburg Palace, through a double refracting prism. He observed that when the prism was in one position, the windows with their golden rays were visible, but when turned round a quarter of a circle from that position, the reflected rays disappeared, though the windows were still seen. In this accidental way was first discovered the extraordinary phenomenon of Polarised Light, which we now purpose to introduce to the reader’s notice. The difficulty of rendering this subject in- telligible in popular language has been felt by all who have approached it; nor do we alto- gether hope to succeed better than others. We shall closely follow the very concise and clear language of Professor Hunt in this attempt, from a conviction that the subject does not admit of a better elucidation than is afforded by him. Polarisation of light may be effected in two ways : — 1st. If a ray of light is reflected from the surface of any body, fluid or solid, but not me- tallic, at a certain angle, it undergoes what has been called plane polarisation . 70 LIGHT. Or, 2d. Polarisation may also be produced by the refraction of light, from several refracting surfaces acting upon the pencil of light in suc- cession ; as by a bundle of plates of glass. In the phenomenon of double refraction, each of the two pencils of light is polarised. What is this mysterious condition of light which is produced by reflection and refraction at peculiar angles to the incident ray ? The reply may be thus given : — 1. An ordinary ray of light will be reflected from a reflecting sur- face, at whatever angle that surface may be placed, in relation to the incident ray. 2. An ordinary ray of light is freely transmitted through a trans- parent medium, as glass, in what- ever position it may be placed, relative to the source of light. By means of the little apparatus represented in the figure, the phenomena of polarised light can be studied in a very beautiful manner. The frame A B encloses a polished glass, and it may be adjusted by means of a pivot. This mirror is placed in such a position, that its plane makes an angle with the vertical of 33° 15 / . A ray of ordinary light a falls reflected in all positions of the re- flecting surface, with respect to the incident ray. 2. A polarised ray of light is not transmitted in all the positions of the permeable medium. LIGHT. 71 on it, and passes feebly through the glass, and is partially reflected downward to c as a polar- ised ray, where there is a blackened mirror which reflects the ray back through the polarising mirror A B, to the mirror at the top of the stand. The graduated adjustments and movements enable the observer to place the mirrors at various angles, and to observe all the pheno- mena of polarisation ; and also to place any transparent object in the line of the ray, and observe the effect of passing a beam of polarised light through it. Perhaps when the reader considers the opera- tion of this little instrument, which is called the Polariscope, he will be better able to comprehend this difficult subject ; and it well repays the student for the trouble involved in the mastery of its difficulties ; for probably no discovery in connexion with light has led to such important results, and few promise so much for the future. The effect of passing a polarised ray through a thin slice of any transparent substance placed in the apparatus in question, is most remarkable. Its whole molecular constitution appears revealed, as if by the touch of a magic wand. The surface of many substances, such as thin films of sulphate of lime or mica, becomes painted with the most brilliant and beautiful colours, and by turning the plate round, we can make them 72 LIGHT. dissolve and reappear, in a manner extremely singular. Rings of the most charming colours are seen in some substances, and these are marked with a figure resembling a Maltese cross, in black and white. The polarised ray is also a most subtle analyser, and is capable of de- tecting adulterations or frauds in substances, in the detection of which chemistry would almost fail. One of the best examples of this is M. Biot’s application of it to detecting the qualities of sugar. The crystals of sugar ex- hibit different rings, when the origin of the sugar is the beetroot, from those which it shows in sugar from the sugar-cane. When once the phenomena exhibited by a transparent solid or fluid, in a pure state, are known, they are invariable; whereas, the smallest addition of any other substance gives rise to an immediate change in the appearance, when seen with the assistance of the polarised ray. Almost every substance in nature, in some definite position, appears to have the power of producing this change upon the solar ray. The sky at all times furnishes polarised light, which is most intense when it is blue and unclouded, and the point of maximum polarisation is varied according to the relative position of the sun and the observer. A knowledge of this fact has led to the highly ingenious and interesting instru- LIGHT. 73 ment of Professor Wheatstone called the Polar Clock, by means of which the true time can be easily determined by examining the polarised condition of the sky. The recent investigations of Professor Faraday have demonstrated the very striking fact, that magnetic force and light have a direct relation to each other. He has proved that magnetism has the power of influencing a ray of light in its passage through transparent bodies. A polar- ised ray is passed through a piece of glass or crystal, or along the length of a tube filled with some transparent fluid, and the line of its path carefully observed. If when this is done, the solid or fluid body is brought under powerful magnetic influence, such as we have at command by making a very energetic voltaic current circulate around a bar of soft iron, it will be found that the polarised ray is disturbed, and in fact that it does not now permeate the medium along the same line. From his elaborate ex- periments, Professor Faraday is led to conclude that magnetism and the light act on each other through the intervention of matter. We have now gone through the principal facts relating to the Constitution and Pheno- mena of Light. We ought not, however, to leave the reader to the supposition that the subject is wholly embraced in these few pages ; 74 LIGHT. very little has in fact "been told of the wonders of light, and that only in very general terms. Yet even in this there is no want of subject for deep thought, or of food for the most cultivated and imaginative understanding. It will he well for all of us if the contemplation of the wonders of nature lead us to more exalted ideas of its Author, and teach us to keep in mind as we study these phenomena — how wonderful and how wise are all the Divine works. PART II. SOURCES OF LIGHT. CHAPTER, L CELESTIAL LIGHT. The sun, the lantern of the world, as it was named by Copernicus, as the primary source of celestial light, must first receive notice at our hands. This glorious orb, the diameter of which is not less than 770,800 geographical miles, or more than 112 times greater than that of the earth, exercises an universal influence of the most powerful kind over every part of the earth's surface : not merely by the all-powerful laws of gravitation, but by the less sensible but equally important effect exercised by its beams upon the earth and its inhabitants. From what has been already said as to the time necessary for a ray of light to shoot across the vast interval between this luminary and the 76 LIGHT. earth, it follows that we never really see the snn in its true position. It is always a little in advance of its apparent place. Its earliest beams, announcing sunrise, inform us of that event after it has taken place by some little time, and its golden circle sinks below the horizon a little before the last rays it emitted vanished from our sight. This huge sphere — whose mass is, according to recent calculations, 359,551 times greater than that of the earth, and 738 times greater than that of all the planets combined — presents an object for admiration and thought almost too vast to be fully embraced by the mind. In order to afford some assist- ance to the reader in conceiving an idea of the magnitude of the sun, it has been computed that if the solar sphere were hollowed out en- tirely, and the earth placed in its centre, (as one might suspend a marble in the centre of the dome of St. PauPs Cathedral,) there would still be room enough for the moon to describe its orbit around the earth, even if the radius of the latter were increased 160,000 geographical miles. The sun rotates on its own axis in 25^ days, and is supposed to have a progressive motion in space, accompanied with the glittering band of planets over which its influence is extended, from the burning planet Mercury to the dimly lighted Neptune. Its special interest for us is LIGHT. 77 not, however, its astronomical history but its relation to our earth as a source of light. M. Arago, who was long occupied in attentive examination of the physical con- stitution of the sun, has given his view of its nature in the following terms : — “ As far as the present state of our astronomical know- ledge enables us to speak, the sun is composed — 1st. Of a central sphere, which is nearly dark ; 2d. Of a vast stratum of clouds, sus- pended at a certain distance from the central body, which it surrounds on all sides ; 3d. Of a Photosphere , or in other words, a luminous atmo- sphere enclosing the cloudy stratum, which in its turn envelopes the dark nucleus. The total eclipse of the 8th July, 1842, afforded indica- tions of a third envelope, situated above the photosphere, and formed of dark or faintly illu- mined clouds. These clouds of the third solar envelope, apparently situated during the total eclipse on the margin of the sun, or even a little beyond it, gave rise to those singular rose- coloured protuberances which so powerfully ex- cited the attention of the scientific world in 1842.” Sir John Herschel also affirms the pro- bable existence of this third envelope in the following terms : — “ Above the luminous surface of the sun, and the region in which the spots reside, there are strong indications of the exist- 78 LIGHT. ence of a gaseous atmosphere, having a some- what imperfect transparency.” These views of the structure of the sun, which are well received by most of the distinguished philosophers of our own time, are as yet little known, and, startling as they are, have attracted but little notice from the student of nature. Humboldt, however, quotes an author living in the middle of the fifteenth century, who appears to have had some clear conceptions of the real nature of the sun. He says the body of the sun itself is only an earth-like nucleus, surrounded by a circle of light as by a delicate envelope ; that in the centre there is a mixture of water- charged clouds and clear air, similar to our atmosphere, and that the power of radiating light to our earth does not appertain to the earthly nucleus of the sun’s body, but to the luminous covering by which it is enveloped. This certainly is a curious instance of the anti- cipation of a truth. Modern science, therefore, does not permit us to consider the sun as a solid globe of burning material, emitting light and heat just as a red- hot ball of metal does, but attributes the light and heat of this great luminary, not to its solid nucleus, but to its gaseous envelopes. The remarkable phenomenon of solar spots confirms this supposition in a very striking LIGHT. 79 manner, and gives us a sight through the brilliant coverings of the sun of the dark and strange mass within them. What may be the real nature of this dark body constituting the mass of the sun it is impossible to conjecture, though there would seem no reason to believe it to be composed of other elements than those which constitute our own globe. It is, however, surrounded by a “ mottled ocean of flame,” through which it is occasionally seen, consti- tuting the phenomenon in question. These spots have been occasionally visible to the naked eye. One such was seen in 1779. They are quite black, and generally of an angular outline, and are frequently surrounded by halos, wdiich ex- hibit the same figure on a larger scale. It is remarkable that they are always situated within a certain distance from the sun’s equator. They also have sometimes a very permanent cha- racter, remaining for five or six months unal- tered, and they are occasionally so large that the earth, if placed in the opening, would not fill up the gap. Though it is difficult to account satisfactorily for the occurrence of these remarkable spots, the most generally received view as to their nature is, that they are actual perforations in the gaseous envelopes of the sun, through which its solid sphere becomes visible. By some philosophers 80 LIGHT. they are thought to be due to ascending currents from the solid surface of the sun, by which clouds of flame are torn asunder, and the dark interior is laid bare to sight. The same phenomena may often be seen in a sea-fog, when a current of air, rising from a part of the land over which it sweeps, thrusts aside its white folds and shows the solid ground again. Sir W. Herschel considered that these spots, together with several other phe- nomena seen in the solar atmosphere, indicate an intense evolution of light. Others have, however, connected their frequency and occur- rence with a diminution of light and heat. Cer- tainly, if upon mere theoretical grounds it were justifiable to speak upon such a subject, it would seem at least highly probable that such a vast rent in the atmosphere of the sun, from whence our light and heat proceeds, as would admit this globe of ours with room to spare round its margin, would be attended with a corresponding loss of light and heat, for it does not appear that we receive these forces from the solid mass of the sun itself, but from its envelopes. A very remarkable series of experiments was prosecuted by Arago, the result of which demon- strates in the most conclusive possible manner, that it is to the gaseous envelope of the sun that this earth and the planets of our system are in- debted for their light and heat. The remark- LIGHT. 81 able phenomena of polarisation of light, alluded to in a preceding chapter, constituted the deli- cate medium by which this philosopher derived his intelligence of the source of solar light. An ignited solid body, such as a red-hot iron ball, or a luminous fused metal, yield only ordinary or unpolarised light in rays issuing in a perpen- dicular direction ; while the rays which reach our eyes from the margin under very small angles, are polarised. When the polariscope, by which ordinary and polarised light can be distinguished, was applied to gas flames, there was no indication of polarisation, however small were the angles at which the rays emanated. If even the light be generated in the interior of gaseous bodies, the length of way does not appear to lessen the number or intensity of the very oblique rays in their passage through the rare media of the gas, nor does their emergence at the surface and their passing into a dif- ferent medium cause polarisation by refraction. “ Now,” observes Humboldt, “ since the sun does not either exhibit any trace of polarisa- tion when the light is suffered to reach the polariscope in a very oblique direction, and at small angles from the margin, it follows from this important comparison that the light shining in the sun cannot emanate from the solid solar body, nor from any liquid substance, but must 82 LIGHT. be derived from a gaseous self-luminous enve- lope. We thus possess a material physical analysis of the photosphere.” In other words, this simple but valuable instrument informs us, without any doubt, that the light which we thus examine by its assistance is of that kind which would be emitted by a gaseous body in a state of combustion, and we have therefore the strongest possible reason for believing that such is really the nature of that source of light whose genial influence pervades our world and others beyond our own. An important series of experiments has been instituted with a view to measure the different intensities of light, and a department of physical science has thus been opened, which promises to lead to very important conclu- sions. This study is called Photometry. It enables us to compare the sun’s light with the light of other celestial bodies, and also with the sources of artificial light which we are employing. A very simple experiment affords us a rough means of comparing the intensities of different kinds of light. A beam of sunshine may be admitted into a darkened room through a small circular hole in a plate of metal fastened in a window-shutter, and a small cylinder of any opaque material being placed in the beam, so LIGHT. 83 as to cast a shadow upon a screen — the distance of a candle from the same cylinder (or one of the same size), placed at the same distance from the screen, must he varied until the shadow in the line of the candle becomes equally intense with the shadow in the line of the sunbeam. We have thus a means of intimating the rela- tive intensity of the light from the sun and of that from the candle, and a standard is thus attained. The more intense the light, the more black and intense are the shadows. From a comparison thus instituted, Dr. Wollaston inti- mated the direct light of the sun as being nearly 1,000,000 times greater than that of the moon, and very many million times greater than that afforded us by all the fixed stars taken collec- tively. He estimated the light of the sun to be equal to that of 5,563 candles, at twelve inches’ distance ; or, in other words, 5,563 candles at the distance of twelve inches would illuminate an object as brilliantly as if it were in the blaze of sunlight. But when we consider the impos- sibility of arranging even 500 candles at such a distance, it will appear evident that no ade- quate conception of the sun’s light is afforded by such means, nor are we able by candles to make the least approach to the splendour of sunshine. The light of the full moon is esti- mated in the same way, as equal only to G2 84 LIGHT. the x^tli part of the light of a candle placed at the distance of a foot. By a more accurate arrangement of apparatus, Dr. Wollaston obtained a certain estimate of the intensity of solar light, as compared with star- light. He found that it would require 20,000 millions of such bright stars as Sirius, to equal the light of the sun, or that that orb must be 140,000 times further from us than he is at present, to reduce his noon-day splendour to that of this star. As compared with sunlight, it was found that moonlight is 881,072 times weaker than it. Yet from what is known of the distance of the star Sirius from our planet, it is supposed that it exceeds our sun’s brilliancy in actual intensity at least 63 times. How vast then must be that long interval which reduces the light of an orb 63 times more brilliant than our sun to such a state of weakness that our luminary is 20,000 million times more bright than it ! All the instruments adapted for the measure- ment of light are of very imperfect construction, and discrepancies of various kinds arise in con- sequence of this defect. Arago has, however, adapted his ingenious polariscope to this pur- pose, and there seems reason to believe that with its assistance more certain and accurate conclusions may be obtained. Probably the LIGHT. 85 advancing art of Photography will, in process of time, become available to the science which has given it birth ; and help us to important dis- coveries in this nearly untrodden field of philo- sophical research. A comparison between solar light and the two most intense kinds of artificial light which man has hitherto been able to produce, yields, ^ according to the present imperfect condition of photometry, the following numerical results : — MM. Fizeau and Foucault have, by a series of ! ingenious experiments, compared the intense I light produced by the flame of the oxyhydrogen lamp directed against a surface of chalk, called I Drummond’s light, with the sunlight, and they P found the sun to be 146 times brighter even than | this dazzling source of artificial light. A still i more conclusive evidence of the superior bril- liancy of sunlight was afforded by placing the Drummond light in the sunshine, and looking at the sun’s disc, when it was actually found that the blazing ball of light of man's creation ? only appeared as a little black spot when viewed in the superior effulgence of sunshine. The electric light was found to be far superior in intensity to the Drummond light ; for when very many plates were used in the galvanic batteries employed for the production of the light, it was found that bright sunlight was only about 86 LIGHT. three times more intense than the artificial light. Before quitting the considerations connected with solar light, we may allude to some of the conjectures which have been made as to its real source, or rather as to the nature of those pro- cesses, which give to the sun’s luminous enve- lopes the power of emitting light and heat. By some it is supposed, that a process of che- mical decomposition takes place in the gaseous envelopes which surround the sun, and its light and heat may be compared with the che- mical processes which man is able to employ for the same purpose. It would, however, appear far more probable that the light of the sun is of the same kind as the electric light, and arises from violent disturbances and con- stant processes of change taking place in its gaseous coverings. Dr. Herschel long ago com- pared these ever-luminous cloud envelopes of the sun’s body with the polar light of our own planet, which is well known to be an elec- trical phenomenon. Yet, after all, these and similar attempts at an explanation of this constant marvel, need themselves to be ex- plained ; they bring us but a feeble and uncer- tain step onwards, while the grand mystery itself remains still far from the grasp of human intellect. LIGHT. 87 Let us now advert for a little space to those lesser sources of light, as far as our planet, at least, is concerned, — the moon and stars. There is certainly a feeling of admiring awe deep-rooted I in our nature when engaged in the contemplation of those glorious orbs which supply our world with all that renders it beautiful in the eyes of man. “ The mild light of the moon’s disc,” ob- serves a great philosopher, “ unlike that of the sun, does not oblige us to cover our eyes, but rather draws them upwards towards heaven. At the same time, it so far overpowers the light of stars, that they no longer attract our notice, and sometimes become almost invisible. Moon- light also shows just so much of earth as to prevent our entirely forgetting it: thus, fancy and thought, rapt in mild enthusiasm, hover indefinitely between heaven and earth.” The sublime language of the word of God expresses the attractive influence over the human imagi- nation in a simple and forcible manner. " If I beheld the sun when it shined, or the moon walking in brightness, And my heart hath been secretly enticed, or my mouth hath kissed my hand : This also were an iniquity to be punished by the judge : for I should have denied the God that is above.” * Job xxxi. 26—28. 88 LIGHT. The impression of a starlight night on the mind of man, in the solitudes of nature, is extremely interesting. The expansive vault rising above the surrounding woods and mountains, embraces all that is known to him of the earth’s surface. His ideas of measurement are, indeed, far too limited to grasp the expanse of heaven, and yet it is the most imposing object he knows. The stars, to him, are only points of light, but the clearness and purity of that light is not without its influence. The contrast between the bright vault of heaven and the dark earth, the silence and the accompanying repose of mind, are so familiar to our ‘ senses, that we are none of us strangers to the impression.* The intensity of the light of the moon in the southern hemisphere has been estimated by Sir John Herschel, at 27,408 times brighter than a Centauri, which is the third in brightness of all the stars. And other observers have estimated her light in our own hemisphere to be 3,000 greater than that of the planet Venus, when at her most brilliant aspect. It need scarcely be said that her light is wholly derived from that of the sun. She shines because the sunbeams strike her surface, and are reflected thence to us. Otherwise the moon is but a dark non-luminous body like our own earth. * Oersted. LIGHT. 89 Careful observation has shown that the moon is wholly, or if not wholly, very nearly, destitute of any aerial envelope resembling our own atmo- sphere. Her hard and rugged outline appears unsoftened by any such covering, and the light of stars over which she passes in her orbit, suffers no refraction as it gleams past her edges. Had she an atmosphere even of extreme tenuity, the result would be that less of the light she receives would be reflected to us ; and it would be obvious, when a beam of starlight shot past her edge to the observer’s | eye, that it was refracted to a greater or less extent. No such effect, however, is produced, and it has therefore been generally concluded I that the moon has no atmosphere, and is conse- quently unfitted to be the abode of air-breathing creatures. Superstition, as we have before seen, has long invested moonlight with peculiar qualities. Science has, however, shown that its sensible influence is extremely feeble. It was supposed , that all heat and all the chemical rays which are poured upon its rugged surface from the sun, sink into the planet and are lost, while a small portion of the light is sent back to us. This has been subsequently shown to be an erro- neous opinion. It is found that a very small, and by ordinary means an inappreciable, portion 90 LIGHT. of the heat rays is really reflected to us from its surface, and its actinic or chemical rays are likewise sent hack to the earth, though with a vast loss of intensity. Photographic pictures have actually been taken by moonlight, a result which most distinctly shows that chemical rays really exist in it. But a yet more striking evi- dence of this is manifested by the possibility of taking pictures of this beautiful orb by means of her own light. Before very sensitive photogra- phic compounds were found, it was thought to be impossible to produce this result, and many failures were made. It was, therefore, con- stantly stated that actinic rays did not exist in moonlight. Optical instruments give us the means of catching great quantities of light even from very faint objects. A lens three inches across catches 144 times more light from any given object than the natural pupil of the eye fairly opened ; consequently the image of that object, when formed in the interior of a camera, by means of such a lens, must be 144 times brighter than when formed in the chamber of the eye ; and its image, when formed by a still larger lens than this, would be more brilliant in proportion to the square of the diameter. It hence occurred some time since to Professor Bond of the Harvard University, United States, LIGHT. 91 that although he could not throw increase of illumination upon the pale and distant moon, he might make more of the faint illumination which it naturally possesses available for the purposes of photography, if he converted the magnificent telescope at his disposal into a photographic camera. The object-glass of that telescope is fifteen inches in diameter, and the image of an object formed in its focus would therefore be twenty-five times brighter than the image of the same object formed by a three-inch lens. He consequently made his arrangements in accordance with these considerations. He placed an iodised plate of silver within the dark tube of the telescope, so that its sensitive surface exactly corresponded with the focus of the large achromatic lens; and he made the telescope tube, thus furnished, follow steadily the moon’s motion in the heavens by means of accurately-adjusted clock-work. The result of this interesting experiment has been a signal triumph. No less than three exquisite mi- niatures of the moon were exhibited at Ipswich at one of the sectional meetings of the British Association for the Advancement of Science by Mr. Bond. The most interesting of these lunar miniatures is a small half-face portrait, about as large again as the half of an ordinary crown-piece, 92 LIGHT. taken at that phase of the lunation, because the lengthened shadows cast behind the ine- qualities of the surface are then seen to most advantage. When we look at the moon’s hemi- sphere half in light and half in darkness, the sun is shining upon it in a direction that is transverse to the one in which we are viewing it. The sun is shining from the right, so to speak, while we are looking straight forward ; consequently the shadows which are cast in the direction of the sun’s beams are spread out lengthwise before our vision; and most wonderful objects those shadows are when observed by the telescope under these advantageous circumstances. Ragged fringes of blackness rest behind peaks and ring- shaped elevations of polished silver ; round and oval patches of darkness fill up cup-like de- pressions; index-shaped triangles of jet point out from the back of spots of brilliant light. In the photographic delineation all these sin- gular features stand revealed. The broken ridge of the Apennines, with its serrated sha- dows; the ring-bounded plains of Arzachel, Alphons, and Ptolemy, with their central isolated peaks, secondary craters, and external buttress-like spurs ; the torn and broken cavities around the Plutonian Tycho': all are there. And beyond these the dull-grey patches of the Mare Crispum, Mare Foecunditatis, and Mare LIGHT. 93 Tranquillitatis (seas by name, but dry plains by nature), set round by the curving margin of more condensed brilliancy, where the light is compressed by the foreshortening of the receding portions of the spherical surface. Even minute details of these varied outlines are so accurately and fully given, that fresh objects may be seen when the drawing is examined by the aid of a magnifying lens. The powers of the microscope may be as successfully brought to bear in ex- amining this beautiful picture as those of the telescope are in viewing the moon itself. For once, art seems to have approached very near indeed to the production of a perfect copy of an original that is among the choicest of nature’s works. The light which we receive from the brightest fixed star is some 28,000 times less than that which we receive from the moon. But this light is compressed into a point of invisible dimensions, instead of being spread over a wide surface. There is therefore scarcely a doubt that when the more sensitive materials of the photographist are brought into operation with lenses as large as the great Harvard refractor, delineations of star-groups may be easily pro- cured. Mr. Bond stated at the Ipswich meeting that his father had already succeeded in pro- ducing a perceptible image of the two consti- 94 LIGHT. tuents of the double star Castor upon even the iodised silver plate. It is scarcely possible yet to calculate how great a service photography may render to the astronomer. The subdued brilliancy of the moon is a wise and merciful arrangement, as far as organic life is concerned on our globe. We can readily understand a cause for the diminished splendour of a planet intended only to relieve, but not to disperse, the salutary darkness of our nights. As the inhabitants of our world, both animal and vegetable, are at present constituted, no greater curse could be imposed upon them than perpetual day, or a day alternating with a day of lesser brilliance, but still of sufficient in- tensity to keep the animal and vegetable pro- cesses in constant activity. We shall again have to observe, that light exercises the most im- portant influence upon organic, as well as in- organic nature. It is therefore sufficient for our present purpose to state here in the most general manner, that the light of the sun offers the most exalted yet subtle stimulus to all nature ; and the softer influence of moonlight was never intended to be a substitute for the glare of day, and thus perpetuate the stimulus, but was merely intended to give solace and comfort at night, and to induce, not to interrupt repose. The earth, it should be added in conclusion, is LIGHT. 95 a moon to this satellite, and one of far superior lustre to it. It is a remarkable fact, that we are only acquainted with one side of our satellite, and that the other is unknown to us. So also, if there be inhabitants in the moon, the earth is only known to those who dwell on one side of it, and is concealed from the view of those who are on the other side. Starlight has less importance in its physical effects on our world than moonlight, and our notice of it will merely be of a cursory kind. It is in the tropics that the brilliancy of starlight is best observed. In our own country, favourable states of the sky are very frequently denied to us ; yet even here, on moonless but clear and transparent nights, the stars gleam with wonderful brilliancy. In finer climates, however, the glorious appearance of the star- spangled firmament has long been observed by travellers. Humboldt says as nature has per- mitted the native of the torrid zone to behold all the vegetable forms of the earth without quitting his own clime, even so are revealed to him the luminous worlds which spangle the firmament from pole to pole. The more mag- nificent portion of the southern sky, in which shine the constellations of the Centaur, Argo, and the Southern Cross, where the Magellanic clouds shed their pale light, is for ever con- 96 LIGHT. cealed from the eyes of the inhabitants of Europe. It is only under the equator that man enjoys the glorious spectacle of all the stars of the southern and northern heavens revealed at once. Some of our northern con- stellations — as, for instance, Ursa Major and Ursa Minor — owing to their low position when seen from the region of the equator, appear to be of a remarkable, almost fearful magnitude. One of the most singular and interesting phe- nomena presented to us in the consideration of the light of the stars, is that they are not all of the same colour. There is a real dif- ference in the colour of the proper light of the fixed stars, not due to its being seen through our atmosphere, or to the effects of scintillations. Modern science, aided by the telescope, has re- vealed the wonderful fact, that stars of every hue of the prismatic spectrum are spread over the sky. Stars of blue, red, green, golden, and even of violet hues, are seen through this wonderful tube. Sir J. Herschel gives a list of seventy ruby-coloured stars of the seventh to the ninth magnitude, some of which appear in the tele- scope like drops of blood. In a cluster of stars seen near the Southern Cross, more than a hundred variously-coloured red, green, blue, and bluish-green stars, are so closely thronged LIGHT. 97 together that they appear in a powerful tele- scope, like a superb piece of fancy jewellery. It is also a remarkable fact that the colour of some stars has been known to undergo a ! change. Thus the splendid star Sirius now shines with a perfectly white light; yet it is known that at one time it shone with a light of a red colour. It is therefore probable that some great change has taken place in the light- producing envelope of this distant sun, by which its original red colour was destroyed, and white light emitted. It has already been said that light occupies a definite time in its passage from one planet to another, and mention has been made of the vast interval of time occupied by the passage of light I from one of the remote nebula to our earth. Consequently, if one of these far distant masses of matter were destroyed at this instant, the destruction of its light would not be sensible to us, nor to our descendants for thousands of centuries ; for the light by which we now per- ceive these wonderful creations, is at the moment when it strikes our eye not less than two millions of years old. “ In the contem- plation of the infinite,” observes Sir D. Brewster, “ in number and in magnitude, the mind ever fails us. We stand appalled before this mighty spectre of boundless space, and faltering reason H 98 LIGHT. sinks under tlie load of its bursting conceptions. But placed as we are on the great locomotive of our system, destined surely to complete at least one round of its ethereal course, and learning that we can make no apparent advance in our sidereal journey, we pant with new ardour for that distant bourne, which we con- stantly approach, without the possibility of reaching it. In feeling this disappointment and patiently bearing it, let us endeavour to realize the great truth from which it flows. It cannot occupy our mind without exalting and improving it.” CHAPTER II. CHEMICAL SOURCES OF LIGHT. In a certain sense it is true that all artificial light is derived from chemical sources ; for the mere construction of a common tallow candle is really a process strictly chemical in its nature; and in the manufacture of the better kinds of candles, and of gas, a very high degree of chemical science is concerned. To this, however, we shall again allude. There is this difference in the nature of the sources of light, to which we propose in this chapter to direct attention, — namely, that the chemical origin is more immediate, while in the others it is more remote. The electric light, the oxyhydrogen light, and others to be noticed in this place, are the direct and immediate result of chemical processes set in motion by the person who employs the light : while the light of gas and candles is obtained from a source in which H 2 100 LIGHT. chemical processes have been carried on and completed by other persons. In a word, our present attention will be directed to what may be called the philosophical sources of light, and in the next chapter to the common every-day sources of artificial light. One of the most remarkable of the philo- sophical sources of intense artificial light is the electric light ; it seems, indeed, as will be remembered from a statement in a former chap- ter, to approach more nearly to that of the sun itself than any other kind of light. Its extreme brilliancy and penetrative power have often been displayed in public, and have been made useful in the execution of great public works during the absence of the sun. We shall here endeavour to give the reader a clear and simple view of the theory of the produc- tion of this important light, and also of the practical appliances by which it has been ren- dered serviceable to those who have employed it on a great scale. It has long been known that if we have a sufficiently powerful voltaic battery, and con- nect the two poles by means of wires, a very intense heat can be produced by the passage of the electric current. The compact and powerful battery invented by Professor Grove, which consists of four or five cells with plates LIGHT. 101 of platinum and of zinc, excited by nitric acid and dilute sulphuric acid, may be used for the experiment, and a very interesting result is obtained. The two stout copper wires which are severally attached to the op- posite poles of the battery, may be taken in the hands without risk and joined together. On doing so only a faint spark of light is seen, and but little sensation of heat is felt by the hand, for the electric current finds room enough to pass without resistance through the thick wire employed. But if now a very fine piece of iron or steel-wire is made to connect the two wires, so that the current has to pass through it, the result is most striking. The wire instantly begins to glow with intense heat, and in a few seconds it actually melts into drops of liquid iron. Platinum wire can be sub- stituted for it, and it becomes so hot as to glow with a dazzling whiteness, and if very fine, it appears to fuse. Wires of all the ordinary metals are instantly melted. If now in place of the wires we substitute two pieces of charcoal, cut in the form of a pencil, and hold them at a small distance apart, a still more brilliant spectacle is produced, the most intense light being developed. The charcoal points should first be placed in absolute contact until they glow with heat, 102 LIGHT. and then they may be gradually separated from each other. In so doing a brilliant arc of vivid dazzling white light leaps from one pencil to the other, and constitutes the electric light. So powerful is the light thus produced, that it is impossible to look at it steadily without pain. The points must be kept at a stated distance from each other in order to bring out the light in its utmost power: but if they are too widely separated the current no longer passes, and the light is extinguished. It is a very remarkable fact, that a real transference of matter takes place in this phenomenon, and that one charcoal point actually grows, while the other lessens. In some experiments in America, it was found that when one point was made of plumbago and the other of charcoal, fused particles of the plumbago were actually carried across the arc of flame to the charcoal point, which sensibly increased in length. Let us consider for a moment the nature of this phenomenon. It might be thought it is a mere case of intense combustion of the charcoal, heated by passage of the electric current. But combustion cannot take place in absence of oxygen or in a vacuum, and it is found that the electric light burns with undiminished bril- liancy in the exhausted receiver of an air-pump, and in a hermetically-sealed globe of glass placed LIGHT. 103 under water. Ordinary combustion is obviously impossible under such circumstances. We must therefore renounce that explanation of the cause of the electric light. It is in fact found that the line of flame is really longer and more bril- liant in a vacuum, than in the air. The true cause of the light is the electric current, which, in its passage from pole to pole, renders itself visible ; it would seem as if it were partly con- verted into light by the violence of the effort necessary to leap from one charcoal point to its opposite. Not much heat is produced by this most intense light, and it is consequently not merely the light of incandescence ; for, if it were so, the intensity of the heat developed would be proportionate to the amount of light produced. This experiment has long been familiar to the students of chemistry, and no one could have seen it without becoming at once sensible that a powerful source of artificial light was thus placed in our hands. The late Professor Daniell long ago stated his views on this point in the following sentence of his Chemical Philosophy, — “ When passing between two charcoal points, the duration of the disruptive discharge of the voltaic battery renders it the most splendid source of light which is under the command 104 LIGHT. A series of splendid displays of this light, and of other phenomena developed by the voltaic battery, was carried on some time ago at the Royal Institution. Possessing a battery-power of the enormous number of 2,000 plates, the experimenters were able to exhibit the intense energies of the electric current thus produced on a magnificent scale. They were able to develop a stream of electric light four inches in length, so dazzlingly brilliant as to be intolerable to the unprotected eye. Professor Daniell, with a combination of seventy of the powerful batteries invented by him, produced an intense flame of electric light one inch in length. The first practical exhibition of the electric light appears to have taken place in the year 1843, at Paris. For some time an ingenious person of the name of Achereau had been making applications to different individuals of superior fortune and influence for patronage and support in the introduction of a new description of artificial light. Succeeding at length in obtaining a sufficiently large apparatus, and permission to make his experiments in the Place de la Concorde at Paris, the day was fixed, and a large number of persons — it is said four or five thousand — were present to witness the spectacle. The hour appointed was nine in the evening, and the apparatus was fixed on the base LIGHT. 105 of one of the statues. All that was visible was a glass globe of about twelve inches diameter, with a movable reflector attached to it; and a couple of wires descending from it to some galvanic apparatus at the foot of the erection. Until a little before nine o’clock in the evening all was in darkness, so far as the simple mechanism was concerned; but the Place was illuminated with its usual complement of 100 large-sized gas-burners. The proper signal being given, the galvanic circuit was completed by the junction of the wires, and almost instantly the light of day seemed to burst upon the entire area. Although all the gas-lights were burning, they seemed in the glare of this new source of light to “pale their ineffectual fires,” as in the pure daylight itself. A large number of them were then put out ; but the amount of light did not seem to be in the least diminished : at the distance of 100 yards it was possible to read moderate-sized print with great facility. The astonishment and applause of the populace were equally great, and the exhibition excited for some time much interest in the scientific circles of Paris. We believe that the scheme was afterwards taken up with a view to light the entire city of Paris by means of one vast light, to be called the “ artificial sun.” Owing, how- ever, in all probability, to some defect in the 106 LIGHT. mechanical arrangements of the light, the whole affair was dropped, and seems to have excited little or no attention until lately. The electric light has more recently been exhibited in several parts of London. It was first introduced, we believe, at the extensive rooms frequently employed for public meetings in Hanover Square. The rooms were, as usual, lighted with chandeliers of wax candles, with a considerable number of oil-lamps; the total amount of light being considered to be equal to 200 or 300 wax candles. On the lecture- table was the light apparatus, a rather elegant object, covered with a tall glass shade. All things being made ready, the galvanic circuit was completed, and in a few seconds the whole apartment w r as filled with such a blaze of diffusive light, as caused the dimly-burning candles and lamps to assume the lack-lustre aspect they bear in ordinary sunlight. Every object in this large room was brightly illu- minated; and as an assistant turned the light on and off at pleasure, the transition was as violent as from broad day to evening twilight. The paintings on the ceiling were displayed in a manner not often witnessed on one of our brightest London summer days ; and, what was very remarkable, the tone of the colours was precisely similar to that which they are seen to LIGHT. 107 possess in real daylight. All the delicate inter- shadings of the yellows, greys, flesh-tints, and even of greens and blues , were brilliantly defined, and in all respects conveyed the daylight im- pression to the eye. The light was about equal to that of 700 or 800 standard wax candles, yet a lady’s bonnet might have covered the entire apparatus; and the actual source of light did not occupy an area of more than an inch in every direction, if so much. The rays were then concentrated by a powerful lens, and directed upon some pictures, which were placed for that purpose on the side of the room. The effect was as if a sunbeam had been snatched from the long-retired luminary himself, and thrown in all its pure radiance on the painted canvas : so brilliant was the illumination, that in the surrounding mirrors it was perfectly easy to see the pure colours of the pictures reflected as if by day. By means of a glass prism, a spectacle yet more beautiful was shown: this was the display of the 'prismatic spectrum , the entire number of the rays being present, and in brilliancy not to be distinguished from the same as shown by the decomposition of the true solar light. Perhaps one of the most striking displays of the character of the electric light followed. The charcoal points were immersed in a globe of water, and still 108 LIGHT. the light continued gleaming forth in all its brilliancy. A company was established in London to carry out the electric light on a commercial plan, but for some reason or other it proved abortive, and, like many other beautiful and ingenious inventions, it has been for some time almost forgotten. Experiments made at Manchester on this light were satisfactory in determining the question of its continuance and steadiness for a considerable number of hours. The light was set in action at four o’clock in the afternoon, when a bright sun was shining, and the light was contrasted with sunshine, without any perceptible difference being discernible between them. It was kept in action from four until nine in the evening, and its brilliancy increased from the very first, when it was thought equal to 200 candles, to the middle period of its trial, when it emitted a light equal to 700 candles. At the conclusion of the display, a portion of the solutions produced in the galvanic battery was drawn off and pre- cipitated before the company present, and a white powder produced, which was represented to be of commercial value sufficient to pay the expense of producing the light. Into this as- sertion, however, we shall presently make inquiry. LIGHT. 109 Another invention for electric light is the apparatus of Mr. Allman. This instrument is extremely simple in its construction, and has a clever arrangement for adjusting the charcoal points, and maintaining a uniform and steady light. It is also rather elegant in its appearance. The principle adopted hy Mr. Allman in the adjustment of the points, is ingeniously made to depend upon the in- tensity of the current itself without the aid of any auxiliary power. The annexed diagram shows his arrange- ment as applied to a bracket lamp, a a is a permanent magnet centred upon a spindle at c, and placed within the coil of wire B ; the electricity circulating in the direction of the double-headed arrows produces a deflection of the magnet a a , which, being connected with the lower charcoal point d , draws it away to the proper distance from the upper point e. The current, after pass- ing through the coil, passes up the stand through 110 LIGHT. the upper and lower points back to the battery. Now, as the electric current causes the separa- tion of the points (and too great a separation would break the circuit), and as the magnet a is so balanced that the points are kept in contact when no current is passing, it follows the current would never destroy itself, but a proper distance would constantly be maintained and a continuous light be produced. The annexed figure is an exact copy by photography of the arrangement for adjusting the distances of the points, as shown at the Great Exhibition. The difficulty of preserving a uniform dis- tance between the points, is not the only diffi- culty in the management of the electric light, for it is found that the battery cannot be depended LIGHT. Ill upon for uniformity of action for any stated time, but is extremly liable to fluctuations in intensity, and indeed to vast losses of power, owing to some unascertained causes. Very recently a new commercial company called the Electric Light, Power, and Colour Company, has commenced operations in London. This company proposes to pay for the whole expenses of the light by the production of a variety of valuable pigments from the fluids of the battery employed. It is stated that several chemical substances are used in the battery, the compounds resulting from which produce the pigments in question. The light has been publicly exhibited, and has as usual attracted admiration and surprise by its brilliancy; but until it has been tried for a greater length of time, and the statements of its inventors practically demonstrated on a large scale, the present attempt must be classed with others which have preceded it, and received with much caution. Before passing to the con- sideration of the cost, and probable success of this source of light in an economical point of view, we may mention that the vast national works recently undertaken at Paris, under direc- tion of the Emperor Louis Napoleon, have been carried on at night, chiefly by the assistance of the electric light, and a brilliancy of illumina- 112 LIGHT. tion not far inferior to daylight has been the result. It is now a well-ascertained fact, that both in its power and intensity the electric light leaves nothing to be desired as a source of artificial illumination. It is unquestionably superior in this respect to every other source of light; and its cleanliness, the absence of heat, and the facility of applying it where other sources of light would be inapplicable, as for example for works carried on under water, render it a most desirable and valuable dis- covery. But it also has many serious if not fatal objections to its general use. In the first place the light-producing power cannot be accumulated. We can collect an exhaustless store of gas or tallow, and burn a light from them without fear of running short. But the electric light is in direct communication with its source, and that source must be kept in constant activity or it will fail, and there is no known means by which we can concentrate the power and let it out by degrees. Again, the galvanic battery is not an instru- ment of constant action, nor of easy and com- modious management. A domestic servant could not keep it in order, nor repair it, nor regulate it. And if that were possible, the construction of the battery is such, that its power is never LIGHT. 113 precisely alike, nor the same from one hour to another. There are also enormous losses of electric power in consequence of non-insulation of the wires, and of yyant of perfect metallic contact in the working parts of the mechanism employed in the light. Lastly, it is a most expensive light. Admitting all that has been said as to the intrinsic value of the products of the battery, there is not an adequate commercial demand for them to make their production pay, nor, in fact, are they really of the value assigned ; or, if they are, they can be obtained in other ways and at a far inferior cost. The electric light also, is not a light of what we may call a comfortable or useful kind. It is a violent glare, brilliant and dazzling to the eye, but it is not so efficacious in lighting up the detail of ob- jects as a far feebler light applied closer to them. It is therefore inapplicable to domestic use, and to lighting the streets. But for lighthouses, or for shedding a brilliant light over a large area at one time, it is unequalled ; and for such purposes the cost of its production may very probably be of minor consideration, and more regard would be given to the certainty and uniformity of the light than to its comparative cost, unless this were very large indeed. In these views it may be satisfactory to the reader to learn that almost all the most eminent chemists and philosophers of I 114 LIGHT. the present day coincide. It is therefore not very probable, to say the least, that the electric light will ever displace our simple and homely taper, or our still more simple, cleanly, and com- modious gas lights. The oxyhydrogen or Drummond light very closely approaches the electric light in its intense and brilliant powers of illumination. The late Captain Drummond invented this light in order to distinguish the stations chosen for the angular points of the triangles, in the Ordnance Survey of the United Kingdom. These stations are many miles asunder, and it is necessary to reflect the rays either of the sun, or of some powerful light, in order to mark out distinctly the respective positions of the different points in carrying on the survey. At night the best artificial illuminations proved insufficient to throw a clear ray over the great distance to which it had to travel, and a more powerful source of light became necessary. Captain Drummond conceived the happy idea of producing light by the intense incandescence of an incombustible substance, such as chalk or lime. Until his invention, light had always been derived by the combustion of solid or liquid substances ; but the light called after him has the peculiarity of being evolved from a body, which is merely heated to whiteness by LIGHT. 115 a small flame which itself is almost without perceptible light ! In the original arrangement of his apparatus, a ball or disc of lime about a quarter of an inch in diameter was placed in the focus of a parabolic mirror at the station to be rendered visible, and while in that position a flame arising from the passage of a stream of oxygen gas through alcohol was directed upon it. The most dazzling light was the immediate consequence. The cistern containing the alcohol was supported on a stand behind the reflector, and was connected by an india-rubber tube with the inner part of a hollow stem, supporting the upright wire, at the top of which was fixed the ball of lime nearly on a level with the cistern. The spirit ascended in the stem, and afterwards through three or more tubes to the ball. The vessel containing the oxygen gas was connected by a flexible tube with an orifice in a cylin- drical box in the same stem, from which it ascended through three flexible india-rubber tubes to the ball, after passing with friction through three small cylinders. The whole apparatus was attached to a stand which carried a mirror, and adjustments were provided by which the ball could be placed exactly in the focus of the mirror. The intensity of the flame thus produced was from sixty to ninety times i 2 116 LIGHT. as great as that of an Argand burner. Such was the brilliancy of the light thus developed, that stations upwards of sixty miles from one another were very distinctly seen in hazy weather. Captain Drummond suggested the intro- duction of this powerful light for lighthouses, and proposed that a stream of hydrogen gas should be substituted for the alcohol, and burnt after mixture with oxygen. He proposed to carry the gases from separate vessels and enter a small chamber through several apertures. The united gases were then passed through several pieces of wire gauze and made to issue in two streams against the ball or disc of lime. To prevent the latter from wasting too rapidly in one place, it was made to revolve once in a minute; and in order to keep up a constant light, it was proposed to have an apparatus by which a number of balls might be successively made to fall in the focus of the mirror. It is well known to chemists that a mix- ture of oxygen and hydrogen gas in certain proportions is very explosive, and will in- stantly take fire, and burst with a loud report the vessel in which it is contained unless it is very strong. The result of this explosion is the production of a few drops of water, which is composed of oxygen and hydrogen. The oxy- hydrogen light is therefore really produced by LIGHT. 117 the ignition of the gases which form water. Yet it is very remarkable that a mixture of these gases when passed through a small tube burns in a tranquil manner, with merely a hissing or crackling sound, and still more so that it forms a flame so blue and pale as scarcely to be visible. The heat of this flame is, however, the most intense and consuming of any known to science. It constitutes, in the form of the oxyhydrogen blowpipe, the most powerful source of intense heat which we possess. This instrument, which is now universally employed instead of the apparatus employed by Drummond for developing an intense artificial light, is constructed in the following manner. The figure shows the bellows used. Hydrogen gas is now commonly replaced by coal gas, which is much more economical and easily procured. This gas and oxygen are con- veniently retained in large gas holders made of macintosh cloth provided with press-boards. In order to send out the gases with sufficient 118 LIGHT. pressure, two or three weights of fifty-six pounds each are placed upon it. An ingenious arrange- ment is made in order to prevent the gases ex- ploding or passing back through the tubes and kindling the reservoirs of gas, as they re- peatedly did when mixed at some distance from the point of the blowpipe. A double pipe is now used, shown in the cut, which carries the sepa- rate gases and does not permit them to unite until just as they reach the nozzle of the blowpipe. The whole arrangement for the light is shown in the next diagram. Thus arranged, the gases are brought from their separate receptacles and con- ducted through an upright tube, to the narrow tube out of which they issue and are set on fire. At a little distance from this point is placed the ball of lime, which when incandescent burns with such intense radiance as to be visible at a distance of sixty-nine miles in a direct line. So intense is the flame that the most refractory substances, such as pipe-clay, silica, and platinum, are melted with the greatest facility, and the latter metal is even dissipated in a state of vapour. With an adequate supply of gases, this light LIGHT. 119 may be continued for an indefinite period ; but it is found that unless the ball of lime be a is the lime cylinder. b the safety tube. h the tube from the hydrogen-holder. o the tube from the oxygen-holder. slowly moved so as to expose a fresh surface to the flame, its brilliancy becomes diminished — it is supposed, owing to some slight fusion of its particles under the intense heat. In most of the popular scientific institutions the oxyhydrogen light has taken the place of all other sources of intense light for the purposes of optical dis- play, as, for example, for the microscope, dis- solving views, &c. We are not aware that this brilliant light has been applied to lighthouses ; it is probably rather of too complicated a nature, 120 LIGHT. and also a difficulty might be found in keeping up the requisite supply of the gases necessary for it. Mr. Children has somewhat modified Drum- mond’s light in the following manner. The flame of spirits of wine, containing a small pro- portion of spirits of turpentine dissolved in it, is thrown on a surface of quicklime by a stream of oxygen gas. The light yielded by this arrange- ment is nearly as brilliant as the Drummond light, and is free from the danger of explosion. Measured by one of Professor Wheatstone’s pho- tometers, this light when at its full brilliancy was found to equal that of about 120 of the best platted- wick wax candles. The apparatus con- sists of a copper lamp with two tubes lying close together, and each containing a wick formed of flat cotton rolled up into a cylinder; and a cylinder of lime, about f ths of an inch long, and ^th of an inch in diameter, enclosed in a thin copper case. The pipe conveying the oxygen gas from the gasometer terminates in a small jet inclining upwards, which lies between the two wicks slightly parted to receive it, and within rather less than ^th of an inch from the circular disc of lime, and about ^th of an inch above the lower edge of the copper case. This apparatus has been successfully employed to illuminate the dissolving views, physioscope and “LIGHT. 121 microscope, at a public institution, and a large screen twenty-two by eighteen feet was bril- liantly illuminated by it over every part of the surface. It is most difficult to determine the precise amount of credence to be given to the statements made in America not very long since, as to the water-gas of Mr. Paine, — an invention which must be classed with those of the present chapter, as to its nature. The following ac- count is chiefly derived from the pages of the Journal ot Patents, and appears to be a candid statement of things actually witnessed : — “ The apparatus consists of four pieces, all placed upon a pine table or shelf, and in no way connected with anything else. “ First — A common magneto-electric machine, consisting of two permanent horseshoe magnets, about twelve inches long; these were placed horizontally on a mahogany frame, about four inches apart, one being placed above the other. Between the ends of these magnets were a pair of helices, and these so attached to a wheel above that they could be set into a rapid rotary motion. “ Second — A large open-mouth glass jar, capable of holding twelve quarts; this was a little more than half filled with water. (We tasted the water, to satisfy ourselves that it was 122 LIGHT. water.) Within this jar was placed a common bell glass, open at the bottom, and reaching within four inches of the bottom of the large jar. The top of the bell glass was closed tightly with a brass cap, which extended over it, so as to rest upon the sides of the outer jar. Passing through the cap of the bell glass were two wires, which extended nearly down to the bottom of the bell glass, and these ter- minated in a circular metallic box, one and a half inch long, and one inch in diameter; this box was hollow, and perforated with small holes in the upper part. The electrodes, or points of connexion between the poles, were in this box. The water in the jar and bell glass reached some six inches above the elec- trodes. “ Third— A quart glass jar, half filled with spirits of turpentine; a tube or gaspipe passed from the top of the bell glass above mentioned, and into this jar of turpentine; terminating at the bottom of the turpentine. From the cap which covered the jar of turpentine, another tube or gaspipe passed to a jet or burner about twelve inches from the jar. “ Fourth — A common glass tumbler, half filled with water. “ The above comprised all the apparatus used by Mr. Paine, with the exception of three wires, LIGHT. 123 or rather flat strips of copper, by \< hich he connected the magneto-electric machine with the jar of water. These wires were connected in the following peculiar manner: — The end of one wire was screwed to the negative pole of the magnetic machine, and the other end to the first of the wires, coming up from the electrodes through the top of the bell glass. A second wire was screwed to the positive pole of the magnetic machine, with the other end termi- nating in the glass of water above mentioned. The third wire was screwed to the bell glass, or rather the second wire which passes through it from the electrodes within, and terminating in the same glass of water, although the wires in the glass did not touch each other. On turning the machine, instantly large bubbles of gas arose from the electrodes, and filled the jar in less than a minute ! After taking out a stopper from the bell glass, and allowing several jars full of gas to escape, in order to expel the common air, and prevent an explosion, these were stopped, and the gas forced on through the gaspipe into the turpentine, and through this to the jet or burner. Between the jar of water where the gas was generated, and the jar of turpentine, a jet issued from the pipe. This was lighted, and proved to be hydrogen gas. The flame, in front of a window, was so pale 124 LIGHT. that it could not be perceived. We could see it by placing a dark body behind it. While this was burning, the gas was forced along through the turpentine to the other burner. A flame was applied to this, and a brilliant light was shown ! ” This light excited much attention when first brought forward, and it is said that the Ame- rican patent was sold for nearly one million sterling. It has also been patented in this country. It is, however, obvious to any person in the least conversant with chemistry, either that the inventor has purposely, or in ignorance, sup- pressed a part of the truth. It is simply impos- sible to decompose water, without generating oxygen as well as hydrogen gas ; and conse- quently a very great error is committed when it is stated that the oxygen is suppressed, or that water can be decomposed without evolving this gas. That both hydrogen and oxygen can be readily obtained by decomposing water has long been familiarly known; and it has also been known that, by having a tube with a partition down the middle and placing one electrode in one division and the other in the other, these gases can be had separately. Every student in chemistry is aware of this, and most persons also know that hydrogen gas can be LIGHT. 125 rendered luminous by passing it through oil of turpentine. It would therefore seem either that some facts are kept back in the account given of this light, or that, like many other inventions, it is merely a revival of experiments long since performed in the laboratory. A more interesting because a truthful and reliable series of experiments, has been carried on at Paris. The apparatus employed for this light is very simple, consisting principally of one or more cylinders of iron arranged horizontally in a furnace, and charged with wood charcoal, which is heated in an intense degree. Steam is passed through these retorts, and is decom- posed into several gases in conjunction with the air and charcoal in the retorts. Carbonic oxide, hydrogen, and a small quantity of carburetted hydrogen are obtained from these, and the gas is then ready for use. The flame produced is not very luminous, but is immediately ren- dered so by being burnt in a burner in contact with platinum, which becomes so intensely in- candescent, as to yield a very useful and constant light. This gas is very pleasant to use, being free from smell; and only yielding water by its combustion, it is far more wholesome than any other light of its kind. In point of cost, how- ever, it is probable that it must yield to the superior economy of coal gas. 126 LIGHT. The last light to which we shall direct the reader’s attention is the Bude light. It has long been known to chemists that a most dazzling light is produced by burning any in- flammable substance, such as phosphorus, car- bon, oils, or even iron wire in oxygen gas. Intensely active chemical decomposition is in- stantly established on surrounding a burning substance, such as a candle or an oil lamp, with an atmosphere of pure oxygen gas, and this is attended with the evolution of great heat and vivid light. The Bude light proceeds upon this well-known principle. It is simply a good Argand oil lamp, fed with a constant stream of oxygen gas, instead of atmospheric air. The increase of light is very remarkable* and may be easily shown by connecting a tube to the ordinary Argand lamp used in our drawing- rooms, and causing the other end to be attached to a bladder filled with oxygen. This light has been several times publicly tried, and has been found to answer well as a powerful, steady, and intense illuminator. The four lamps at the corners of Trafalgar Square were originally designed to exhibit this light, but are now supplied with ordinary gas. In point of economy it is probably inferior to the electric light, and it is certainly many times more costly than the ordinary gas light. Of LIGHT. 127 late, the Bude light has ceased to be employed for public purposes. In reviewing the different lights here shortly noticed, it must be confessed that not one of the number appears to have much probability of ever coming into general nse. The contrast between the applicability of these lights, and that of our ordinary sources of artificial light to economical uses, very closely resembles that between the rival powers of steam and electro-magnetism. Just as steam must always be a cheaper source of power, since it is the direct product of two abundant raw materials, coal and water ; so gas, oils and fats, as a source of artificial light, must always be cheaper than the electric or similar lights: for these are directly produced from abundant raw material, whereas the Electric, Drummond, and other lights, are a step beyond these in production, and require the use of manufactured articles for their development and support. This fact tells with a fatal force against the almost innumerable schemes for new artificial lights, arising from chemical sources, with which the present age abounds. CHAPTER III. DOMESTIC SOURCES OF LIGHT. The necessity for a cheap and commodious artificial light must have been felt at a very early period in the history of mankind, and it appears early to have led to the adoption of a very efficient source in the combustion of animal fats and vegetable resins. Two ele- mentary substances may be said indeed to constitute the source not only of the simple and rude artificial lights adopted by uncivilized man, but also of the most elegant and refined kinds which modern skill and science has placed at our disposal. These elementary bodies are hydrogen and carbon. We shall presently ob- serve in what a great and interesting variety of forms these elements present themselves to our notice as sources of artificial light. It will, in the meantime, be useful to furnish materials for correct ideas on the general subject of flame and light. By the terms ignition and incandescence, observes Professor Brande, we express a pro- LIGHT. 129 perty which bodies possess of giving out light, whenever their temperatures are raised up to a certain point. The quantity of light thus emitted increases with their temperature. At first it is dim and feeble, then it becomes dingy red, and the bodies are said to be red hot, then bright red, then, as the body becomes more intensely heated, of an orange or yellow tint ; and at length it acquires such brilliancy as * to be painful to the eye, when it is said to be at a white heat. A series of interesting experiments have been instituted in order to determine, by a thermome- tric scale, the temperature at which light begins to be emitted by heated bodies. Sir H. Davy, who gave much attention to this subject, esti- mates the temperature of a substance which in the dark is just feebly luminous, to be about 800° Fahrenheit, a dull red visible in day- light is supposed to be about 1000°, a full red heat 1,200°, an orange heat 1,700°, and a white heat 3,000°. These experiments, however, scarcely admit of accurate determination, and probably only exhibit an approximation to the truth. It is, however, obvious from these results, that all our sources of domestic light are merely contrivances for raising the temperature of substances to a very high point ; for we are not satisfied with the most common source of K 130 LIGHT. light, unless it emits at least a bright yellow illuminating ray ; and to do so, from what we have already observed, it is necessary to raise the temperature to at least 1,500° above the boiling point of water. It is therefore to the intense heat generated even by the wick of the feeblest candle, that its luminous property is to be attributed — at least, in part. But it is to be carefully observed that a flame of very high temperature is not necessarily luminous. In the last chapter it was noticed that the destroying flame of the oxyhydrogen blowpipe, the most intense source of heat pro- duced by art, is yet so little luminous that it could scarcely be seen at the end of a dark room. The flame of a jet of hydrogen gas, obtained by dissolving zinc in dilute sulphuric acid, is scarcely visible, yet is intensely hot. As a direct source of light, either of these flames would be useless ; yet it has been already seen that one of them at least can be made to furnish one of the most intense and dazzling kinds of light which we possess. The manner in which this was accomplished shows us very distinctly what it is which really constitutes the bright- ness of a flame. By placing a ball of lime in the flame of the oxyhydrogen blowpipe, a light little inferior to daylight is instantly produced. The flame falling upon the solid LIGHT. 131 matter of the ball, heats it up to an incandescent point, and a most dazzling stream of light is procured from it. This experiment renders it certain that light, in its artificial sources at least, is principally due to the intense incandescence of solid matter . The reader can readily satisfy himself of this. If some strips of sheet zinc are placed in a common Florence flask or bottle of any sort, to the mouth of which a perforated cork is fitted, and supplied with a glass tube drawn out to a point, and some ordinary sulphuric acid mixed with five or six times its bulk of water be poured upon the zinc, hydrogen gas is immediately produced and escapes through the glass tube. After allowing it to escape for a minute or two, for fear of an explosion should any air be mixed with it, the flame may be kindled by a taper. It burns with a bluish light, which, as we have before remarked, can scarcely be per- ceived in the daytime. If now a small coil of very fine platinum wire be placed in the midst of this flame, it is instantly heated to a very intense degree, and there is a considerable amount of light emitted. Yet careful weighing of the wire before and after would not detect any perceptible diminution in its weight. This simple experiment thus affords us addi- tional proof that light is really the result of K 2 132 LIGHT. the intense incandescence of solid matter diffused in the flame. In whatever way we heat a solid substance beyond a certain point, if it is not decomposed or does not evaporate before it attains that degree of temperature* light is pro- duced. Our domestic sources of light are there- fore a means of intensely heating minute portions of solid matter to a temperature at which they become luminous. The process called combustion is that by which this result is effected. It is a strictly chemical process, familiar though it may appear, and its results are strictly chemical compounds. The flame of a rushlight exhibits a chemical experiment on a humble scale, and the heated air which rises from it contains the gaseous products of that experiment. Ordinary com- bustion may be simply expressed as a combi- nation of the burning body in an act of intense chemical decomposition, with the oxygen of the air. The carbon and hydrogen, which we have named as forming the main sources of all our artificial lights, unite at a high temperature with the oxygen of the air, and form carbonic acid, carbonic oxide, and watery vapour. In the tiny flame of the smallest taper, as well as in the larger sources of domestic light, carbon and hydrogen are thus united with oxygen ; and by their union develop an intense heat, and LIGHT. 133 emit light. The products being gaseous, go off imperceptibly. But they may be readily made manifest to the eye. If we hold a cold tumbler, inverted over the flame of a candle, its sides will become covered with steam, representing the result of the combination of the hydrogen of the tallow with the oxygen of the air. If we would detect the carbonic acid, we must hold a jar over the flame, and then pour some lime-water into the jar and shake it up, and immediately it becomes milky from the union of the carbonic acid emitted by the flame (from the. union of the carbon of the tallow with the oxygen of the air) with the lime, to form car- bonate of lime or chalk. Such is the chemical nature of the art of combustion. Let us now notice its phenomena. It has already been said that solid matter heated to incandescence, is necessary to the pro- duction of light. In the flame of our domestic sources of light, the brilliancy is due chiefly to very finely divided particles of carbon which are blended with and burned in the flame. And the more perfectly the ignition of these particles is accomplished in the flame, the more vivid and intense is the resulting light : hence the superior lustre of the lights fed with oxygen gas, as the Bude light. It is possible to examine what has been appro- 134 LIGHT. priately called the structure of flame, (see figure,) by aid of a very few pieces of simple apparatus. A piece of thin glass, a small piece of fine glass tube, a piece of wire gauze, and a wax taper or a jet of gas, are all that are necessary to demonstrate in a satisfactory way the nature of the flame of a candle ; and, in fact, constituted all the appa- ratus employed by Sir H. Davy in the researches which led him to the valuable discovery of the miners’ safety lamp. In a can- dle, the material forming the body of it being melted by the heat of the burning wick, is drawn up by capillary attrac- tion, through the ascending tubes formed by the juxtaposition of the fibres of the cotton, and in the wick it is converted by the heat into vapour which is given off and ignites ; thus a constant fusion and evaporation of the material takes place, and is sustained by the heat developed in the act of combustion. Now, it is very remarkable, that it is from the exterior of the flame almost exclusively, that its light is derived. The centre of the flame of a candle is quite black. If we place a piece of LIGHT. 135 thin glass over the flame, we can look down into it, and it is seen as a ring of light having a dark central portion. The cause of this is that the process of combustion takes place only at the surface, and not in the centre of the flame. It is therefore not a solid flame, but a hollow cone, the circumference only of which is on fire. If we take a small glass tube and carefully put one end of it through the outer part of this cone into the dark interior of the flame, some of the gaseous matter of the burning material will rise through the tube, and may be kindled at the other end. All the early forms of artificial light for do- mestic use are constructed in such a manner as to show complete ignorance of the hollow nature of flame, and consequently were very imperfect in their results. The lamps used by the Romans emitted but a feeble yellow light, and filled the air with smoke, contaminating the walls and clothes. Candles were very slowly introduced ; and, though superior to lamps in some respects, were very feeble in their illuminating powers. No person, however, seems to have really inves- tigated the cause of the defects in lamps, until about the year 1780; but in that year M. Argand, a native of Geneva, promulgated an invention of great advantage. It has been before explained, that the interior of an ordinary flame consists of gas which is not inflamed, because it is debarred 136 LIGHT. from mixing with the oxygen of the atmosphere. Argand, therefore, caused a circular wick to be constructed, so burnt in a hollow burner that the air not only came to the outside, but also to the inside of the flame ; a draught of air being produced by a glass chimney, which, protecting the flame from draughts, caused little or no smoke. So excellent was this principle proved to be, that every succeeding inventor made it a basis of improvement, few attempting to adopt any other sort of burner ; their ingenuity being chiefly expended on other parts of the lamp. The most elegant improvement was the annular table lamp ; the oil reservoir of which, consisting of a circular tube placed below the light, casts comparatively no shadow, which all lamps upon the old construction did. The rays of light were the more equally diffused by the inter- vention of a large ground-glass shade. Candles, lamps, and gas constitute the three great sources of domestic artificial light: and they represent, curiously enough, the three forms in which combustible material occurs — namely, solid, fluid, and gaseous. Candles of all de- scriptions consist of solid inflammable matter. Lamps of all sorts are supplied with it in a liquid state, and our streets and houses are lighted by tubes and burners to which is led a constant stream of gaseous combustible matter. LIGHT. 137 One of the most accurate and complete notices of the various kinds of candles which have been published of late years, exists in the Juries’ Report of the Great Exhibition ; and to it we would refer the reader for much interesting and minute information on this subject. Taking this report as our authority, we shall give a short notice of this important manufacture. Not less than one and a quarter million cwts. of tallow were imported into the United King- dom in 1850. This valuable commercial sub- stance deserves and has received a most careful investigation by scientific chemists, and the results have been most important. It has been shown by Chevreul that fats and oils are not simple substances, but contain and may be made to yield several distinct chemical compounds possessing distinct qualities. These are chiefly fatty acids existing in combination with a substance called oxide of glyceryl, and they may be obtained in a state fit for making excellent candles, far superior to the common tallow and equal to the best wax, by an in- genious combination of chemical and mecha- nical applications. The most extensive works for carrying on this process are those of Price’s Candle Company; and the manner in which fats are treated there, and made to yield the 138 LIGHT. most beautiful and valuable commercial products, deserves our attention. About 20 tons of palm oil are placed in a large lead-lined vat, and fused by a steam-jet. The fluid mass, after being allowed to settle, has now to be exposed to the combined action of concentrated sulphuric acid and heat, and for this purpose is pumped up into the acidifying vessel, in which its temperature is raised to 350° F. The means of heating is a jet of low- pressure steam, which, in its course from the boiler, passes through a series of iron pipes heated in a furnace. The quantity of acid used is in the proportion of 6 lbs. for 112 lbs. of palm oil. In this operation the palm oil is decomposed and becomes much blackened. Withdrawn at that period, it is seen that an important change has been effected by the action of the acid, as the mass now readily crystallizes to a tolerably solid fat. The fat is now drawn off from the acid and transferred to the washing tank, where .it is boiled up with water by means of a steam-jet. After one or two washings, the blackened fat is withdrawn and pumped up to the supply tank, which commands the stills. The stills, which are made of copper, are heated by an open grate ; each still is capable of holding five tons of fat. When charged, the temperature is LIGHT. 139 raised to 293° *5 C. (560° F.), and low-pressure steam passed through the mass; this steam is previously heated by passing through a system of iron pipes placed in a furnace. The current of steam carries with it the vapour of the fatty acids, and thus facilitates the process. The mixed vapours of fatty acids and water pass together to a series of vertical pipes, which retain a temperature above 100° C. (212° F.), where the fats only condense while the steam passes to a second refrigerator, cooled by a current of water ; here it is condensed along with the minute quantity of fat carried over by it. A separating tank, from which the water escapes at the bottom, whilst the fats float on the top, serves to recover this small quantity. After continuing the distillation for a certain period, the residue in the still is transferred to another still formed of iron pipes, set in a furnace, and there submitted to a much higher temperature, and a jet of steam more strongly heated. The residue left in these iron stills is a sort of pitch, and is applied to the same uses as ordinary pitch ; by this means an additional quantity of fatty acids is obtained. The fatty acids, as they run from the still, are used to a great extent for the manufacture of candles without pressing, and form what are called Composite candles, which possess all the 140 LIGHT. advantages of being self-snuffing, but are more fusible and softer than the pressed stearic acid candles. A large proportion of the distilled fats, however, is pressed to make a better sort of candle, and for this purpose 50 hydraulic presses are employed. The fats are spread by ingenious machinery on woven mats, and submitted to powerful cold pressure, between iron plates ; the oleic, or metoleic acid, runs out, and is collected, and chiefly exported to Germany, where it is em- ployed in soap-making. After cold-pressing, the fat acids are subjected to hot pressure, in hydraulic presses, confined in a chamber heated by steam. The pressed cakes, after the removal of the edges, are melted in contact with a little diluted sulphuric acid, and run into blocks. The moulding of the cheaper descriptions of candles is effected by ingenious machinery invented by Mr. Morgan, of Manchester, and improved by the engineer of the Company. By this machine 18 candles are moulded at one time ; the wicks, 60 yards long, are wound on 18 separate reels, one for each mould. As one set of candles is pushed out by a series of plungers, they draw with them into the moulds the wicks for the next lot; these wicks being held temporarily with one clip, whilst the LIGHT. 141 candles are held with another, are cut off close to the candles by a traversing circular cutter. Compound forceps, having 18 holders, now seize the wicks at the open end of the moulds, and hold them in their places ; the plungers then return and draw the wick tight. The moulds, which during the operation have remained in a horizontal position, are now turned in a vertical direction, the small end downwards, and are then passed on a railway to the person who is to fill them, they being heated to the proper temperature by their transit through a hot closet. They are then passed to other parallel railways, and left to cool : after remaining a sufficient time to allow of the solidification of the candles, the moulds are brought back in succession by means of turn-tables to their first position. The forceps (which during the moulding have remained where they were placed) are now removed, and the frame of moulds again turned in a horizontal position. Eighteen plungers or pistons are made to press forward the loose bottoms of the moulds which correspond to the small end of the candle. In pushing these forward the candles are pressed out, and thus the cycle of operations is com- pleted. It must be added that the return-stroke of the piston brings back the bottoms of the moulds against shoulders provided to keep them from falling out. 142 LIGHT. Pressed cocoa-nut oil is largely employed to mix with the pressed acids of palm oil to make the best composite candles. So enormously has the trade of making candles extended since the introduction of these inte- resting and valuable chemical discoveries into the manufacture, that the once threatened ex- tinction of this useful form of light by the predominance of lamps appears now to be ren- dered impossible. There can be little doubt, however, had not some means of producing a brilliant, cheap, cleanly, and self-snuffing candle been discovered, that oil lamps, or some other means of domestic illumination, would have been introduced into general use. Several different kinds of candles are now in general use, which are composed of dif- ferent products obtained from the chemical de- compositions of fat, and from different methods of treating these products after they are obtained. Wax candles are still used by the wealthier classes ; but it is very probable that in a little time this costly article wull be universally replaced by the equally brilliant, pure, and cleanly substances obtained by chemistry from the most apparently unlikely sources. Long since Liebig, in one of his popular works on chemistry, wrote the following sentence: — “ It would certainly be esteemed one of the greatest LIGHT. 143 discoveries of the age if any one could succeed in condensing coal gas into a white, dry, solid, odourless substance, portable and capable of being placed upon a candlestick, or burned in a lamp.” This discovery, if not yet really made, may be considered as very nearly accomplished ; for it has been found that coal-gas can be dis- tilled at a low temperature, so as to yield a considerable quantity of a product called Paraf- fine. This substance when pure is white, solid, inodorous, and burns with a brilliant and smokeless flame. It has been obtained in large quantities by the distillation of peat, and has actually been made into candles and applied to economical purposes. At present, however, it is not by any means certain that Paraffine candles can be made so cheaply as to be a substitute for composites, much less probably for gas. We shall advert now to the liquid sources of artificial light. In reality, however, a candle scarcely differs from a lamp for burning oil. The fat requires to be first liquefied before it can ascend the wick and be consumed. Whereas, in the lamp, the liquid material is already sup- plied to the wick, and retains that condition irrespective of the heat of the flame. The great improvement effected in the construction of lamps by Argand has already been noticed ; and upon that part of the lamp to which his 144 LIGHT. discovery refers — namely, the wick and apparatus for supporting it, and the central perforation for the admission of fresh air — there has been no important alteration made since his time, neither does any appear possible. The mechanical in- genuity, therefore, which has expended itself in so many different ways on this subject, has been occupied almost exclusively with the means for supplying the lamp with oil, the form of the lamp, and the adjustment of the glass chimney to the wdck. The common table lamp, which has often assumed a very elegant appearance, is an instrument of very simple construction. It consists essentially of a hollow metal ring sur- rounding the flame, and supported by two branches which convey the oil poured into it to the wick, and keep up the supply as it is consumed. This ring is, in some lamps, called the Sinumbra, or shadowless lamps, so con- structed as to present a thin edge to the light ; and thus the rays pass above and beneath LIGHT. 145 it, casting little, if any, shadow on the table ; and this effect is assisted by the diffusive or dis- persive power of a ground glass shade which is placed over the ring. The figure represents the arrangement which is now generally adopted in lamps made on this principle. A certain amount of the light of the lamp was, however, always lost, even in the best arrange- ments of this form of table lamp, in consequence of the ring and its supports being placed so near to the light. It was therefore an object much sought after to arrange the reservoir for the oil in such a way as not to interfere with the light, but to be either above it, behind it, or below it. An ingenious form of lamp call the hot oil lamp has been invented, in which the reservoir for the oil is placed near the top of the chimney, leaving a clear space of some inches in height for the light. This lamp is calculated to burn the commonest kinds of oil, which is rendered very limpid and fit for brilliant combustion by the heat to which it is exposed from the light below it. This lamp is in other respects inconvenient, and it is also very easily put out of order ; it has therefore been only partially adopted, yet it is capable of giving a brilliant light, and casts no shadow around it on the table ; the ceiling, how- ever, is almost in darkness. Avery common lamp, at one time universally 146 LIGHT. employed, was that shown in this sketch, and called the fountain reservoir lamp. In this lamp the reservoir of oil is placed above and behind the light* The oil-vessel was generally formed of brass, and was connected by a horizontal tube with the wick and burner. This tube conveyed the oil to the light, and the supply was regulated by a valve acted on by a button outside. The arrangements of the burner did not differ from those of the table lamp. In this lamp, the reservoir being always placed on a bracket against the wall, the shadow was of no consequence ; and the light passing in LIGHT. 147 that direction, was often caught by a reflecting mirror and directed to some other point. For many years this lamp constituted the chief means of illumination for churches, public buildings, and shops. It was so easily cleansed, regulated, and was so simple in its parts, that it for a time maintained ground even against its formidable and ultimately successful opponent, coal gas. There have been many kinds of oil lamps invented, with the oil-vessel at the foot of the apparatus ; and this is, after all, the best form of lamp for ordinary use. But its progress to perfection has been attended with a variety of difficulties, and for a long time appeared almost hopeless. A variety of apparatus has been applied in order to force up the oil from the reservoir at the base of the lamp to the burner. In some lamps a small pump was employed, which required to be occasionally moved by hand, and in the interval the oil was forced up to the wick. In others a column of a heavy fluid, such as a solution of salt in water, was made to act on a column of oil, and keep it up to a level with the wick. And another kind of lamp had an air-chamber in communication with the oil-vessel, the compressed air in which forced up the oil to the proper height for the wick. All these inventions, however, proved equally un- successful for ordinary purposes. l 2 148 LIGHT. The Carcel, or clockwork lamp, was the first great improvement in this kind of lamp, and was the origin of the simpler apparatus which has since taken its place. In the Carcel lamp the oil is pumped up from the base of the lamp by clockwork, which acts on a kind of piston. The mechanical part requires to be wound up once in the evening, and the supply thencontinues very constant. The oil, on reaching the wick, flows over and drops down again, and is ultimately directed into the reservoir. This is a great advantage, tending to promote the steadier combustion of the wick, and keeping the passages free from accumulation of impurities. This light was first introduced in Paris, where it became extremely popular. But the expense of the mechanical arrangements proved fatal to its general introduction. For a long time it seemed impossible to construct a lamp on this principle — that is to say, with the piston in the base, for the purpose of forcing up the oil, so economically as to bring it into general use. The French manufacturers, hownver, devoted extraordinary attention to the subject, wdiich was one of much interest to them, in consequence of the cheapness of vegetable oils in that country, and the little progress made in the application of coal gas to domestic and even public illu- mination. LIGHT. 149 After various attempts to introduce a weight, and then a spring, in order to force down the piston and thus cause the ascent of the oil, the Moderateur lamp was invented; and since it combined all the advantages of the Carcel lamp, and could be manufactured at an extremely low price, it rose rapidly in popular favour in France, and has been attended with similar success in this country. This lamp has taken an extra- ordinary hold on public taste. It consists essen- tially of a circular reservoir, constituting the foot of the lamp, within which a spiral spring works, acting upon a piston, and so driving the oil up a small tube to the apparatus for holding the wick. This spring is wound up by a small key at one side of the lamp ; and, when once wound up, will continue in action for many hours. The oil ascending the tube, saturates the wick and flows over at the top, dropping in a succession of little drops into a central hole leading into the reservoir, into which it sinks through an ingenious valve when the spring is again wound up. The lamp requires a very tall chimney in order to produce a strong draught of air over the wick, and it is capable of burning oils of such inferior quality as to be useless for other lamps. The extraor- dinary cheapness of these lamps, in addition to the elegance of their form, has probably 150 LIGHT. contributed much to render them as popular as they are at this moment. Many of the large French shops are entirely lighted by these lamps, to the exclusion of gas. Among ourselves, however, while gas retains its present price, it is very improbable that this or any other kind of oil lamp will ever prove a very formidable rival to it. A few years ago a great rage for lamps which burned rectified spirits of turpentine, called camphine, existed in England, and threatened the extinction of all other artificial lights. The intensely white dazzling star of light produced by this lamp, was so remarkably superior in brilliancy to the yellow flame of the best oil lamps, that its popularity does not seem won- derful. The camphine was contained in a reservoir of glass below the burner, which drew it up by the capillary attraction of the wick. The wick required much attention ; and, in order to consume the carbon of the camphine more completely, a small button of iron was placed in the middle of the wick, the effect of which was to throw a powerful stream of air upon it within, whilst the peculiar form of the chimney caused an equally strong current to pass over it ex- ternally. It was to this part of the lamp alone that its success was due. So many complaints, however, soon arose as to the smell of the lamp, LIGHT. 151 its smokiness, its explosiveness, and its general offensiveness, that camphine lamps were very rapidly discarded from general use, and took their place with the numerous other inventions which had preceded them, and, like them, had been popular for a time, and were then thrown aside and forgotten. A very pretty kind of lamp was for some time in general use in Berlin, and has been attempted to be introduced here. It is fed with a dangerous mixture of strong alcohol and turpentine ; and this, rising by capillarity into a metallic wick, bursts into vapour with the heat of the wick, and then emits flames of gas at the summit. This light is a very pleasing, cleanly, and brilliant light ; but a more dangerous one can scarcely be imagined, since the least accident to the reservoir would imperil the destruction of the building in which it was used. The same must be said of the gas lamps invented by a Mr. Halliday, and which were much in use in London a few years ago, until a lamentable accident occurred with one of them, and thus they terminated their public career. Next to the steam-engine, the introduction of gas for domestic and public illumination must be regarded as one of the most valuable inven- tions of modern times. To a brief discussion of this interesting subject we propose to devote the LIGHT. 153 remaining portion of the present chapter. The history of gas-lighting would he rather out of place in this little work : hut as to what may be termed the philosophy of this admirable invention, it is in every way worthy of being generally known with accuracy. The term “ gas ” is very commonly understood to refer to only one or two elastic substances, and very little idea seems to be generally entertained that there exists as great a variety of different gases as of solid and fluid substances. The gas used for illuminating purposes is not a simple substance, but in the state supplied to us for combustion generally contains at least two gases in its com- position, and is occasionally contaminated with others. We shall, however, consider gas here as the generic term for the substance used for lighting, after making this statement : that it consists chemically of a gas called light carbu- ret ted hydrogen, and a small proportion of a very brilliant burning gas called olefiant gas. Occasionally, hydrogen gas and carbonic oxide gas, both of which are combustible, but burn with only a feeble blue flame, are mixed with the other gases, and deteriorate the product for all purposes of illumination. The gas which in the schoolboy’s experiment of heating pounded coal in a clay- pipe, is emitted from the tube and burned, is of very 154 LIGHT. complex nature, and is quite unsuitable for artificial lights until some of its ingredients are removed ; and the difficulty of effecting this in a cheap, rapid, and simple manner proved at first a great obstacle to the introduction of gas. For it was found to be so bad in smell, and so des- tructive to furniture, and even injurious to health, that it appeared almost a vain attempt to apply it to commercial uses. Careful chemical in- vestigations into the nature of this valuable though offensive substance led to discovery of the means necessary to fit it for universal em- ployment, and to render it the admirable and almost indispensable source of public and private artificial light. All that appears complicated in the history of the gas-manufacture is really due to the existence of impurities in the gas ; for if it were possible to use it at once in the state in which it leaves the heated retort, no manufacture could be more simple. Such, however, is not the case, as we shall now proceed to show. There may be said to be three stages in the production of gas in a fit state for use. In the first, it is extracted full of impurities from the raw material, coal ; in the second, it is deprived of these impurities; and, in the third, it is stored up in a large reservoir, from which it is discharged into the mains. The apparatus concerned in the first of these stages, is the retort. This LIGHT. 155 vessel is a cylinder of iron or of fire-clay, closed at one end, but open at the other, to which a close fitting cover is attached ; and immediately above, at this end, rises the tube which carries the gas from the retort onwards. The coal is thrown by a scoop into the retort, which is heated by a furnace to a low red heat, and the cover is then tightly screwed on, as shown in the figure. The heat is applied to the re- tort, thus charged and sustained at the same temperature for from five to eight hours. By that time the whole of the gas of the coal has distilled over, and there remains only the coke. The gaseous particles of the coal liberated by the heat to which it is exposed rise through the vertical tube. Such is the first stage of this manufacture. The retort is now opened, the coke removed, a fresh charge of coals introduced, and the whole process is recommenced, and so on continually. The impure gas is conveyed from the retort into a horizontal tube called the hydraulic main. In this tube the gas deposits some of its first impurities, tar and ammoniacal liquor ; and as 156 LIGHT. the tube from each retort dips into it, an escape of gas backwards is entirely obviated, and each retort is isolated from the rest. The gas is, however, still very impure, and charged with the vapour of tar and ammonia : this is removed by causing it to pass through a series of tubes exposed to the trickling of cold water, and thus rapidly cooling down the gas, and causing the con- densation of its tarry and ammoniacal vapours. Other substances intimately blended with the gas now require to be withdrawn from it. Of these the most pernicious are sulphuretted hy- drogen gas, and hydrosulphate of ammonia. The removal of these ingredients may to a small extent be effected by washing the gas in water ; but it is much more rapidly accomplished by means of lime. This substance has a power- ful affinity for these deleterious gases ; and since it is both cheap and abundant, it is the best which can be employed for this purpose. The gas is caused to pass through what is called milk of lime, being a watery mixture in which much lime is suspended. Or it may also be purified by passing it over iron gratings on which slaked lime is sprinkled. Both methods are in common use. The lime absorbs the fetid gases, and is renewed when necessary. The gas now is much more pure, but still retains a slight trace of ammoniacal gas. This gas is very LIGHT, 157 soluble in water, and is removed by causing it to pass through a vessel filled with pieces of coke, over which a stream of water trickles, and in its passage dissolves the ammonia out of the coal gas. The gas is now in a state fit for use, and is ready to be supplied to the numerous consumers. But it is necessary to have an accurate register of the quantity made, before it is transmitted to the great reservoirs or storehouses from whence it flows to the streets of cities. This is effected 158 LIGHT. by a beautiful apparatus called a Station Meter — an instrument resembling in its essential principles the ordinary gas-meters employed by the private consumer. The station meter accu- rately indicates the amount of gas made in any given time, and it is then sent on into the gas-holders. These enormous iron chambers, shown in the diagram, constitute the most sin- gular feature of our gas-works. They are simply air-tight iron vessels closed at the top, and open at the bottom, which dip into water, and thus prevent the gas from escaping at the sides. The gas enters by one tube which pierces the gas-holder from below, and takes its exit by another. The capacity of these chambers will appear surprising to the. reader unaccustomed to such calculations. At one of the large metropolitan gas-works are eighteen large gas-holders, the largest of which is ninety- five feet in diameter, forty feet in height, and is capable of containing a quarter of a million cubic feet of gas ! That such enormous vessels are necessary may be understood when it is stated that at these works a million and a half cubic feet of gas are produced every twenty-four hours during the winter months. The gas raises the chamber, which is counterpoised by weights, to its highest point, and is then cut off and directed into another gas-holder. On extraor- LIGHT. 159 dinary occasions — such as that of a general illumination, or the ascent of a balloon — it is necessary to use additional retorts, in order to keep the supply equal to the enormous demand at such times. The ordinary gas is often so diluted with hydrogen and carbonic oxide as to yield a very feeble light. It has therefore been sought to enrich it with some material which will com- municate to it a greater light-giving power. And this has been accomplished in some degree by Mr. Lowe’s invention, which causes the gas to pass through a sponge saturated with naphtha. The gas companies, however, have not adopted this plan, and there is, perhaps, some difficulty about the use of it. Gas is burned in an almost infinite variety of ways, and great ingenuity has been expended upon the various kinds of burners.. Those in most common use are the Bat’s- wing, (see figure,) which is used for the street-lamps; the Fish- tail, which is employed for shops ; and the Argand burner, which is used for churches and public buildings. Mr. Leslie has invented a very singular kind of burner, which is very economical and emits much light. It consists of a circular hollow ring, from which a number 160 LIGHT. of fine tubes spring, the united jets of which constitute the flame. The gas is thus exposed on all sides to the air, and undergoes very com- plete combustion. Ingenious persons have at different times pro- duced inventions for making gas at home, and many of the arrangements for this purpose are apparently practical in their nature. But it is an invariable rule that production on the small scale is never so economical as on the great, and such attempts at domestic gas manu- facture have generally ended in failure. In the metropolis, indeed, where, even to small con- sumers, gas is sold at the rate of four shillings and sixpence per thousand cubic feet, there is little inducement to set up such an apparatus. But in the country, enterprising persons have found it a profitable undertaking — when they have a large demand for light in their houses, and do not object to the annoyances inseparable from the manufacture. PART III. EFFECTS OF LIGHT. CHAPTER I. EFFECTS OF LIGHT ON MAN AND ANIMALS. Unlike those rays with which it is generally associated, and always when in the state of the sunbeam, light produces no sensible impression upon our bodies. It is caught by the eye and so renders itself perceptible, but it falls on any other part of our frame without apparent effect. Sound is received by the ear in a somewhat similar way ; but the air is a tangible substance, and has a direct physical impression upon the surface. When we cannot hear sound, we can feel the touch of a passing wind. But the rays of light, or, if we adopt the language of the un- dnlatory theory, the waves of this wonderful medium, have no sensible power over us if we close the only organ by which they enter to cause sensation. Heat-rays, on the contrary, are immediately perceived by any part of our M 162 LIGHT. bodies ; and, as is familiarly known, they pro- duce actual destruction of organized matter if concentrated upon any part to excess. It is probably to this cause that we are to attribute the general ignorance of the real effects of light upon the human and animal frame. As they are not sensible, they are commonly thought to be without result. As science advances, it will in all probability be discovered that no constituent of the solar beam is without its effect upon the organization of man and the animal world. It will be found that not only the heat-rays, but those of light, and of actinic force, exercise an important influence upon life and well-being; and that the one cannot be dissociated from the other without a serious and injurious result following. Yet, in the present state of knowledge, it is extremely difficult to define accurately what effect is produced upon animals by the pre- sence of solar light, and what results follow its absence. So strong is the vital force, if so it may be called, so vigorous the powers residing in the organization of animals, that often, under the most apparently unfavourable circumstances, life and health are maintained, and experiments fail to produce sensible results, even though pushed to a very extended limit, and practised LIGHT. 163 with every precaution. We have, however, certain results obtained by accurate observers, which exhibit, in an interesting and remarkable manner, the effect produced by the absence of light ; and thus, by a negative way of demon- stration, show us the power and influence of this medium on the animal economy. It is a subject of very familiar observation that the colour of the animal surface is in- fluenced by light. No one can doubt this who examines the colour of the bared neck and chest of a sailor, with that of his arms where they are constantly covered by clothing. And indeed, though in a less marked degree, the same contrast of colour is visible in the skin of every individual. The colour of the upper part of the neck is very many shades deeper than that of the chest; the hands are darker than the arms or feet. And in those persons whose avocations expose them for a large part of the day to the light of the sun, the surface becomes sometimes of a deep orange colour. The complexion of the residents in a large town, such as the metropolis, where but a feeble power of daylight generally exists as com- pared with the open country, is for the most part extremely pale and fair, while that of persons residing in the sun-lighted districts around is ruddy and brown. There can be no question M 2 164 LIGHT. that this colouring effect is due to light, as may he easily proved by placing a small piece of black plaster upon an exposed part of the skin, and at the end of a week comparing the colour of the surrounding parts with that of the skin covered by the membrane. It will then be found that the protected patch is two or three shades paler than the surrounding surface. The seat of colour thus produced on the animal surface by the agency of light, has been very well shown by the researches of physio- logists. Mingled with the minute cells which form the epidermis or scarf-skin of the body, are tiny cells called pigment cells, the office of which is to form the matter which gives colour to the surface. The production of these cells is very much due to the influence of exposure to light. And in the minute brown spots called freckles, which are so rapidly produced in the skin of persons of very fair complexion in exposure to sunlight, these minute cells are rendered visible to us by their aggregation in % those places. It is therefore to a certain stimulus communicated to these cells by the light of day, that the dark colour of the surface of which we have spoken is due. Those who have lived in tropical countries for any considerable period, have these pigment cells greatly developed under the influence of LIGHT. 165 the light and heat to which they are exposed. This hue is, however, temporary, or at least in a great degree ; and if the individual removes into colder countries, it will in great measure disappear. But if he remains a resident in the tropics, it is very likely that the next generation will not only retain this swarthy hue to a certain degree, but that the complexion of successive generations will ultimately become extremely dark, and resemble that of the native inhabit- ants of the region. It is a well-known fact, for instance, that a colony of Jews from Por- tugal, which settled at Tranquebar, about three centuries ago, and has since kept itself distinct from the surrounding tribes, cannot now be distinguished as to colour from the native Hindoos. Some physiologists have supposed that the deep blackness of the Negro's skin depends upon a similar cause operating through successive generations ; but others maintain that there is a specific difference in the skin, and that a thin filmy black membrane is there found which is wanting in other races than the negro. The new-born infants of the negro, and other dark races, do not, however, exhibit nearly the same depth of colour in their skins as that which they present after the lapse of a few days, when light has had time to exert its influence upon their surface. And in those individuals of the 166 LIGHT. coloured tribes who keep themselves most ex- cluded from the influence of light, it is proved that the colour of the skin is not nearly so dark as in others more exposed. Thus the chiefs of some tribes and their families, who are but little exposed to the sun as compared with the com- mon people, are known to have a complexion several shades lighter than the rest. A curious experiment has been performed with the cockroach, which lends additional sup- port to the facts here narrated. This insect, which in its ordinary state is intensely black, has been taken in its early stage of exist- ence, and carefully reared up in total darkness. The result has been, that instead of assuming an inky hue when at its full growth, it has been nearly white ; showing in the most marked manner, the necessity of a certain amount of light for the development of its characteristic colour. As we shall again have to remark, the vegetable world presents us with a precisely parallel example of the absence of light effecting a complete bleaching of a plant. €t The influence of light upon organized crea- tion,” observes Professor Hunt, “ is well shown in the sea. Near the shores are found sea- weeds of the most beautiful hues, particularly on the rocks which are left dry by the tides ; and the rich tints of the Actinice , which inhabit LIGHT. 167 shallow water, must have been often observed. The fishes which swim near the surface are also distinguished by the variety of their colours. Whereas, those which live at greater depths are grey, brown, or black. It has been found that after a certain depth, where the quantity of light is so reduced that a mere twilight pre- vails, the inhabitants of the ocean are nearly colourless.” The late Professor Forbes, whose dredging researches have made us acquainted with so much that is interesting, of the nature and habits of the subaqueous world of organized beings, has established the fact here alluded to on most certain ground. He found that animal life reached only to a depth of about 300 fathoms, and that the number of animals existing under the surface of the sea gradually diminishes with the depth of it, beyond five and thirty fathoms. That this is chiefly due to the absence of light, the rays of which are unable to penetrate beyond this depth, can scarcely be questioned. A still more interesting fact was developed by these researches, and has since been con- firmed by other naturalists in our own waters. It was observed that the vegetables and animals which near the surface, as has just been remarked, assume brilliant colours, gra- 168 LIGHT. dually lost their colour as a greater depth was reached, and in the lowest region were colourless. It is impossible therefore to resist the conclusion that light is really necessary to the production of colour in animals, and has also an intimate connexion with their vitality. “ There is,” writes the author just referred to, “ a remarkable correspondence between the geographical position of a region, and the colours of its plants and animals. Within the tropics, where * The sun shines for ever unchangeably bright/ the darkest green prevails over the leaves of the plants ; the flowers and fruits are tinctured with colours of the deepest dye, whilst the plumage of the birds is of the most variegated des- cription and of the richest hues. In the people also of these climes, there is manifested a desire for the most striking colours, and their dresses have all a distinguishing character, not of shape merely, but of chromatic arrangement. In the temperate climates, everything is of more subdued variety, the flowers are less bright of hue; the prevailing tint of the winged tribes is a russet brown, and the dresses of the inhabitants of these regions is of a sombre character. In the colder portions of the earth, there is but little colour; the flowers are LIGHT. 169 generally white or yellow, and the animals exhibit no other contrast than that which black and white afford. A chromatic scale might in fact be formed, its maximum point being at the equator, and its minimum at the poles.” Advancing from the consideration of the influence of light upon colour, we approach a still more interesting and perplexing subject, namely, — its effects on animal life. It is dif- ficult to reconcile the mind to the belief that such an agent as light, producing an impercep- tible effect on all our other senses except the sight, is really capable of producing a sensible effect upon the organization of the animal world. Yet it is very well ascertained that such is the case. It is easy to understand that if we deprive an animal of food or of air, its existence will very quickly terminate. But in what way can it be supposed that sunlight has a relation to animal life, or that the withdrawal of it is attended with the most serious results ? To this inquiry no satisfactory reply can yet be given. If an infusion of any green leaves, made with cold water, be carefully strained off, and equally divided into two white-glass bottles, of which the one is placed in the sun, and the other in a case in which it is totally excluded from light, a means will be afforded by which the in- fluence of the sunbeam upon animated beings will 170 LIGHT. be beautifully demonstrated. On examination of these bottles after the lapse of a week, or less if in summer, by placing a drop upon the field of a microscope, it will be found that the con- tents of the bottle kept in the dark have undergone little alteration ; while a drop from the other phial will be found teeming with myriads of animalcules in the highest state of activity. Here is a remarkable evidence of the intimate dependence of the lower form of animal life, upon the stimulus of the sunbeam for their starting into existence. This fact may also be illustrated by another experiment. If some of the minute creatures called Water-fleas — very abundantly to be met with, we regret to say, in much of the pipe- water supplied to houses in the metropolis — be placed in two separate vessels, one of which is placed on the window-sill, and the other in the dark, it will be found that those which have enjoyed the full daylight undergo their trans- formation much more rapidly than those which have been kept in the dark. These little creatures, when arrived at their full growth, throw off their horny casing every two or three days, and this change of apparel is really necessary to their welfare ; since if it does not take place, their bodies become covered with minute plants, which attach themselves to their LIGHT. 171 surface, impeding tlieir motions through the water and preventing their breathing with free- dom. If now they be secluded from the light, they are not able to renew their shells with the usual frequency, and cannot therefore rid them- selves of those troublesome appendages in the same way as they are able to do in the light. In another way we have a means of judg- ing of the connexion of light with animal life ; and the experiment to which we are about to advert has a very considerable economical im- portance in silk-growing countries. If equal numbers of silk-worms’ eggs be preserved in a dark room and in one exposed to common day- light, a much larger proportion of larvse are hatched from the latter than from the former — a result which clearly shows the important influ- ence of daylight on the animal vitality. This subject has received very remarkable illustrations from other sources, and particularly from the curious investigations of Dr. Edwards. He instituted some careful experiments on tad- poles, which, as is familiarly known, are frogs in an early stage of their development. He placed a number of these active little creatures in a large box perforated with holes, through which a free circulation of water was permitted to take place by plunging it to the bottom of the river Seine. The box was so constructed that 172 LIGHT. no light could penetrate it; and the tadpoles, nourished with proper food, increased in size, hut did not undergo their metamorphosis. After some time the box was taken up and its contents examined, when it was found that it was full of large tadpoles, not one of which had made the smallest approach to a change of form. These animals ordinarily remain only for a brief period in this rudimentary condition, and then are metamorphosed into frogs. Now, the arrest of this process, in the absence of light, indicates, in the most forcible manner, the necessity for the direct stimulus of the solar beam to the perfect development of this creature. A still more remarkable case, in which the absence of light appears to be connected with a partial arrest of development, is that of the Proteus Anguinus , a creature which has received a graphic notice at the hands of Sir II. Davy. In a conversation supposed to be taking place in the grotto of the Madalena, at Adelsberg, in Illyria, many hundred feet below the surface, one of the speakers says, “I see three or four creatures like slender fish, moving in the mud below the water.” The Unknown replies : “ I see them ; they are the Protei : now I have them in my fishing-net, and now they are safe in the pitcher of water. The animal is of a fleshy whiteness, and transparent in its natural state ; but when LIGHT. 173 exposed to light its skin gradually becomes darker, and at last gains an olive tint. It is abun- dantly furnished with teeth, from which it may be understood that it is an animal of prey ; yet in its confined state it has never been known to eat, and it has been kept alive for many years by occasionally changing the water in which it was placed. They have also been found at a place thirty miles distant from this cavern, thrown up by water from some subterraneous cavity. In dry seasons the Protei are very seldom found in the lake of the Madalena, but after great rains they are often abundant.” “The Unknown” says, “ I think it cannot be doubted that their natural residence is in an extensive subterranean lake, from which in great floods they are some- times forced through the crevices of the rocks into the place where they are found.” These singular creatures have no organs of vision, but in their place are two small dots which occupy the position of eyes, but have no power of perception residing in them. The entire absence of colour, and the imperfect development of its organs, in at least their intermediate condition, between those of a reptile and those of a fish, seem to be the result of the absence of light. Those who consider that light is indispensable to animal organization assume the very opposite of Sir Humphry Davy’s theory, and believe 174 LIGHT. that the Protei are found in crevices, near the surface to which the most faint rays of light forcibly penetrate, and are then washed down to the lake below by the rains. There can be little question that sunlight has its direct influence over the human body. Divine authority long ago pronounced — “ Truly the light is sweet and pleasant ; It is pleasant for the eye to behold the sun.” — E ccl. xi. 6, 7. and the influence of sunlight on the body fully illustrates the truth that it is not only pleasant to the sensations, but also good in its results. Some facts have been collected on this subject which will probably be received with surprise by those who have not given their attention to such investigations. “Numerous facts/'’ observes Dr. Carpenter, “ collected from different sources, lead to the belief that the healthy development of the human body, and the rapidity of its recovery from disease, are greatly influenced by the amount of light to which it is exposed.” Thus it was remarked by Dr. Edwards, that persons who live in caves or cellars, or in very dark and narrow streets, are apt to produce deformed children ; and that men who work in mines are liable to disease and deformity, be- yond what the simple closeness of the atmo- sphere would be likely to produce. Part of this difference is doubtless owing, however, to the LIGHT. 175 relative purity of the atmosphere in the former case, and the want of ventilation in the latter. But other instances might he quoted which exhibit a marked variation under circumstances otherwise the same. Thus it has been stated by Dr. Wylie (who was long at the head of the medical staff in the Russian army), that the cases of disease on the dark side of an extensive bar- rack at St. Petersburgh, have been uniformly, for many years, in the proportion of three to one to those on the side exposed to strong light. And in one of the London hospitals, with a long range of frontage looking nearly due north and south, it has been observed that residence in the north wards is much less conducive to the welfare of the patients than in those on the south side of the building. It has also been observed that the more the body is exposed to the influence of light, the more freedom is there, other things being the same, from malformation or irregular functional action. Thus Humboldt has remarked, that among several nations of South America who wear very little clothing, he never saw a single individual with a natural deformity ; and Linnasus, in his account of his tour through Lapland, enumerates constant exposure to solar light as one of the causes which render a sum- mer journey through high northern latitudes so peculiarly healthful and invigorating. 176 LIGHT. A very remarkable instance of recovery from disease lias been related by Baron Dupuytren, an eminent French surgeon. A lady, residing in Paris, had suffered for many years from an enormous complication of diseases, which had baffled the skill of all her medical advisers, and her state appeared almost hopeless. As a last resource, the opinion of Dupuytren was requested upon her case, and he, unable to offer any direct medical treatment essentially differing from all that had been previously tried in vain, suggested that she should be taken out of the dark room in which she lived, and away from the dismal street to a brighter part of the city, and that she should expose herself as much as possible to the daylight. The result was quickly manifest in her rapid improvement, and this continued until her recovery was complete. An equally singular instance has been related by Southey, in the case of his own parent. Enough has now been said to show that light really exercises a most important, salutary influence upon the animal world. And it is remarkable that even those humble members of the lower grades of organized beings, to whom a direct organ of sight has not been granted, still appear sensible of the influence of light. Some animalcules, foi instance, which are without eyes, court the sunshine, and remain within its LIGHT. 177 direct rays by choice, as though they really felt a pleasure in its effects ; others, again, shun the more powerful light, and hide themselves where only a slight ray can reach them. The eye is, however, the great source of our direct enjoyment of the influence of light, and its obscuration by disease or defective structure is naturally felt to be one of the most painful trials to which we are subject. For although blind persons have a dim perception of some undefinable influence upon them when placed in the sun, still the eye alone constitutes the medium by which the pleasantness of light is perceived by us. The manner in which the light is received by this organ, and communi- cated to the brain in the faculty of vision, is so full of interest, that we shall briefly notice it before putting an end to the present chapter. Our knowledge of the form, size, colour, and position of objects around us, is acquired through the medium of an optical instrument, which impresses upon the optic nerve an exact picture of surrounding objects, resembling that which is formed by the camera obscura. This optical instrument is the eye ; and without entering into the more difficult and less under- stood subject of the physiology of vision, we will describe the apparatus so wonderfully con- trived for its performance, and so surprisingly N 178 LIGHT. acute and accurate in the perceptions which it conveys. Externally the eye consists of a tough membrane, called tbe sclerotic coat, the visible part of which we call the white of the eye. This membrane forms the outer envelope of all the beautiful apparatus at the back of the eye ; it contains in front the clear and trans- parent part called the cornea . This cornea is an exceedingly tough membrane, though pellucid as glass, and forms the outer part of the arrange- ments through which the light must pass before it reaches the sensitive surface on which its impress is perceived. Lining the interior of the sclerotic coat, except where the cornea is fitted into it, is a layer of bloodvessels and nerves, called the choroid coat, the inner surface of which is covered with a black pigment. Within the choroid coat is the sensitive membrane, called the retina , and occupying the cavity thus formed is what is called the vitreous humour, a clear fluid substance, penetrated in every direction by delicate fibres of membrane. Imbedded in the front of this humour is the crystalline lens, by means of which the picture is formed on the retina. The front of this lens is covered to a greater or less degree by the beautiful structure called the pupil or iris, which has a contractile power, and serves to moderate the intensity of the light thrown into the eye. At the back of the LIGHT. 179 eye is the great nerve which carries the percep- tion to the brain. A very interesting experiment, and one that throws much light upon the nature of the appa- ratus forming the eye, is to take the eye of a bullock recently slaughtered, and very carefully to cut a hole in the upper part of the ball, and look in upon the retina through the hole, while the eye is directed so as to receive the rays of light from a landscape outside the window. It will then be seen that a picture of outer objects is formed on the retina just as it is in a camera obscura ; and also that it is inverted as in that instrument. Whatever, therefore, be the pro- cess by which we see things in their natural position, and not turned upside down, as in the camera, it is, nevertheless, very certain that an inverted picture is really formed by the optical apparatus of our eyes. It is generally thought that the correction of the image is a process performed in our minds, of which we are quite unconscious. It might be thought, as we are provided with two such instruments, that a double picture of objects around us would be received by the mind. This is not the case, as every one knows. And that we do not see two pictures with our two eyes is due to the intersection of the optic nerves, which, after leaving the right n 2 180 LIGHT. and left eye, meet and become intimately blended together before they enter the brain. The stereo- scope, which is now so fashionable and enter- taining an instrument, furnishes an illustration of this. By this apparatus, which will be noticed in the concluding part of this work, two separate pictures are seen as one, and that as if it were a solid instead of a mere flat representa- tion of objects. The human eye may be taken as a type of the most perfect optical instrument with which we are acquainted. When in a healthy state, how rapidly does it carry information to the brain, and how accurately are its adjustments adapted to our wants ! If the philosopher has to use a telescope or a camera to get the picture of distant objects or those of near ones, he can- not instantly do it. The instrument must be adjusted to the differing distances of these objects. But this process is instantly accom- plished by the eye ; we are unconscious of delay or difficulty in the act of glancing from a friend at our side to the very extreme verge of the horizon, and our sight is equally clear and well-defined in both cases. The most sensitive substances yet used in photography for receiving pictures by light fall far short of the velocity of vision. We may except, perhaps, the process of Mr. Talbot, by which a faint impression of LIGHT. 181 the path of a rifle-hall was obtained; but all the ordinary photographic substances are much slower in action. Another remarkable feature in this apparatus is, the instantaneous oblitera- tion of the image, leaving the eye as fresh and capable of taking new impressions as before. With this no known photographic process can rightly compare, although some have a faint resemblance to it. And, last of all, there is the wonderful faculty of receiving ideas of colour in all the countless variations of tint in which it presents itself to our notice. To deprive the eye of this faculty alone, and to leave it all the rest, would go far to render this world a dreary and uninteresting abode for man. The size, form, movement, and distance of objects would be still as perceptible as before, but a dismal monotony of tint would destroy his pleasure, would render his own works distasteful to him, and unfit him for the right appreciation of those of the Divine Hand. CHAPTER II. EFFECTS OF LIGHT ON THE VEGETABLE KINGDOM. Few facts can be affirmed with such certainty as that light is all-important to the vegetable world. Whatever doubts might be raised as to its direct influence on man and animals, there exists none as to its influence on plants in their higher states of organization. The Fungi ap- pear to be but little affected by light, and some indeed appear to shun its effects, luxuriating in dark recesses and even in deep mines where daylight never penetrates. But all the higher plants are absolutely dependent on the sun- beam for life and health. Repeated experiments have illustrated and confirmed this statement. Under the influence of light the first process is accomplished by which inorganic matter is transformed into an organic compound, adapted ultimately to form part of the organized being. LIGHT. 183 This is simply and beautifully illustrated in the subjoined remarks by Professor Draper. “ If we expose some spring water to the sunshine, though it may have been clear and transparent at first, it presently begins to assume a greenish tint, and after a while flocks of green matter collect on the sides of the vessel in which it is contained ; in these flocks, whenever the sun is shining, bubbles of gas may be seen, which if collected prove to be a mixture of oxygen and nitrogen, the proportion of the two being vari- able. Meanwhile the green matter rapidly grows ; its new parts as they are developed being all day long united with air-bells, which disappear as soon as the sun has set. If these observa- tions be made upon a stream of water, the cur- rent of which runs slowly, it will be discovered that the green matter serves as food for thousands of aquatic insects, which make their habitations in it. These insects are endowed with powers of rapid locomotion, and possess a highly organ- ized structure, and in their turn they fall a prey to the fish which frequent such streams.” Such is the beautiful arrangement of cir- cumstances in the organized world, that while one animal preys on another, which probably feeds on the vegetable world, all are ultimately, directly or indirectly, dependent on plants for nutrition. No animal possesses the power — so 184 LIGHT. extensively bestowed on vegetables — of causing the inorganic elements to unite into the simplest organic compound. And plants only have this faculty while under the influence of light. Thus, the sunbeam gives to plants energy to produce organic compounds fit for the nutrition of animals ; and animals feeding upon these plants, or upon others who feed on them, receive such com- pounds in a state lit for the service of their bodies. Since therefore without light no plant can carry on its peculiar functions, so it can be shown that light is really the grand agent upon which the phenomena of life more or less directly depend. It is easy to show that light is necessarily connected with the development of the little flocks of green matter just referred to, and in the following manner. If some of the same water which when exposed to the sunshine became in a few days filled with delicate green matter, spangled over with air-bubbles, be taken in a fresh state and put into a dark cupboard, none of this green matter will make its appearance. On examining the green matter produced in the light, by means of a good microscope, it will be found to consist of a number of tiny cells in dif- ferent stages of development, and these cells constitute the simplest germs of living vegetable matter. LIGHT. 185 It is not, however, to be for a moment sup- posed that light possesses in itself a creative power over matter, nor that it constitutes the principle which we call life. To believe either of these to be its action in the case in question, would be erroneous as well as in other cases, although at first it might almost seem that the sunshine called into being the little cells, or at least gave them life. These little cells have an independent existence, and have a life given them by God, and, insignificant as they appear, are not less of His creation than the most noble tree waving in the forest, nor than man himself. If we very carefully distil some water in absolutely clean vessels of glass, and only admit to the surface of it air which has been carefully purified from all foreign matters, then no amount of exposure to light and warmth will develop the flocculent green matter, or any other form of life in it. The water remains as clear and pellucid as at the first, and the microscope in vain searches for the faintest indication of organic existence in it. It is there- fore obvious, that though light is all-important to the development of vegetable life, it is neither capable of creating it, nor is it the principle of life itself. The term, germ , is appropriately applied to those minute particles of organized matter, which when developed under favour- 186 LIGHT. able influences produce these green flocks. Their origin is one of the many mysteries of the great realm of organized creation. Up to this point we have only considered the influence of light in giving activity to the tiniest forms of the world of plants. We shall now notice the effect of the three several principles associated in the sunbeam upon the vegetable world. These we have already noticed as Light, Heat, and Actinism. In estimating the influ- ence of light upon animal life, it has not been found possible to observe the separate effects of these three agencies. But with plants a series of very interesting researches has been made, and has brought out several very remarkable facts, as to the relative effect of these principles upon the growth and functions of the vegetable world. Mr. Hunt’s very careful experiments on the influence of light upon plants furnish us with some singular results, and appear to promise im- portant information to those who cultivate plants for commercial purposes, as well as to amateur florists. The earliest stage of vegetable growth is the germination of the seed, and it was found that light exercises a marked effect on this pro- cess. It is well known to gardeners that germi- nation takes place best in the dark, and they are in the habit of placing their pots of seed for LIGHT. 187 some days in the dark until germination is fully established. This fact was shown in an inter- esting way by Mr. Hunt. Some seeds of the common cress and some turnip seed were placed upon moist earth, and very slightly covered with fine sand. One half was screened from the light by a blackened board, and the other freely exposed. The result was, that under the shaded half of the board germination took place two or three days before that which was exposed to the light. A more striking result followed the next ex- periment. Part of the board was covered with a glass trough containing a chemical solution of a deep yellow colour, and therefore calculated to stop out most of the rays of actinism, while those of light and heat passed pretty freely through it. The other half was covered by the « blackened board. In four different experiments the seeds refused to germinate at all, and in the fifth, ten days after the seeds in the dark had germinated, half-a-dozen of the seeds showed feeble symptoms of the effort to germinate. From this and the preceding experiment it is therefore obvious, that light deprived of the power of chemical action, and excess of light under any circumstances, arrest the development of the plant, by preventing the vitality of the germ from manifesting itself. 188 LIGHT. It seems now an interesting object of inquiry as to whether the chemical agency or actinic rays of light would exert any effect on this important process in the plant ; and in order to ascertain the fact, a box was prepared in which was placed moist flannel, and this was kept wet by an under layer of water. One half could be completely screened from the light, and the other half exposed to any influence which it was thought desirable to try. Seeds of several plants were placed on the flannel, and the box was exposed to sunshine in a warm room, one half of it being covered by a glass trough filled with a deep blue solution, and the other half merely screened from light. The actinic rays could easily penetrate the blue liquid, but those of light were almost excluded. In every in- , stance the seeds placed under the blue liquid, and therefore freely enjoying the actinic rays, germinated in one half the time occupied by those placed in the dark. This most interesting discovery was not long announced before it was applied in practice on the large scale, by Messrs. Lawson of Edin- burgh, who are extensively engaged in business as Seedsmen and Florists. It is their practice to test the germinating powers of all seeds which come into their warehouses before sending them out for sale, and it is an object to discover with LIGHT. 189 as little delay as possible the extent to which the vital principle is active in the seed, since its commercial value is to a considerable extent influenced thereby. The plan formerly adopted was to sow the seeds in a hotbed and note the results. It was usually from eight to fourteen days before the commercial value of the seed could be ascertained by its germination. With a view to ascertain the effects, if any, of the blue rays in this process, a case was made the sides of which were formed of blue glass, and the case was connected with a stove by which its temperature was kept to the right degree. Shelves were fitted up in the case, on which were placed small pots wherein the seeds to be tested were sown. The result was very remarkable and gratifying. In from two to five days the seeds germinated, instead of from eight to fourteen days as before. In this way an extraordinary saving of time was gained, and a most important practical application of this scientific fact was thus accomplished. From these experiments two conclusions ap- pear to be obviously deducible. First, that light, properly so called, (or the luminous ray,) prevents germination — -the earliest stage in the life of a plant. And secondly, that actinism, (or the chemical ray,) quickens this process in a most manifest and remarkable manner. 190 LIGHT. So soon as the plumule of the seed hursts through the soil, all this action of the solar rays is reversed. Now the rays of actinism alone are most prejudicial to its progress, while those of light are essential to its vigorous growth. If the young plant be allowed to grow under the influ- ence of the blue rays, it will for some time grow with great rapidity, but it produces feeble and succulent stalks which soon perish. No leaves are formed, and the young stem remains flaccid and soft, but at the same time becomes enor- mously elongated. If, however, the plant be exposed to sunshine, after the first stage of its groAvth is completed in germination, it grows not so rapidly, but with great solidity and vigour. Leaves are formed, the stem elongates, and becomes rigid with the development of woody tissue ; in due time the flowers appear, to be succeeded by the production and maturation of the fruit. To all this, light and heat, and probably actinic influence, though in a less de- gree, are necessary. Advancing from the germinating state of the plant, we shall notice the influence of light upon it in its period of increase and development, and then during the completion of the functions of its existence. It is a most familiar fact that a plant growing in a dark cellar produces very flaccid stalks almost destitute of any leaves, LTGHT. 191 and of a pale yellow colour or white. By arti- ficially covering up the poisonous celery plant at an early period of its growth, and cutting off its supply of light, the plant produces pale suc- culent stalks, and becomes harmless and even salutary for economical purposes. The same plant growing in a hedgerow, under a bright sky, would be of a dark green colour, almost woody in the stalk, and highly poisonous and acrid if it were tasted. If a plant which has been bleached, or in technical language, etiolated, by being allowed to grow in the dark, be placed for a short time in the sunshine, a most remarkable change in its appearance would take place. Its pale yellow stalks, Tull of juices, but of soft and flabby structure, would soon become of a green colour, and assume a rigid character. Its leaves stunted, fleshy, and ill-grown, would also turn to a dark green, and all fresh leaves would as- sume their proper vigour and form. And it would be rapidly seen that the whole organi- zation of the plant was materially improved in vigour and in colour. This fact is occasionally illustrated on a grand scale in the American forests. Over the vast forests of that country clouds sometimes spread and continue for many days, so as almost to interrupt the rays of the sun. In one instance, 192 LIGHT. just about spring-time, the sun had not shone for twenty days, during which time the leaves of the trees had reached their full size, but growing in a comparative absence of light, they were of a pale whitish colour. One forenoon, however, the sun broke through in full bright- ness, and the colour of the leaves changed so fast, that by the middle of the afternoon the whole forest for many miles in extent exhibited its usual summer dress. The production of the green colouring matter of vegetables is therefore one of the results of their free enjoyment of the pleasant rays of sun-shine. Connected with this is a still more important and necessary process, which has been appropriately called the Respiration of Plants. With the performance of this function in the animal body, light has no direct con- nexion ; but it is indispensable to it in plants. Without light a plant cannot breathe. We can easily prove that a plant breathes when in the light, and that in so doing it de- composes carbonic acid and gives out oxygen, by taking a small tumbler, and collecting the gas found on the leaves of aquatic plants growing under water in our wayside ponds. If we examine this gas we shall find that it consists chiefly of oxygen, and this oxygen was derived from the decomposition of car- LIGHT. 193 bonic acid gas, which is very soluble in water, the plant taking the carbon to form its woody tissues, &c., and discharging the oxygen which was united to it in the carbonic acid gas. This may be also seen by placing a living aquatic plant in a vessel of water and covering it with a tumbler containing a quantity of carbonic acid gas mixed with air. In a few days, if the plant be placed in the sun, it will be found that the carbonic acid has all disappeared, that the plant is heavier by reason of the carbon it has as- sumed, and that the air in the tumbler contains a sensible amount of oxygen in excess of that which it held at first. The same process takes place over the vast green surfaces of our fields and meadows, and not less in the humblest blade of grass than in the broad leaves of the largest trees ; and this process constitutes the all- important function known as the Respiration of Plants. As the sun declines, the plant takes up less and less of the carbonic acid of the air, and of that brought up in solution by its roots from the dewy ground, until at nightfall this process almost ceases, and remains all night in a nearly quiescent state. But at sunrise it again wakes into activity, minute streams of carbonic acid pour upwards to the leaves, and these are de- composed by a force which no chemist can 0 194 LIGHT. either imitate or comprehend, pure oxygen flies from the leaves into the air, and the carbon is despatched throughout the whole economy of the plant, to form its wood, and to fulfil its part in the other functions of growth. To man and the animal world the results of this function in plants are most salutary. Car- bonic acid gas is not only the refuse gas of the animal economy, but is actually noxious to our existence. We could not long continue to breathe it even in a diluted state without dangerous or even fatal consequences. An admirably arranged series of operations thus displays itself, and has often attracted the won- der and admiration of philosophers : vegetables live in that which is poisonous to us, and the respiration of plants becomes the antidote to that of man and animals. We know that all the carbon which forms the masses of the magnificent trees of the forests, and of the herbs of the fields, has been supplied from the atmosphere, to which it has been given by the functions of animal life, and the necessities of animal existence. Man and the whole of the animal kingdom require and take from the atmosphere its oxygen for their support. It is this which maintains life, and the product of this combination is car- bonic acid, which is thrown off as the waste LIGHT. 195 material, and which deteriorates the air. The vegetable kingdom, however, drinks this noxious air ; it appropriates one of the elements of this gas (carbon), and the other (oxygen) is liberated again to perform its services to the animal world. It is not possible to conceive a more perfect or more beautiful system of harmonious arrange- ment than this, making the animal and vege- table kingdoms mutually dependent — the exist- ence of the one ceases when the other is destroyed. If the vegetable world was swept away, animal life would soon become extinct ; and if all ani- mal existence was brought to a close, the forest would fall, and the flowers of the field, which now clothe the earth with gladness, would perish and decay. The animal kingdom is constantly producing carbonic acid, water in the state of vapour, nitrogen, and, in combination with hydrogen, ammonia. The vegetable kingdom continually consumes ammonia, nitrogen, water, and car- bonic acid. The one is constantly pouring into the air what the other is as constantly drawing from it, and thus the equilibrium of the elements is maintained. Plants may in fact be regarded as compounds of carbon-vapour, oxygen, hydrogen, and nitrogen gases, consoli- dated by the powerful influences of the solar ray; and all these elements are the produce of 0 2 196 LIGHT. the living animal, the conditions of whose exist- ence are also greatly under the influence of those beams, which are poured in unceasing flow from the centre of our system. Can we have more complete proof of the loftiest design and most perfect order than these phenomena afford us ? Plants, in some instances, supply us with a sensible evidence of the influence of light upon them. The well-known phenomenon of what is called the sleep of plants, furnishes us with a familiar indication of the powers of sunshine and daylight over their organization. In tro- pical climates this may be observed in the most remarkable degree. Humboldt observes, that in South America the mimosa and tama- rind-trees close their leaves, in a clear and serene sky, from twenty* five to thirty-five minutes before the setting of the sun. It seems that, accustomed during the day to an extreme brilliance of light, the sensitive and other legu- minous plants, w T ith thin and delicate leaves, are affected in the evening by the smallest decline in the intensity of the sun’s rays ; so that for vegetation, night begins there before the total disappearance of the solar disk. Again, it is a well-known fact, that plants bend toward the light ; so that if a plant is kept constantly with one side exposed to the light, as in the window of a sitting-room, it will direct all LIGHT. 197 its branches and leaves toward the window. But it is also now known that there are particular rays of light which have precisely the opposite effect on plants. Thus it has been found that they will actually bend away from the red rays. It is extremely difficult to account for this fact, but it appears to rest upon good evidence. Let us now notice the influence of light upon plants in, what we may be allowed to term, the last stage of their existence — namely, the deve- lopment of the flower, and the production of the fruit. In the ingenious experiments of Mr. Hunt, on the influence of the three prin- ciples associated in the sunbeam upon plants, he states that he could rarely succeed in getting plants to flower under the influence of any of those coloured substances which cut off the rays usually called the calorific rays. For instance, under intense yellow, deep blue, or very dark green glasses, however carefully the plants were attended to, there was seldom any attempt to produce flowers. By removing the glasses of these kinds of colour, and substituting red glass, the plants flowered and produced seed without difficulty. It has, therefore, been thought that the red rays exercise some very peculiar influence upon plants at their flowering and fruiting season, and that, in fact, they are 198 LIGHT. essential to the completion of those processes in the system of the vegetable. In this we have evidence of the fact that there is no principle of the sunbeam which has not some part to fulfil in connexion with the functions of the vegetable kingdom. At the earliest epoch of plant life the rays of actinic influence are of the most importance, since they quicken, in a marked degree, the process of ger- mination in the seed. Then in the next stage, when the solid structures of the plant are to be formed — when it is to live and breathe for the benefit of the world of animals, and in so doing to derive bulk and vigour from the act, the pure rays of light, properly so called, stimulate it, and keep the process in full performance. And, finally, when the flower, the crown of the plant, is to be formed, and to ripen into the fruit which is to provide for the perpetual reproduction of the plant, then the red rays which are associated with the heat of the sunshine have their effect, and, this accomplished, the act of vegetable growth is complete, the plant, if an annual, withers away, and its seed, falling on the earth, awaits the germinating influence of a future spring to recommence the recurring series of changes In harmonious accordance with this wonder- ful and interesting chain of mutual dependences, LIGHT. 199 it has been found that the three component principles of the sunbeam differ in their activity at different periods of the year. Thus in the spring, the chemical principle is the most active ; as the summer advances light and heat rays are in excess ; and as the autumn comes on, light and chemical rays both diminish, and the red rays are in excess. In the spring, observes Professor Hunt, when seeds germinate, and young vegetation awakes from the repose of winter, we find an excess of that principle which imparts the required sti- mulus. In the summer this exciting agent is counterbalanced by another possessing different powers, upon the exercise of which the structural formation of the plant depends; and in the autumnal season these are checked by a myste- rious agency, which we can scarcely recognise as heat, although connected with thermic mani- festations, upon which appears to depend the development of the flower and the perfection of the seed. The general influence of light upon plants is also necessary in order to perfect the production of the various vegetable secretions. The more plants are exposed to the light the more perfect is the performance of their functions, and the more strongly developed are their peculiar odours and resins. Hence the most odoriferous 200 LIGHT. herbs are found where the light is strongest — as sweet herbs in Barbary and Palestine. The peach, the vine, and the melon ripen best, and are of the most delicious flavour, under the brilliant sunshine of Cashmere, Persia, Italy, and Spain: and all our spices are the pro- duction of countries where a blaze of sunshine is the rule, and not the exception as in our own climate. PART IV. APPLICATIONS OF LIGHT. CHAPTER I. HISTORY OF THE ART OF SUN-PAINTING. Men of science had so long been accustomed to consider light as one of those agencies the effects of which were not to be estimated by the ordinary means within their control, that the discovery of its singular power of leaving per- manent impressions after it had itself vanished was deferred almost to our own era. This seems the more remarkable when it is remem- bered that philosophers were well acquainted with the power of light over vegetation, and it seems strange that those who saw a blanched leaf grown in the dark turn to a healthy green when placed in the sunshine should not have been led to a discovery apparently so obvious, as that the rays of the sun have a powerful chemical influence upon matter. A few isolated facts were known to chemists in connexion with the influence of light upon chemical 202 LIGHT. substances, but they were not arranged in any definite system, and attracted only a pass- ing notice. It has been stated, but the statement requires confirmation, that the Indian jugglers, many ages ago, were acquainted with this subject. It is said that they possessed a secret which resembled our present photographic processes, and that they were able to copy the profile of an individual, in a very brief space, by the action of light. If this was ever true, it no longer remains so, for no such art is now practised. The alchemists, in the midst of all their false reasonings and fanciful experiments, seized upon an important fact connected with this subject, and in the early age at which they lived, found that one of the preparations of silver, — the chloride, or horn silver, — on being exposed to the light, turned quite black. They saw at once that this change of colour was due to the solar beam, and they made it, as in so many other cases where a fact was discovered by them, the basis of endless speculations and profitless investigations into the constitution of the metals, in the vain hope of transmuting the silver into gold. The crystallization of certain chemical sub- stances is, in a remarkable way, influenced by light. So far back as 1722, Petit showed that LIGHT. 203 solutions of saltpetre and of sal ammoniac crys- tallized more readily in the light than they did in darkness, and subsequent observers confirmed this by a series of ingenious experiments. Thus M. Chaptal found that when a number of deli- cate crystals were shooting up the sides of a vessel containing a saline solution, the effect took place only on that side of the vessel which was exposed to the light. He was able to cause the crystals to form on any side he pleased, by exposing it to the light, and of arresting it by placing a dark screen over other parts. This can also be observed by any one who will plstce a lump of camphor in a large, dry glass bottle, and after a few weeks it will be found that the side of the bottle directed towards the light will be covered with crystals, while that side turned to the wall will remain quite free from them. From this experiment it is obvious that light exercises a very sensible influence on the forma- tion of crystals, though it is one which as yet no philosopher has succeeded in satisfactorily explaining. It would appear that the eminent Swedish chemist, Scheele, was the first who gave the attention to this subject which its interest and importance deserved. In 1777, Scheele published some discoveries which he had made in the chemical agency of light, and these 204 LIGHT. may be considered as the earliest systematic investigations into this subject. He states that he found a solution of silver in nitric acid, which is a nitrate of silver, become quite black on being poured on a white surface, and then exposed to the light. He found also that this solution remained unaltered in the dark, and even when heated, and thence he drew the correct inference that it was the chemical agency of the solar beam which brought about this remarkable change in the substance. He also very accurately stated the important fact, that this change in colour was really a chemical decomposition, and that the silver was reduced to a metallic state by the agency of sunlight. But a still more interesting and valuable experiment was made by this great chemist, which may be given in his own words : “ Fix a glass prism at the window, and let the refracted sunbeams fall on the floor. In this streak of coloured light put a paper, strewed on its sur- face with chloride of silver, and you will observe that this substance grows sooner black in the violet ray than in any other of the other rays of the spectrum.” This result was very far in advance of Scheele’s age, and but for the pre- occupation of his powerful mind with his favourite theory of phlogiston as a universally pervading substance, he might have been led to other and LIGHT. 205 still more brilliant discoveries as to the nature of the sunbeam. It need scarcely be here repeated, that it is in the violet rays of the spectrum that the actinic rays of light are chiefly found, and in making this discovery Scheele had little conception how closely he was on the verge of one still more important. Count Rumford performed an elaborate series of experiments with a view to determine the chemical influence of light, but all that can be said of them is, that they only gave a negative result in his hands; and other observers have shown the existence of important errors in his conclusions. It was not until 1802 that Ritter of Jena once more revived the question of the chemical agency of light, and carried it to demonstration by some interesting experiments with the solar spectrum. Ritter found that the chloride of silver darkened rapidly even beyond the violet ray of the solar spectrum, where no rays of light were visible to the eye; and he thence drew the conclusion, which subsequent philosophers have confirmed, that there exist in the rays of light rays of chemical power, which produce no impression on the organs of vision. Subsequently, Dr. Wollaston announced the same fact, and distinctly stated, that the chemical effects attributed to light are not owing to any of the rays usually perceived. 206 LIGHT. In 1804, Dr. Young produced what may appropriately be considered a photographic picture, and conducted his experiment in the following manner. He formed an image of the rings of colours produced by the pressure of plates of glass together, by means of the solar microscope, and threw this image upon paper dipped in a solution of nitrate of silver, placed at the distance of about nine inches from the microscope. In the course of an hour, portions of three dark rings were visible on the paper. From this period the progress of photographic discovery was more rapid, but it took what may be called a popular, rather than a scientific direction. The application of light for the pro- duction of pictures, was one of the earliest ideas developed by the discovery of the chemical rays of the sunbeam. The possibility of commanding the pencil of nature, in order to delineate her own forms and beauties, seems very soon to have fascinated observers, and much attention was given to this subject, although at first with but very indifferent success. The earliest published account of any attempt to produce images by the decomposing powers of light, was that of Mr. Thomas Wedgwood and Sir Humphrey Davy. In June, 1802, a paper by these authors appeared in the Journal of the Royal Institution, entitled, “ An Account LIGHT. 207 of a Method of copying Painting upon Glass, and of making profiles, by the Agency of Light, upon Nitrate of Silver.” Mr. Wedgwood made use of white paper or white leather for the purpose of preparing his sensitive surface, and describes his process in the following manner: “ White paper, or white leather, moistened with a solution of nitrate of silver, undergoes no change when kept in a dark place, but on being exposed to the daylight it speedily changes colour, and, after passing through different shades of grey and brown, becomes at length nearly black. The alterations of colour take place more speedily in proportion as the light is more intense. In the direct beams of the sun, two or three minutes are sufficient to produce the full effect ; in the shade, several hours are required ; and light transmitted through different-coloured glasses, acts upon it with different degrees of intensity. Thus it is found that the red rays, or the common sunbeams, passed through red glass, have very little action upon it; yellow and green are more efficacious, but blue and violet light produce the most decided and power- ful effects.” The methods of applying these phenomena to practical uses are thus adverted to : “ When the shadow of any figure is thrown upon the prepared surface, the part concealed by it re- 208 LIGHT. mains white and the other parts become speedily dark. For copying paintings on glass, the solu- tion should be applied on leather, and in this case it is more readily acted upon than when paper is used. After the colour has been once fixed on the leather or paper, it cannot be re- moved by the application of water, or water and soap, and it is in a high degree permanent. The copy of a painting in the profile, immediately after being taken must be kept in an obscure place. It may indeed be examined in tlie shade, but in this case the exposure should only be for a few minutes ; by the light of candles or lamps as commonly employed it is not sensibly affected. No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by light have as yet been suc- cessful. They have been covered with a thin coating of fine varnish, but this has not de- stroyed their susceptibility of becoming coloured, and even after repeated washings, sufficient of the active part of the saline matter will still adhere to the white parts of the leather or paper as to cause them to become dark when exposed to the rays of the sun.” Thus far it is evident that Wedgwood and Davy had attained considerable progress in the practical application of the chemical rays of light to ordinary uses ; and as we look back LIGHT. 209 from the present perfection which photogra- phic art has reached, it is impossible not to perceive with a feeling of surprise, the large amount of knowledge of photography possessed by the authors of that process. They had seized the important fact that red rays protect the sen- sitive surface, while blue rays rapidly affect it. And they had noticed, without being able to explain it, the superior sensitiveness of white leather to white paper,— a fact due to the pre- sence of a small portion of gallic acid in the leather, which substance at a later period, in the hands of Mr. Fox Talbot and Mr. Read e, has contributed more to the progress of photo- graphic art than almost any other that could be named. They failed in only one respect, and that was in the art of securing the permanency of their pictures, and it is interesting to notice the candour which led these early photographers to confess their inability to retain that which they had succeeded in producing. It seems remarkable that so eminent a chemist as Davy should have failed to have found some chemical solvent for the undecomposed silver salt in the leather or paper. Had this been accomplished, and the papers washed, for example, with a solu- tion of hyposulphite of soda, the whole process of photographic printing would have been rightly claimed by them as a completed art. P 210 LIGHT. Attempts were made by Wedgwood and Davy to obtain an impression of objects by means of the camera obscura ; but the images formed by that instrument were found to be too faint to produce in any moderate time an effect upon the nitrate of silver. Sir H. Davy, however, suc- ceeded in impressing the images of objects upon prepared paper by means of the solar micro- scope. It was also found at that time that paper prepared with the moistened chloride of silver was much more sensitive than that made with the nitrate. From this period to 1814, the progress of photography seems to have been arrested ; and probably the failure of Wedgwood and Davy to render their pictures permanent contributed very much to the neglect of the whole sub- ject. These pictures were regarded merely as ephemeral scientific curiosities, and there was little idea of the fact that their production in- volved the principles of an art destined to assume considerable proportions thereafter. In 1814, M. Niepce, of Chalons on the Saone, turned his attention to the chemical agency of light, and discovered the existence of a very remark- able property possessed by light in altering the solubility of many resinous substances. At a later period M. Daguerre, whose name has since become celebrated in the daguerreo- LIGHT. 211 type, directed his inquiries in the same direc- tion, and in 1827, Niepce and Daguerre became fellow-labourers in the same field of investi- gation. In 1827, M. Niepce communicated an account of his discoveries to the Royal Society, and accompanied it with specimens of pictures drawn by light upon metal plates. Some of these plates were in the state of advanced etchings. Some of them — the earliest specimens in exist- ence of light-drawn pictures upon metal tablets — are now in this country. They are upon pewter plates. One picture represents the ruins of an abbey, and remains still very distinct. Another is Niepce’s first successful attempt in obtaining an image from nature : it is a court- yard seen from an upper window. Several copies of prints also exist. Some of these were in possession of Dr. Robert Brown, of the British Museum; and we have been informed that one is in the hands of a publican at Rich- mond, who has repeatedly received offers of large sums of money for it. In 1829, Messrs. Niepce and Daguerre entered into an agreement to pursue, for their mutual benefit, the researches which they had respec- tively begun. “ The discovery which I have made,” writes M. Niepce, “ and to which I give the name of Heliography, consists in producing p 2 212 LIGHT. spontaneously, by the action of light, with gra- dations of tints from black to white, the images received by the camera obscura.” His plates were prepared in the following manner. Some powdered bitumen of Judea, or asphaltum, was put into a wine-glass, and the essential oil of lavender was dropped upon it, drop by drop, until it could absorb no more, and a solution of the asphaltum thus obtained was then applied to the highly polished surface of a silvered plate, which was afterwards dried and gently heated. The plate thus prepared was then fit to place in the camera, and after being exposed for a sufficient length of time it was removed: but no trace of a picture was visible on its surface. The next operation was the disengage- ment of “ the shrouded imagery, and this was accomplished by a solvent.” The solvent con- sisted of a mixture of oil of lavender and oil of white petroleum. “ Into this liquid,” writes M. Niepce, “ the tablet is plunged, and the operator, observing it by reflected light, begins to perceive the objects to which it had been exposed, gradually un- folding their forms, though still veiled by the supernatant fluid. The plate is then lifted out and held in a vertical position, till as much as pos- sible of the solvent has been allowed to drop away, and it is afterwards gently washed by a LIGHT. 213 stream of distilled water.” This process, though very wonderful and interesting as the first step in photographic art, was extremely tedious and uncertain. An exposure of two or three hours was necessary to produce an impression from an engraving, even under the influence of a bright sun, and in the camera an exposure of six or eight hours was not too long. This process was subsequently improved and shortened by M. Daguerre; and he made use of the vapour of petroleum instead of the oil, for the development of the picture — an application which had its influence probably in directing that discoverer to the curious process known by his name. Iodine was first employed upon silver tablets by M. Niepce, but without the smallest perception of its photographic proper- ties, being merely used by him to darken his pictures w T ith. The application of this sub- stance to the silver plate, with a view to exalt its sensitiveness, is therefore wholly due to Daguerre, and in fact it constitutes the ground- work upon which the whole process of the daguerreotype, in its most refined form, as now practised, substantially depends. It was not until after the death of M. Niepce, that M. Daguerre completed the elegant art of taking pictures on the silver plate. He seems to have been dissatisfied with the slow- 214 LIGHT. ness of the action of bituminous substances under the influence of light, and at a period anterior to Niepce’s death, to have given much attention to the photographic properties of the compounds of silver. In June, 1839, the dis- covery of M. Daguerre was reported by M. Arago to the French Academy, and a few months afterwards a bill was passed, securing to the inventor a pension for life of 6,000 francs. In granting this pension it was very distinctly stated, and with characteristic expres- sion, that France presented the art to the world, herself rewarding its inventor. The inventor, however, does not seem to have acquiesced in this liberal gift, and steps were not only im- mediately taken to secure the patent-right in England and elsewhere, but vigorous measures were adopted against all persons disposed to avail themselves of the generous offers of France. The early daguerreotypes, though very infe- rior to those now taken, were still remarkably beautiful, and incomparably superior to any other photographic pictures then known. The method adopted by M. Daguerre was the fol- lowing : — a silvered plate was first highly cleansed, and brought to the finest possible surface, having a mirror-like brilliancy ; it was then exposed to the vapour of iodine, placed in LIGHT. 215 a pan beneath it, and in a few moments its surface, combining with the vapour thrown up from the iodine, became beautifully coloured. This colour represented the compound formed, which was an iodide of silver, and the plate was now ready for exposure in the camera. The exposure being completed, the plate was then removed from the holder, and though still unaltered in appearance, yet it was in reality impressed with a picture, the outlines of which could be ren- dered visible by an after process. This process, called the “ development,” was probably the result of an accident, for it is very improbable that Daguerre could have discovered it by a series of direct* experiments. He had already found that a picture could be developed on bitumen by vapours, but the application of the vapour of mercury to the silver plate did not then occur to him ; yet this was really all that was necessary to bring out the impression on the silver plate, and eventually he discovered its power. The development of Daguerre’s pictures, by holding the silver plate over the vapour of mercury in a box, was one of the most attrac- tive parts of his process, and excited, when it was first seen, almost enthusiastic admi- ration. To this hour it is scarcely possible to witness an experiment more curious than the gradual disclosure of the hidden picture 216 LIGHT. on the silver plate, when placed in the mer- curial vapour. To a person who has never witnessed it, it is a wondrous and enchanting spectacle, and its discovery in the ignorant ages of the world might have exposed its in- ventor to the danger of martyrdom in the cause of science. This process was much quicker than that of M. Niepce, but it was still too slow for taking portraits. Few persons could sufficiently settle their features to endure the glare of sunshine for half-an-hour, or even longer; had it not therefore been possible to shorten this amount of exposure, by increasing the sensitiveness of the plate, the daguerreotype would Very pro- bably have soon been discarded for all practical purposes. The attention of philosophers was, however, now fairly aroused as to the importance of the new-born art of photography, and many attempts were made to communicate a superior degree of sensibility to the silver tablet. This was ultimately accomplished by Mr. John God- dard, who applied the vapour of the curious element called bromine, in addition to that of iodine, to the silver surface. The result was most remarkable. Instead of an hour being required in dull weather for an impression, the time was actually reduced to a few seconds ; and as thus improved, the art of daguerreotyping LIGHT. 217 lias now attained such rapidity and perfection, that pictures are often taken in one, two, or three seconds, and landscapes almost instan- taneously. Enthusiastic as Daguerre was in the capabilities of his art, this was a point of perfection he had scarcely dreamed of. He lived, however, to see it accomplished, and many likenesses of the inventor himself were taken by this wonderfully rapid improved me- thod. The process, however, was still a patent, and its validity was never doubted until of late years. While M. Daguerre was busy with silver plates, our countryman, Mr. Fox Talbot, had long been occupying himself with the applica- tion of paper to photography, and he had already attained some interesting results. In 1839, Mr. Talbot described a process to the Royal Society, for the preparation of a sensitive paper, able to take impressions from objects applied to its surface. In this respect it does not appear that any great advance was made upon what had already been long since accomplished by Wedg- wood and Davy. Soon afterwards, however, it was announced that Mr. Talbot had discovered a new and highly remarkable art, which has since been called by his name — Talbotype. By this art, paper was so prepared that pictures could be taken upon it in a few minutes in the 218 LIGHT. camera. The surface of the paper was covered with a layer of iodide of silver, and the picture was brought out by the application of a solution of gallic acid. But the pictures thus produced were of this remarkable nature ; that instead of being, like those obtained by the daguerreotype, exact representations of the lights and shadows of objects, these were reversed. That is to say, the white objects of nature were shown in the picture as black, and the black as white. Thus the dark earth of a landscape would be white or whitish, while the bright sky would be quite black. This sort of picture was called a “ negative,” and very probably its inventor beheld it with no little dismay. In reality, however, it is this property of talbotype pictures which really constitutes all their value ; for this negative picture becomes the matrix of others, which have the lights and shadows as in nature. To this point, however, we shall return, further on. By means of ordinary sensitive paper placed under the negative, a true copy was obtained with lights and shadows correct, and thus the chief value of the talbotype was developed, for it became possible to multiply copies of the original picture indefinitely. The same could not of course be accomplished by the daguerreotype in consequence of the opacity LIGHT. 219 of the material upon which the pictures are received It is to he regretted that Mr. Talbot, like M. Daguerre, secured his process by a patent, and thus probably did more to interfere with the improvement and development of the art he had discovered, than could have been at first anticipated. With the completion of Mr. Talbot’s inven- tion, the progress of photographic art for a short time seemed to cease. Both kinds of photo- graphs, those on silver plates and those on paper, seemed now as perfect as their nature would permit of, and since the patent-rights of both M. Daguerre and Mr. Talbot were vigorously maintained, and amateur photographers were threatened with legal proceedings, the art was for a time completely at a stand-still. After the lapse of some years, however, certain serious doubts began to be entertained, whether either Daguerre or Talbot had any valid legal claim to the exclusive pursuit of this art ; and as ex- perimenters grew more bold, the art once more revived, and, in spite of the restrictions under which it laboured, soon attained a most impor- tant position in this country and in France. A valuable method of covering daguerreotypes with a delicate film of gold, and thus protecting them from the injurious influence of the atmo- 220 LIGHT. sphere, was discovered by M. Fizeau, and several persons found also a means of colouring the surface, chiefly by application of colours in the form of a fine dry powder to the plate, the sur- face being rendered very slightly adhesive by a wash of water containing a little isinglass. And this art is still extensively practised. A variety of different effects were produced upon the pic- tures, to which we shall again have occasion to refer. But essentially tlie art of the daguerreo- type remains at this moment in the same state as when it received the important addition referred to in a previous page. In fact, it is apparently incapable of improvement, unless a method shall be discovered of impressing the plate with the natural colours of objects. To a certain extent this has been accomplished by M. Niepce de Saint Victor, nephew of M. Niepce, the first photographer, but only by a process essentially different to that of the daguerreotype. It is very remarkable that the French, to whom we are indebted for the beautiful art of daguerreotype, have for many years almost wholly thrown it aside, and have devoted extra- ordinary care and skill in attempting to perfect the art of the talbotype, for which they are indebted to us, and which we have apparently disregarded. The severity of the patent restric- tions over this art in England, more than any LIGHT. 221 other cause, operated to keep it from making any progress amongst us, whereas in France the rights were either not respected or did not legally exist. There were certain features in the talbotype process which rendered its perfection as an art very desirable and indeed promising. The inexpensiveness of the material in which the pictures are taken; the capability of multiplying copies indefinitely, as with an ordinary wood engraving, though by a scientific process ; and the artistic and pleasing character of the pictures produced, were essential recommendations to the vigorous prosecution of the art. The principal defects in the talbotype process arose from the uneven and coarse structure of the paper, even when manufactured with extraordinary care and expressly for photographic uses. In conse- quence of this, the same exquisite definition and sharpness of outline as that resulting from the use of metal plates as in the daguerreotype, could not be obtained. And though by extreme care some of the English and French paper- makers have now succeeded in producing paper of wonderful beauty and evenness of texture, still there is a degree of spottiness about talbo- types from paper negatives, which is immedi- ately perceptible to the eye of an experienced photographer. 222 LIGHT. Many attempts to remedy this defect have been made; of these the most successful has been the application of melted wax to the paper, which so fills up the interstices, as to give it a more uniform and homogeneous aspect. M. Le Gray of Paris, a very distinguished photographic artist, introduced with great success a modifica- tion of the talbotype, which has since been called the Waxed Paper Process. The paper in this process is saturated with wax before being mixed with chemical solutions. The first grand improvement, however, in the talbotype process consisted in the substitution of glass plates for paper. This application is due to Sir J. Herschel, who at a very early period in the history of this art devoted his powerful intellect to a careful investigation of the whole subject. M. Niepce de Saint Victor shortly afterwards proposed to cover glass plates with a thin layer of albumen, or white of egg, in which the sensitive iodide of silver might be retained; and a most valuable and charming addition to photographic art was thus made, for albumen pictures on glass plates have all the exquisite delicacy of the daguerreotype, and the combined advantages of the paper negative, admitting of numberless copies being taken from one picture : the albumen process was, however, very slow in operation. LIGHT. 223 The last great improvement was that for which credit is due to our countryman, Mr. Frederick Scott Archer, — the collodion process. The peculiarity of this process, which, together with other methods, will be fully detailed further on, chiefly consists of the application of a thin layer of a solution of gun-cotton in ether to the surface of a glass plate. This layer is then made sensitive, and to such a remarkable degree, that it far surpasses the daguerreotype, and possesses, besides the advantages of the albumen process, delicacy and sharpness of definition, with the capability of infinite multiplication of copies. The collodion process and the daguerreotype are now in active competition for taking portraits, and each will probably long remain fashionable, since there are peculiar beauties possessed by each, which are not shared by the other. For landscape collodion, albumen and paper seem to be chiefly preferred. In this brief sketch of the history of photographic art, it is hoped that nothing of material consequence has been omit- ted, and as little irrelevant matter as possibl introduced. CHAPTER IT. THE DAGUERREOTYPE. The earliest photographic pictures were taken upon a pewter plate, by M. Niepce; and for many years the whole energies and attention of both Niepce and Daguerre were directed to the production of pictures upon metallic surfaces. The result, as we have already seen, was more or less successful; and anterior to the dis- covery of the daguerreotype, metal tablets, either of copper silvered on the surface, or of other metals, were alone employed for this purpose, but merely as a support to the sensitive substance, which in Niepce's case was a resinous coat- ing. For all practical uses, a plate of glass would probably have answered as well, and it does not appear that M. Niepce attached im- portance to the metallic tablet as a photo- graphic agent. LIGHT. 225 M. Daguerre, however, seems soon to have perceived that the metal might itself be made sensitive to light, under peculiar conditions of its surface ; and by a series of long and earnest researches, at length caught the happy idea which led to the discovery of his beautiful art. His process is one extremely simple in its details ; and when once mastered, it is a very certain and successful method of obtaining pic- tures by the agency of light. We shall first describe what may be termed the chemistry of the process, and then the way in which it is practised by the artist. The sensitive film in the daguerreotype is an iodide and bromide of silver, formed on the surface of the plate, and after receiving the impression of light, the image is made manifest by the application of the vapour of mercury. The plate is of copper, and is silvered over either by the electrotype process, or by the ordinary method of plating. The silver sur- face is brought to the very highest state of polish of which it is capable. After the polish- ing is completed, it is fit for use ; and the sooner it is used the better, since the surface is very apt to become injured by contact with the atmosphere. A very interesting part of the process is now to be gone through, before this mirror-like plate is able to retain on its surface Q 226 LIGHT. the pictures it so brilliantly reflects. A box of peculiar construction, fitted with two earthen vessels and a sliding frame, called the coating- box, is now employed. The plate is laid face downwards on the frame over the pan contain- ing iodine, and the cover of the latter is with- drawn for a short time. When the plate is lifted, its surface is seen to have undergone a beautiful and remarkable change. Its surface now exhibits a beautiful golden-yellow colour, and by prolonged exposure to the vapour this hue deepens into red, and finally into blue and steel-grey. This depth of colour is not, how- ever, permitted to take place in practice ; and after the plate has taken a golden-yellow or rose- colour, it is placed for a few moments over the second pan, which contains the vapour of bromine. Here the colour deepens a little, and it is then replaced over the iodine for a short time, at the expiration of which it is in its most sensitive state. The plate is now transferred to the dark frame of the camera, and exposed to the picture produced by the object-glass. The light having acted on it for a sufficient length of time, the plate is removed from the camera. At this time the image is in a latent and invi- sible condition ; and it requires to be exposed to mercurial vapour before it becomes visible. For this purpose it is immersed in a box filled LIGHT. 227 with this vapour, at a gentle heat; and in about two or three minutes the picture comes out with extraordinary brilliancy. In order to remove from the surface of the plate the delicate film of iodide and bromide of silver still upon it, the plate is washed in a solution of hyposulphite of soda. It is then abun- dantly washed in cold water ; and finally, in order to render the image permanent, it is covered with a solution of gold, which deposits a very thin film of this metal all over it. The plate is now dried over a lamp, and the picture is finished. In giving the practical details of this process, it will be necessary, in the first place, to speak of the required instruments; and, first of all, of the lens and camera, by means of which the picture is formed. “ The camera,” observes Mr. Glaisher, “ is the principal instrument of the photographist, as by its means light is made to become a chemical agent. “ This instrument, the invention of Baptista Porta, towards the end of the sixteenth century, was simply a dark chamber furnished with a single double-convex lens, fitted in a tube, whicli was of use in the focal adjustment of the image. So constructed it was first applied to the copy- ing of different objects, the outlines of which, as Q 2 228 LIGHT. given by the camera, were traced upon tlie paper by a pencil ; but the image was reversed. After a time this, inconvenience was rectified by the use of a mirror ; with an instrument of this kind, A. Canaletti, at the beginning of the eighteenth century, made the drawings for his tine pictures of Venice, which he conse- quently executed with a perfection of perspec- tive so great as to be regarded as an illusion. “ By degrees a lens concave towards the object and convex towards the image was adopted ; the picture was thus rendered clearer, but the colours of the spectrum were not corrected. At length an achromatic lens was used, the flint glass being towards the object. M. Daguerre went a step further, and determined those rela- tive proportions of the camera, which, for the most part, are still adopted ; the interior being carefully darkened, for the purpose of avoiding casual reflection upon the field of view.” The common photographic camera and lens are repre- sented in the figure on page 232. The right construction of a photographic lens is a point of vital importance to success in this art ; for however perfect may be the skill of the artist, it will be impossible to obtain a good result with an imperfect or defective lens. The most important points to be obtained in a good photographic lens, are, that the image it forms LIGHT. 229 should be distinct, that it should be free from coloured fringes, and that it should be as flat as possible. How this may be secured we shall now very briefly explain. The first object is secured by giving a proper curvature to the glasses of which the lens is composed. The second is secured as follows : —From the fact that different substances dis- perse the coloured rays of light in different proportions, it has been found possible to pro- duce an achromatic combination through which the rays of light in their passage become recom- bined, and the image is free from coloured fringes. As the dispersion of actinic rays is somewhat different from that of light rays, a different pro- cess is required to make these rays reunite with the light rays. It is obviously of the greatest importance that those rays of light which produce the impression on the plate — the aetinic-r— should fall exactly in the same plane as those which are merely luminous. To illus- trate this assertion, let us state that by using a single lens of crown-glass in a camera obscura, we may obtain a tolerably good picture, to all appearance, on the ground-glass plate; but when we put a piece of prepared paper exactly in the same place as the ground-glass, in the expecta- tion that the picture will be equally sharp and clear upon it, we find on development of it, that 230 LIGHT. it is all indistinct and confused. The cause of this lies in the fact that, in passing through this single lens, the rays have become separated, and the chemical, or actinic, rays do not con- verge to a focus in the same plane as the lumi- nous. There is, therefore, in a non-achromatic lens what is called its luminous or visual focus, where the rays of light converge to form the picture ; and there is a second focus, called the chemical or actinic, where that class of rays converges ; and this is generally a little nearer to the lens than the other. It is very important that these two foci should be made to coincide, or come together; and in a good achromatic lens this is accomplished by the compensating variations of density in the two glasses. The reader will therefore now be able to under- stand what is meant, when photographic artists speak of the coincidence of the two foci ; and will perceive that it is simply effected, by a careful achromatic arrangement of the glasses used, for which he will, of course, be chiefly dependent on the care and skill of the optician. In some of the very large glasses for taking portraits made by Voigtlander of Vienna, in which the focus is very short, it is found extremely difficult to produce the necessary coin- cidence of these foci, seeing that the amount of correction differs with the distance at which LIGHT. 231 the object is placed. In these lenses, therefore, it is necessary that the artist should, by a series of experiments, learn how his lens works, other- wise he will never be sure of the best result. In order to assist artists in estimating exactly the relative difference in the places occupied by the visual and chemical foci, several very ingenious instruments have been used. That called the focimeter, is perhaps one of the most useful. It consists merely of segments of a circle, numbered and placed at a fixed distance apart, upon a central axis. An image of this instrument is received by the camera, and a particular num- ber in the series is selected by the artist, and made to show most distinctly upon the ground- glass. If now the same number shows with equal distinctness on the plate when the photographic picture is taken, it follows that the two foci exactly fall in the same place ; but if not, and if some other number of the series is most distinct, then it shows the exact amount of difference between the two foci. This arrange- ment may be imitated by placing a number of phials in a diagonal line, with a letter fixed on each, then proceeding as before. In the best lenses for landscapes or portraits, made by Mr. Ross of London, the skill of the optician effects all this arrangement by the proper correction of the glasses, and the artist may be 232 LIGHT. sure that his photographic picture will be just as sharp as that which he saw depicted on the ground-glass of the camera. But the lenses for portraits are not so rapid in action as those of Voigtlander. The second important point in the construction of a good lens, is, that the picture should be quite flat and not curved ; that is to say, that it should be equally clear and sharp all over the ground-glass, and not sharp in the middle, and very confused toward the edges. This defect arises in a great measure from the fact that the lenses used, being segments of spheres, tend to produce a partly spherical, or curved image, which does not therefore coincide with a flat surface. By employing a meniscus lens, of which the one surface is convex and the other concave, this tendency to throw a curved image is almost wholly overcome. And in making achromatic lenses for photographic use, it is customary to LIGHT. 233 make one lens of crown-glass, which is biconvex, and the other of flint-glass, of a biconcave figure. By this means the two foci are made to coincide, and the curvature of the image is corrected by these lenses in combination, cemented together with some transparent cement so as to form a solid lens. A lens thus formed, (though really consisting of two lenses joined together by cement, it is called a single lens,) is almost always used by photographers for producing landscapes and pictures of still life, as the image it throws is very flat and perfect all over the field. But a single lens requires a longer time to produce its effect on the photographic surface than a double lens ; that is, than two such lenses placed near each other in a brass tube. This combination is called a double achromatic lens, and it gives a most brilliant and beautiful image on the ground-glass, and when photo- graphically used, acts with great force upon the sensitive surface. The single and double lenses are always mounted in brass tubes, which are connected with the front of the camera by a screw fitting into a ring. For landscape, or single lenses, the adjustment of the focus so as to, get a bright well-defined picture, is generally managed by a rack and pinion connected with the back of 234 LIGHT. the camera. But for portrait-purposes the rack and pinion adjustment are applied to the brass tube itself, and the lens may thus be very gra- dually made to advance or retire, and thus pro- duce the sharpest image of the object taken. The manufacture of these lenses is an art peculiar in itself, and requiring much care and experience. It has been carried to a high point of excellence by Voigtlander of Vienna, by Ross of London, and by Lerebours, Secretan and Chevalier of Paris. Many other good lenses are also made, both in England and France, but popular opinion generally concurs in the choice of instruments made by these opticians. The cost of a single lens varies from two pounds to sixteen : and of a double lens from five pounds to fifty pounds. For a portrait lens, occasionally employed by the writer, thirty-six pounds was the manufacturer’s charge. The lens employed for the photo- graphic picture which appears in this work, was a double achromatic, made by Voigtlander. If much mathematical knowledge and prac- tical experience are necessary in the construction of the lenses used in photography, but little of either is requisite for the manufacture of a very good camera. The lens, it can never be too often said, is the important instrument in photography ; the camera may be made by any LIGHT. 235 skilful carpenter. With a view, however, to ob- viate any accidental defects, it will probably be always best to procure the camera from those who devote themselves chiefly to the production of this instrument. The camera consists chiefly of the following parts: — a box with a sliding trunk, a groove behind for the reception of the frame holding the ground-glass, and a set of frames adapted for holding the prepared plates, or paper fitting into this groove. “ The photographic camera in its most simple form,” observes Mr. Archer, “is nothing more than a box, into the middle of one end of w T hich an opening is made to insert a lens, whilst the other end is open, and furnished with a groove, into which a dark frame containing the prepared plate is placed to receive the impression of outward objects refracted by the lens. So long as the parts work true, and the main box is constructed square and light-tight, it matters not if the outside is rough and unpolished. It would serve the purpose intended as well as if made in the most costly workmanship. “ Before photography appropriated the camera to its own use, it was a mere toy; a simple arrangement to produce an image of outward objects, which we gazed upon, regretting that the power was wanting to fix and retain the delicate and faithfully coloured impression pre- 236 LIGHT. sented to the eye. The regret then expressed cannot now be entertained; for although the pictures we can produce are defective, and wanting in colour, still they are beautiful im- pressions of natural scenery, and other objects. “The camera, in fact, has now become a valuable scientific instrument, hardly to be recognised under its new form and complicated arrangements, for much thought and labour have been expended in endeavouring to bring it to perfection. It has assumed many forms, and been modified in its various parts to suit the views of the operator, and to work the process proposed to be carried on by its aid.” The most simple kind of camera now in use is that made upon the French pattern; and for photography, as it is usually practised, no better form is required. It is a box with a double body, the back division of which slides out from the main body ; the bottom of the latter is pro- longed backward, forming a support for the sliding division, which latter is fixed, when required, to the bottom, by means of a thumb- screw running in a slot. It is furnished with a dark frame and a focus glass, both of which slide into a vertical groove at the back, when the plate or paper is to be exposed to light, or the focus obtained. The dark frame of this camera should, when LIGHT. 237 used for prepared collodion plates, be furnished at the bottom with a movable glass ledge, formed of two pieces of glass, one wider than the other, cemented together, thus forming a ledge of one-eighth of an inch deep, on and against which the prepared glass plate rests during exposure in the camera. This glass ledge should, each time a picture is made, be removed and cleaned, in order that moisture and dirt may be got rid of ; also a slip of glass should be cemented along the top of the dark frame, for the upper part of the prepared plate to rest against. The glass used in this dark frame should be cut a little less in width than the frame it is intended to go into ; one-sixteenth of an inch less on each side. These precautions ensure cleanliness, and prevent stains on the plate, from contact with the sides, which often arise after the dark frame has been in use a short time. For convenience of arrangement, and in travelling, where economy of space is desirable, what are called folding cameras have been in- vented. Instead of having rigid sides, like those of a box, these cameras have very thin sides, which are ingeniously hinged together and fold quite flat, while at the same time they admit of being raised and made as firm as possible, by fitting into them sliding frames for this pur- 238 LIGHT. pose. The economy of room effected thus is very surprising ; for a large camera, which when set up appears like an enormous trunk, yet when folded in takes the more reasonable size of a dressing-case. One of the best of these cameras is one made by Ottewill ; the writer has found them extremely portable, well made, and useful in actual operation. These instruments are, how- ever, more costly than the rigid cameras. A very valuable and interesting part of a good photographic camera is what is called the sliding front ; and we would strongly advise the addition of this part to any camera in which it may not exist. It is an arrangement for cutting off the superfluous amount of fore- ground, which constitutes an annoying difficulty in the use of the ordinary instrument. The part of the camera-front bearing the lens is made to slide in a dove-tail in the other part of the front, so that the operator is able to raise or depress the lens to the extent of about one inch or one inch and a half. In raising or depressing the lens, we also raise or lower the horizontal line of the landscape (or other object at which the camera is pointed) upon the ground-glass. It is absolutely essential in order to obtain correct perspective — for ex- ample, in copying a building — that we should not raise the camera at an angle, but that it LIGHT. 239 should be perfectly horizontal. Now, suppos- ing the operator to be standing on the ground, it will be found that if the camera is horizontal, one half of the picture will be occupied by the object we wish to have represented, and the other half by the foreground, which we could just do as well without, and it is very probable the effect of the picture will be spoiled, by some of the upper part of the object not being repre- sented. Now this difficulty is completely over- come in this case, by raising the object-glass a little higher, when part of the ground will be cut off, and the whole of our object represented, without having to move the camera out of its horizontal position ; consequently, the true perspective will be retained. In the best cameras there is also an arrange- ment for sliding the lens laterally, as well as in a vertical direction; and this is a most convenient plan for use. In actual practice, the amount of time required in adjusting the camera is generally far greater than that actually employed in taking the picture, for it will be obvious that too great care cannot be exercised in so selecting the point of view as to secure the best possible picture, whereas the mere time of exposure is ordinarily very short in comparison. The best constructed cameras for portraits are supplied with a number of nice mechanical 240 LIGHT. arrangements at the back, by which the posi- tion of the plate is accommodated to that of the sitter. It is found that if the plate be placed in a vertical position while taking the portrait of a person in a sitting posture, that only certain parts of his figure will be sharply de- fined (or in focus), the other parts, as the feet, or knees, or hands, are confused and indistinct, while the face and head are very sharp and clear. This arises from these parts of the figure not being in the same vertical plane, and their focus is con- sequently at a different distance from that of the other parts. By setting the back in which the sensitive plate is held on a hinge, the plate may be thrown into an inclined position, corresponding with that of the sitter’s body, and in this way all the parts are brought into a true focus, and appear equally distinct. There is also a method of giving the plate a slight lateral inclination, which is occasionally useful. Most of the French cameras of this kind, are also fitted with two pinions, working in racks at the bottom of the camera, by which the sliding part may be advanced or withdrawn without touching the lens for alteration of the focus. One of these is a coarse, the other a fine adjustment. But it would be an endless task to describe the varieties of cameras, invented and used with more or less success of late years. LIGHT. 241 One of the great difficulties of practising the art of photography, has always been the neces- sity for having a darkened room, in which to bring out the pictures. For the daguerreotype a dark room is necessary, when exposing the plate to the mercurial vapour ; and for the talbo- type when applying the washes for developing the picture, either on glass plates or paper. This difficulty is of course removed at the establishments of the photographic artists, be- cause there a room is properly fitted up for this purpose ; and these rooms are generally the most interesting part of such establishments ; resembling more the laboratories of the old alchemists, than of modern chemists. But in taking landscapes, away from these conveniences, the want of a dark room is much felt. Mr. Archer and other inventors have con- trived a camera of rather a complicated form, in which all the operations of preparing the plate and developing the picture can be carried on, even in the sunshine and in an open field. This camera consists of a box fitted with sleeves at the side, through which the operator puts his hands ; and contains the baths for preparing the plate and washing it ; and also the bottles and glasses required. The operator places his face to the back of the instrument and peeps in to see what he is about. Yellow R 242 LIGHT. light, which is found not to injure the sensitive plates, is admitted from above, and the progress of the picture can thus be examined. This camera is chiefly applicable to the collodion process ; but it must be confessed that it is very cumbrous in use, and few persons would be willing to go through all the trouble of employing it, if pictures could be got by any other means. Fortunately, such means exist. A very pretty kind of camera, also intended for the collodion process, has been patented by Mr. Newton. In this instrument the baths for the plates are also placed within the camera ; and the plate, after being covered with collodion, is thrust down into them by a brass rod. When the picture is taken, it is pushed down into an- other bath, where it becomes developed ; and on opening the dark box, we see the picture completely developed. This camera, however, is of very limited use, from the risks which its employment involves both of loss and breakage. The writer has for many years used the daguerreotype in the open fields, without any of these contrivances ; nor does he believe anything necessary to success with the ordinary appa- ratus except a good piece of black velvet, which forms an extemporaneously constructed dark chamber, quite sufficient for use. Hundreds of LIGHT. 243 beautiful pictures have been taken, and finished on the spot, simply by throwing the velvet over the common iodine and mercury-boxes. But for the talbotype processes, whether on plate or paper, it may be safely asserted that no con- trivance short of a dark tent or a dark chamber, will suffice to produce perfect pictures ; and the operator will only waste time and money if he attempts any of the plans proposed for their substitution. For the preparation of the delicate sensitive surfaces of plates or paper, there is no place so good as the dark, quiet chamber at home; and for the subsequent development of the picture, it is the best and most conve- nient spot. In order to place the camera at a suitable elevation towards the object to be taken, almost as many different kinds of stands have been made as there have been of the camera itself. The most firm and convenient arrangement is that of ordinary use, which consists of three legs con- nected together at the top, and firmly attached to a triangular piece of brass, which may be securely bolted to the bottom of the camera. This kind of stand is very strong, light, and portable, and gives great firmness to the instrument placed upon it. For the use of the professional daguerreotypist or photographic artist, a more solid stand is employed, fitted E 2 244 LIGHT. with sundry contrivances for elevating and depressing it. Up to this point, the photographic apparatus described is common to both departments of the art; namely, of daguerreotype and talbo- type. The lens, the camera with its holders, a camera and lens. b dark frame. c coating box. d mercury box. e plate holder for polishing. /plate box. g levelling stand. h washing dish. i polishing butf. and the stand are required alike in each. But here the similarity ceases ; and we shall now occupy the remainder of this chapter with an account of the practical details necessary for the LIGHT. 245 production of a daguerreotype picture. The entire process resolves itself into the following distinct operations, upon the careful execution of each of which the perfection of the result will depend to a greater or less degree : — First, cleansing and polishing the plate ; second, applying the sensitive coatings ; third, obtaining the impression on the plate by means of the camera ; fourth, rendering the impres- sion visible by the aid of mercurial vapour ; fifth, removing the sensitive coating from the surface of the plate ; and, sixth, fixing the picture by means of a coating of gold, and dry- ing the plate. The cut shows the apparatus necessary. Daguerreotype plates are made in large quantities in France, and also in this country. The plates manufactured in England are gene- rally thicker and have more silver on them than the foreign, from which circumstance they can receive a finer surface, and are more applicable for beginners, as they will bear being cleaned a great number of times. The French plates being cheaper than the English, can be employed when practice has enabled the operator to be nearly certain of his results ; they are usually marked 1*40 and 1*30, indicating the quantity of silver on them, and consequently their quality : those marked 1*40 will scarcely admit of being 246 LIGHT. used a second time ; but the other may, perhaps, with care, be polished three or four times with- out removing the silver. When these plates leave the maker’s hands, they are without polish, and retain on their surface the marks of the planishing hammer used in finishing them. They require, therefore, to be very carefully cleaned, and subsequently polished, before they are fit for the uses of the photographer. The best and simplest method of doing this is the following, as it is practised by some of the first establishments for daguer- reotype portraits. Some very finely-powdered tripoli, most carefully freed from hard and gritty particles, is moistened with a mixture of alcohol and water, and is then gently rubbed on the surface of the plate, by means of a soft piece of cotton wool. This rubbing is carried on all over the plate until it assumes a silver-grey colour, and is perfectly smooth and homogeneous in its appearance. It is then dried, and is ready for the final polishing, which is effected by means of rouge and a tool called a polishing buff, consisting of a flat piece of wood covered over with a soft piece of leather purposely made for this art. The plate is gently rubbed with this buff until its surface takes a most brilliant and mirror-like polish, and when this is com- pleted it is nearly fit for use. In order, however, LIGHT. 247 to render the surface as fine as possible, it is gently rubbed over with a fine piece of velvet, and is now prepared for the second part of the process. The second part of the daguerreotype process consists in the alternate and successive appli- cation of the vapours of iodine and bromine to the polished surface of the plate. This may be accomplished by a simple apparatus composed of an earthen pan and cover, containing the iodine or bromine, and holding the plate over it until it has acquired a certain colour. But it is far more convenient to use the well-arranged French coating boxes, which accomplish this object in a very perfect manner. We have already explained that the silver plate must be made to assume a certain colour over the iodine and bromine before it is fit for use. The exact tint preferred by the writer is that bordering on a rose-colour. When the plate has reached this colour over the iodine pan, it is placed over the bromine, and the colour allowed to deepen slightly. It is then taken back to the iodine, and kept over it for almost half the time required in the first instance to produce its roseate hue. It is then extremely sensitive to light. The apparatus most convenient for this pur- pose of rendering the plate sensitive, consists of 248 LIGHT. two deep glass pans with polished slides, and mounted in a wooden box, at the back of which are two openings, corresponding to the two pans, over which is fastened a piece of white paper. In the front of the box, and immediately opposite the back openings, are two small doors opening outwards, and each lined with a piece of looking-glass. Two glass covers, and a series of wooden frames sliding over them on the top of the box, complete the apparatus. From half to one ounce of pure crystallized iodine is placed at the bottom of one of the pans, and the bromine in the other, and they are then closed with their respective covers, and the whole apparatus placed before a window with a moderate light. The plate to be rendered sensitive to light is placed in its required frame at the top of the apparatus, and immediately over the pan con- taining the iodine ; the plate-glass cover is then removed so as to expose the plate to the vapour of the iodine below ; the small mirror is now so adjusted that, by looking into it, the white paper at the back can be seen reflected from the surface of the silver plate, and any change of colour immediately perceived. When the plate has assumed a pale rose-colour, the cover is to be returned over the iodine, and the slide holding the plate shifted over the pan containing the LIGHT. 249 bromine ; the cover is now to be withdrawn, the mirror adjusted as for the iodine ; and the glass cover being removed, the plate is exposed to the vapour till it becomes of a deep rose-colour, when it is returned over the iodine till of a blue tint, and immediately placed in the camera- frame. By this elegant and simple method the silver plate is fitted to receive the delicate impress of light in the camera, and to retain it with wonderful tenacity, as we have elsewhere noticed. The third part of the process — namely, the exposure in the camera — is that in which great 250 LIGHT. judgment and care is requisite, otherwise the most beautifully prepared plate will give but a miserable and ill-looking impression. The figure represents the arrangement commonly used. It is a remarkable and important fact, that the prolonged impression of light actually destroys the picture on the plate. For in- stance, if we take two daguerreotype plates both alike in preparation, and in every other respect, and expose the one in the camera to an object brightly illuminated, for five or six seconds, and the other for five or six minutes, contrary to the expectation of the inexperienced, it would really be found, that the picture obtained by an ex- posure of a few seconds was strong and forcible, while that of a few minutes was almost indis- tinguishable, and of the most unpleasing and hideous appearance. This effect of light unduly exercised, is called Solarization. Its cause is not known with accuracy, but it is a fact so well known, as to be one of the first of the series of annoyances encountered by the amateur photo- grapher. The great object, therefore, to be effected in exposing the plate in the camera, is to hit the exact time when the best effect of light is obtained, and to arrest it before it goes further. On the other hand, if the exposure be of too short a duration, then little or no result is obtained, and only the outlines of a picture LIGHT. 251 are seen on the plate. It is, therefore, obviously a critical moment in the history of the parti- cular plate concerned, whether the exposure it receives is exactly right, neither too much nor too little. One of the most expert operators in the metropolis, whose professional qualifi- cations in this art are of the highest kind, and his practice of the most extensive description, assured the writer that he could not depend upon more than one out of every three pictures taken being first-rate in its quality. This diffi- culty is enhanced, if not in a great measure produced, by the variations which take place in the intensity of solar light. The exposure which five minutes before would have been suffi- cient, is, under a transient blaze of sunlight, too much, or beneath the overshadowing of a cloud, too little. From the morning until the evening a perpetual change in the intensity of light takes place ; and if this were only constant and equable, it would cause but little difficulty. But with a sky frequently covered with clouds, and the light consequently rapidly alternating from an intense glare to a subdued illumination, it is very difficult so to adjust matters as to make sure of a good result. The state of the plate also, in the daguerreotype, is very uncertain ; some plates are more sensitive than others prepared in the same way ; and the same plate may 252 LIGHT. be more or less sensitive, according as it is ex- posed for a moment or two, more or less, to the chemical vapours of the coating box. Long practice alone enables the operator to master these difficulties, but the most experienced artists will always acknowledge their existence, and be ready to confess the frequency of their own errors. Supposing the plate to have been sufficiently exposed in the camera, the dark frame is then closed, and we advance to the interesting stage in which the picture is deve- loped. The apparatus used is called the mercury- box, and is made of wood. The bottom is of iron, slightly dished in the centre: this is for containing a small quantity of mercury ; the bulb of a thermometer dips into this, the stem being bent in such a manner that the scale comes outside the front of the box, the mercury being heated by the spirit lamp. The thermo- meter indicates the temperature obtained. The plates are supported in a groove, placed for this purpose inside the lid ; this process is generally carried on in a dark room, but the box may be so contrived as not to render this necessary, the dark frame fitting the camera and containing the plate being made so as to adapt itself to the top of the mercury-box, so that when placed in it the slide may be withdrawn as in the camera. LIGHT. 253 The plate may he examined from time to time by raising the lid and allowing a light from a lantern to fall upon the plate for a short time. The plate should be allowed to remain in the mercury-box until the picture is fully deve- loped, or until it will take up no more mercury. This is readily seen ; for when the picture has been in the mercury-box too long, the shadows begin to turn grey, and are covered with very minute particles of mercury. The proper time for the plate to be removed is just before this effect takes place. It is better to allow the plate to remain over the mercury as long as possible ; for by so doing the picture acquires a more clear and white appearance. Should there be any small trace of solarization, a prolonged ex- posure to the mercury will sometimes remove it. The heat to be applied to the mercury- box should not exceed 200° Fahrenheit: it is better to bring out the picture at about 150°; it takes a little longer time, but there is less risk of spoiling it, and it will be found that the details are generally better deve- loped by employing a low heat. Should the mercury be made too hot, it will deposit itself in the deepest shadows, and spoil the picture. As the picture comes out, or is rendered 254 LIGHT. visible by the mercury, it will be readily seen whether the plate has been exposed a sufficient time in the camera; for if only those parts most illuminated appear very distinctly, then we shall be certain it has been too short a time in the camera ; but should it have been left the right period, the first effect of the mercury will develop the whole of the picture, but very faintly, and further mercurializing will develop it strongly. But if the plate has been too long exposed in the camera, it will have a blue appearance in those parts most illuminated by the sun; and if the exposure has been still longer, the lights and shades of the picture will be reversed. It is this blue appearance given by the too-prolonged action of light which is technically called “ solarization.” The mercurializing process is the test of the success of all the preceding steps in this art. If the picture fail to appear bright and clear at this stage, we must go back either to the pre- ceeding stage, or to those still earlier, for the cause, and seek to remedy it on the next occa- sion. Supposing, however, the plate to have been well cleaned, made sensitive to the proper degree, well exposed to the light in the camera, and, finally, properly developed by the mer- cury, then the next process may be gone on with. But if it has failed, it is better to LIGHT. 255 clean the plate afresh, and make another trial ; and in this, as in every part of the photo- graphic art, the patience and perseverance of the operator will probably be put to a severe test, and he will have to encounter many vexa- tious disappointments before he is entirely successful. The fifth part of the process consists in removing, by means of a chemical solution, the iodide and bromide of silver from the surface of the plate ; and this can be accomplished without in any way injuring the delicate picture ; but on the contrary, if it be a really good one, it is rather improved. Two ounces of a substance called hyposulphite of soda are dissolved in a pint of common water in a wide-mouthed bottle. The plate is then held at one corner by pliers, and this solution is poured over its surface. The iodide and bromide of silver are washed off in solution by this liquid, and the plate is left clean. To remove the solution the plate is then abundantly washed with common water, and afterwards with distilled water. The sixth and last part is the process intended to fix or render the picture permanent. This is effected by pouring on the surface of the plate a very dilute solution of gold in hyposulphite of soda, and then heating the plate by a spirit lamp underneath. In a few minutes the 256 LIGHT. picture brightens up with great beauty, and is far more intense and lustrous than before. A minute film of gold is, in fact, now deposited all over its surface, and protects it to a certain degree from change by exposure to the air. The solution is then thrown off, and the plate is again well washed, and finally dried by heating it over the lamp and gently blowing upon its surface. The daguerreotype is now complete ; and if all these processes have been skilfully executed, it is difficult to imagine a more beau- tiful or delicate production than the finished image. Its harmony of light and shade, and the wonderful fineness of the lines which com- pose the picture, are a source of wonder and pleasure. In order to protect it from accidental injury — for the softest touch of a feather is likely to damage the plate — these pictures are usually placed in frames under glass, and are hermeti- cally sealed up by gummed paper all round the edges. When thus mounted, the daguerreotype is placed in the hands of the purchaser. At large establishments, so rapid are these pictures taken and finished, that it is not uncommon to take from thirty to forty portraits in a single day ; and in addition to the artist who arranges the picture, such establishments keep a number of work-people employed in all the minor details LIGHT. 257 of the preparation, colouring, mounting and finishing the pictures. There are several varieties of effects produced by different methods on the daguerreotype pic- ture ; the simplest is called colouring. As objects copied by the daguerreotype process are only represented in light and shade, not in the colours as they appear in nature, it has been suggested, after the picture has been set, to colour them by hand, similar to a painting ; and certainly, when done in an artistic skilful manner, it produces a very pleasing effect. The simplest method is to use dry colours, ground extremely fine, with some dry gum or starch. The picture must be well set with gold, and the colour applied or dusted on with a fine camel’ s- hair pencil, taking up a very small quantity of colour at a time, removing the superfluous colour by blowing it off with a caoutchouc bottle; when the desired tint is produced, breathing on the plate will cause the colour to adhere. Mr. Claudet’s method is to mix a small quantity of the colour with spirit of wine, applying it to the plate with a camel’ s-hair pencil ; and if not sufficiently dark, some of the dry colour is applied over it, to which it will adhere. As a general rule, the colours should be applied very cautiously, as it is very difficult to remove them when once on the plate. The best colours to be s 258 LIGHT. used are carmine, chrome yellow, and ultra- marine, by combining which any desired tint may be obtained. To those, however, who admire this art in its pure state, the attempt to apply colour will only be viewed as a species of imperti- nence. It is certain that even in the most skilful hands the contrast between the deli- cate and finished performance of the pencil of nature, and the inharmonious and often coarse effects of that of the artist, generally spoils the result. A very singular effect is produced in daguer- reotype plates by a process called crayon daguerreotype. This peculiar process is under- stood to be a French invention. In No. 1197 of the Athenaeum , Mr. Mayall has described, in the following terms, the method of producing crayon daguerreotypes : — “ First. Take a daguerreotype image on a prepared plate as usual, taking care to mark the end of the plate on which the head is produced. When taken, and before mercurializing, remove the plate and place on it a plate of glass, pre- pared as follows : Second. Cut a piece of thin plate glass of the same size as the daguerreotype plate ; gum upon one side of it a thin oval piece of blackened zinc, the centre of the oval to coincide with the centre of the image upon the LIGHT. 259 plate. Having carefully placed the glass thus prepared, with the centre of the zinc disc, upon the centre of the image, expose the whole to daylight for twenty seconds. The action of the light will obliterate every trace of the image from every part of the plate, except that which is covered with the blackened zinc, and also from the thickness of the glass the action will be refracted under the edges of the zinc disc, and will soften into the dark parts. Third. Mercurialize the plate as usual ; the image will be found with a halo of light around it, gradually softening into the background. By grinding the glass on which the disc is fixed, and by altering the size and shape of the disc, a variety of effects may be produced.” The appearance of these pictures is extremely singular. The fact that the exposure of plates already impressed with an image in the camera obscura to daylight entirely removed the original impression, is one of the practical discoveries made by every daguerreotypist who has acci- dentally lifted the shutter of the plate-holder after removing it from the camera. But such an application of this fact could scarcely have been anticipated. More recently, Mr. Mayall has patented a contrivance which the writer has seen and examined, for producing the same effect during the time that the picture is being taken, s 2 260 LIGHT. A frame something like a fire-screen, with a large perforation in it, serrated on the edges like a star, is placed^ near the lens, and the portrait is received through this aperture. While the picture is being impressed, the frame is acted on by clock-work, and the serrated edges move round. By this means all those parts of the picture which lie outside the face and bust of the sitter are cut off, and the moving of the serrated edges produces an appearance on the plate as if a circular cloud surrounded the picture. Those who have seen some of the portraits by the older masters in which the head of the figure alone appears out of a surrounding cloud, will be able to form a conception of the appearance of a crayon daguerreotype. Great force is given to the picture itself by concen- trating the whole of the interest of it upon the face and upper part of the figure, the rest being softened away in a very singular and charming manner. Another modification of the daguerreotype has been introduced by Mr. Beard, but it appears to be only a revival of an experiment long since performed and rejected by Daguerre himself. It is what is called enamelling daguerreotypes. It is effected by simply pouring over the surface of the picture a solution of resinous substances in spirit, and then heating the plate. The LIGHT. 261 varnish rapidly dries, and leaves the picture covered with a smooth hard transparent coating, which ‘is not easily injured. The pictures are, however, greatly damaged in effect by such applications ; and in time there is a certainty of their decomposition, and the consequent destruction of the image beneath. The attempt to produce coloured impressions without any assistance from the pencil of the artist, was long thought to be almost hopeless. Yet few persons can have practised the daguer- reotype to any extent, without having occa- sionally met with instances of the production of colour. The writer has repeatedly seen it, but all attempts to retain it are futile ; and the application of the washes for removing the sensitive coating and fixing the picture inva- riably destroys all such appearances. On one occasion the writer took a very perfect impres- sion of some large pearls, and mother-of-pearl shells; and the iridescence of these objects was so wonderfully copied on the plate, as to excite the astonishment of all to whom the picture was shown. M. Niepce de St. Victor has, however, suc- ceeded in a remarkable manner, not only in producing coloured pictures, but also rendering them more or less permanent. “ In some experiments,” observes the editor 262 LIGHT. of the Athenceum , u made by Sir John Herscliel, a coloured impression of the prismatic spectrum was obtained on paper stained with a vegetable j uice. Mr. Robert Hunt published some a ccounts of the indications of colour in their natural order obtained on some sensitive photographic sur- faces. These were, however, exceedingly faint indications ; and M. Biot and many others re- garded the prospect of producing photographs in colours as the vision of enthusiasts, — not likely, from the dissimilar action of the solar rays, ever to become a reality. M. Edmond Becquerel, however, published a process by which on plates of metal many of the more intense colours have been produced ; but it appears to have been reserved for the nephew of the earliest student in photography, Niepce, to make the discovery of producing on the same plate by one impression of the solar rays all the colours of the chromatic scale. By this process, called by the discoverer, M. Niepce de St. Victor, 4 Heliochromy ’ — sun colouring— every colour of the original pictures is most faithfully impressed on the prepared silver tablet. The exact preparation of the plates still remains a secret with the inventor : and he in- forms Mr. Malone, to whom some pictures which we have seen were given by him, that it is in many respects different from that published by him in his paper ‘ On the Relation which exists LIGHT. 263 between the colour of certain coloured Flames and the Heliographic Images coloured by Light/ Suffice it to say, that the plate when prepared presents evidently a dark browm, or nearly a black surface, — and the image is eaten out in colours. We have endeavoured, by close examination, to ascertain something of the laws producing this most remarkable effect ; but it is not easy at present to perceive the relations between the colouring action of light and the associated che- mical influence. In one picture a female figure has a red silk dress, with purple trimming and white lace. The flesh tints, the red, the purple, and the white are well preserved in the copy. In another, one of the male figures is remarkable for the delicacy of its delineation : — here, blue, red, white, and pink are perfectly impressed. The third picture is injured in some parts ; but it is, from the number of colours which it con- tains, the most remarkable of all. Eed, blue, yellow, green, and white are distinctly marked, and the intensity of the yellow is very striking. Such are the facts as they have been examined by us ; and these results are superior to those which were given to the world when photo- graphy was first announced. “ M. Niepce de St. Victor is still pursuing his investigations on the production of colours by photographic means. By connecting a silver 264 LIGHT. plate with a voltaic battery, and plunging it into a solution of sulphate of copper and chloride of sodium, a chloride of silver mixed with some oxide of copper, or finely-divided metallic copper, is formed, producing a dark-coloured surface. This is exposed to radiation, and every ray falling on the plate impresses it with its own colour, — it is, in fact, eaten out in natural colours.” At a more recent period we learn from the same source, that “ M. Niepce de St. Victor has laid before the Paris Academy of Sciences, daguerreotypes upon which he had succeeded in fixing, in a manner more or less permanent, colours by the camera obscura. M. Niepce states, that the production of all the colours is practicable, and he is actively engaged in en- deavouring to arrive at a convenient method of preparing the plates. 6 1 have begun/ he says, 4 by reproducing in the dark chamber coloured engravings, then artificial and natural flowers, and lastly dead nature — a doll, dressed in stuffs of different colours, and always with gold and silver lace. I have obtained all the colours; and what is still more extraordinary and more curious is, that the gold and the silver are de- picted with their metallic lustre, and that rock- crystal, alabaster, and porcelain are represented with the lustre which is natural to them. In LIGHT. 265 producing the images of precious stones and of glass, we observe a curious peculiarity. We have placed before the lens a deep green, which has given a yellow image instead of a green one; whilst a clear green glass placed by the side of the other is perfectly reproduced in colour.’ The greatest difficulty is that of obtaining many colours at a time ; it is, however, possible, and M. Niepce has frequently obtained this result. He has observed, that bright colours are pro- duced much more vividly and more quickly than dark colours ; that is to say, that the nearer the colours approach to white the more easily are they produced, and the more closely they approach to black the greater is the difficulty of reproducing them. Of all others, the most diffi- cult to be obtained is the deep green of leaves ; the light green leaves are, however, reproduced very easily. After sundry other remarks, of no peculiar moment, M. Niepce de St. Victor informs us, that the colours are rendered very much more vivid by the action of ammonia, and at the same time this volatile alkali appears to fix the colours with much permanence. These results bring us much nearer to the desidera- tum of producing photographs in their natural colours. The results are produced upon plates of silver which have been acted upon by chlo- ride of copper, or some other combination of 266 LIGHT. chlorine. The manipulatory details, we under- stand, are very easy.” * Upon the veracity of these statements there cannot exist a doubt. But others have been made public which are open to the greatest sus- picion and uncertainty. A person of the name of Hill has announced in America that he has discovered a process by which he can produce coloured pictures at one operation. These pic- tures have been described as wonderfully per- fect and enduring in their nature. It must, however, be acknowledged, that the superla- tive manner in which these productions are described, and the general tenor of the com- munications received, which have not gained the support of any American philosophers of eminence, render the so-called hillotypes very questionable specimens of the art of fixing natural colours. The whole subject is one of great difficulty, and has a far less pro- mising aspect, to the writer's mind at least, than any other department of this wonderful art. The length to which the present chapter has extended precludes our noticing minor modifications of the daguerreotype ; but all the more important facts in connexion with the process have here been detailed. The same * Athenceum , No. 1308. LIGHT. 267 reason also renders it more convenient to defer the discussion of the stereoscope and other applications of the photographic art to the close of our nest chapter and the end of this little work. CHAPTER III. THE TALBOTYPE. The talbotype, in contradistinction to that department of photographic art which we have just been considering — the daguerreotype — may be regarded as a wet process, the other being distinguished as a dry process. No liquid application whatever is necessary in order to produce a daguerreotype picture; whereas to obtain a talbotype, washes of various kinds are necessary from first to last. This is perhaps one of the greatest objections to the practice of it by the amateur, since he requires so many liquids, and incurs so much risk of fracture of bottles, and consequent damage to all within reach of their contents. But, on the other hand, the advantages of economy and of the indefinite multiplication of copies, more than counter- balance these objections ; and, besides, the size LIGHT. 269 of the pictures obtained is vastly superior to the very largest daguerreotypes. Talbotype pictures two feet in length by eighteen inches in depth, are by no means uncommon; and this is a size almost unheard of in daguer- reotyping. For convenience of description, we shall con- sider the talbotype processes under the follow- ing heads : — 1. The original plan of Mr. Talbot, on paper, which has probably never been surpassed, not- withstanding all the modifications of it which are practised. 2. Pictures on albuminized plates; and, 3. Collodion pictures. The talbotype process, as originally con- tained in the patent-specification of Mr. Fox Talbot, consists of the following stages : — 1. The preparation of a paper having in its pores a delicate layer of iodide of silver. 2. Pendering this paper sensitive to light. 3. Exposure in the camera. 4. Development of the picture. 5. Fixing the picture. 6. Printing from the pic- ture. 7. Fixing and finishing the printed copy. The two last stages may be considered as separate from the preceding, since they only relate to the production of the copy, not to that of the original picture, which is called the negative, the copy being denominated the positive. 270 LIGHT. The process for obtaining negatives, as de- scribed in the patent of 1842, is the following “ Take a sheet of the best writing-paper (paper is now made expressly for this purpose), having a smooth surface and a close and even texture. The water-mark, if any, should be cut off, lest it should injure the appearance of the picture. Dissolve 100 grains of crystallized nitrate of silver in six ounces of distilled water. Wash the paper with this solution with a soft brush on one side, and put a mark on that side, whereby to know it again. Dry the paper cautiously at a distance from the fire, or else let it dry spon- taneously in a dark-room. When dry, or nearly so, slip it into a solution of iodide of potassium, containing 500 grains of that salt in one pint of water, and let it stay two or three minutes in the solution. Then dip the paper into a vessel of water, dry it lightly with blotting-paper, and finish drying it at a fire, which will not injure it even if held pretty near ; or else it may be left to dry spontaneously. All this is best done in the evening by candlelight. The paper so far prepared is called iodized paper, because it has a uniform pale-yellow coating of iodide of silver. It is scarcely sensitive to light ; but, never- theless, it ought to be kept in a portfolio or drawer until wanted for use. It may be kept for any length of time without spoiling, or under- LIGHT. 271 going any change, if protected from sunshine. When the paper is required for use, take a sheet of it and wash it with a liquid prepared in the following manner : — Dissolve 100 grains of crystallized nitrate of silver in two ounces of distilled water ; add to this solution one-sixth of its volume of strong acetic acid. Let this be called mixture A. Make a saturated solution of crystallized gallic acid in cold distilled water. The quantity dissolved is very small. Call this solution B. Mix together the solutions A and B in equal volumes, but only a small portion of them at a time, because the mixture does not keep long without spoiling. This mixture is called the gallo-nitrate of silver. This solution must be washed over the iodized paper on the side marked ; and being allowed to remain upon it for half a minute, it must be dipped into water, and then lightly dried with blotting- paper. This operation in particular requires the total exclusion of daylight; and although the paper thus prepared has been found to keep for two or three months, it is advisable to use it within a few hours, as it is often rendered use- less by spontaneous change in the dark.” This paper is very sensitive to light, and it is necessary to prepare it either in a room totally darkened, excepting only the dim light afforded by a common candle, at a distance from the 272 LIGHT. operator, or into which only a deep yellow light is admitted by means of a double curtain of yellow calico. Under such a light the paper will be uninjured. The next process is its exposure in the camera. It may be useful to mention that the slides in which the paper is held are rather different to those prepared for plates, and the paper is in these frames placed against a plate of glass, through which the picture is received. The time of exposure in the camera very much depends upon the strength of the solution used to make the paper sensitive. If that be very strong, it will only occupy a few seconds ; but if very dilute, then it may extend to several minutes. For landscapes, with a lens of small opening, from ten to fifteen minutes may sometimes be required. But the average time is about from three to five minutes. The impression being taken, the paper must be removed in the slide to the dark room, and there be submitted to the next process. The solutions required for bringing out the latent picture, are the same as those employed for preparing the paper, viz., solution of gallic acid, and aceto-nitrate of silver. These solutions should be mixed in equal proportions, say half a drachm of each, to which add half a drachm of distilled water. This mixture is to be applied LIGHT. 273 to that surface of the paper on which the latent image is formed, by either of the methods before described, taking care that the whole of the surface be thoroughly wetted ; it is then laid, face upwards, on a plate of glass or other clean substance, and the picture will be observed to gradually appear in all its details. During the development of the picture, it is necessary to keep the surface wet, otherwise the light parts of the picture sink, and become opaque; this is done by applying, when necessary, a fresh quantity of the mixture of gallic acid and aceto- nitrate of silver. If the picture is very tardy in making its appearance, which will generally be the case in cold weather, it can be much accele- rated by the cautious application of heat, and a better result by this means obtained than would otherwise be produced. The best method of applying heat is by holding the picture over the steam from some hot water, placed at the bottom of rather a deep dish. The appearance of this picture to the eye of the person who for the first time produces it, will probably be very unsatisfactory, even in the rather improbable event of his succeeding per- fectly at first, in all the manipulatory details. The black sky, the white trees and ground, and the sombre look of the whole production will be very unpleasing to him. But regarding it, as it T 274 LIGHT. immediately tell from tlie appearance of liis engraved block, what kind of a picture it will produce, so the practised photographic artist immediately discovers by the appearance of his negative picture, obtained in the manner really is, merely as the matrix of another picture in which all these effects are reversed, he will become speedily reconciled to it. This diagram shows the appearance of a negative and of its positive proof. Just as the wood-engraver can POSITIVE PROOF. NEGATIVE PICTURE. LIGHT. 275 described, what sort of a copy it will produce, and estimates accordingly the success which has attended his experiment. The next step is to render the negative no longer sensible to the influences of the light. In order to do so, it is necessary to dissolve out the iodide of silver from the pores of the paper, and also the nitrate of silver left on the surface. This may be done as follows : — As soon as the picture is sufficiently developed the gallo-nitrate should be washed off* as quickly as possible, otherwise the paper will blacken all over, and the water used for washing should be once or twice renewed. It may remain in water about ten minutes ; the proof should then be taken out, pressed between folds of clean blotting-paper, and placed in a strong solution of hyposulphite of soda. As soon as the whole of tlie yellow iodide of silver has been dissolved, it should be taken out, well washed in abundance of common water, and, to insure the removal of the whole of the hypo- sulphite of silver, left in a quantity of water for some hours ; and then, upon being dried with blotting-paper, it will be found to be very per- fectly fixed. If it is not convenient to operate at once with the hyposulphite of soda, which process requires a great quantity of water and long soaking, it may be fixed for a few days very well, until the operator has time to use the t 2 276 LIGHT. hyposulphite, hy a single wash with a solution of bromide of potassium, containing ten grains to the ounce of water ; it should then be dipped in water and dried by blotting-paper, between the folds of which it should be kept until the hypo- sulphite can be applied. The whole process for obtaining negatives has thus been fully described as it is commonly practised, and the directions given are those contained in the best manuals on photography. This process on paper has many admirers for landscape-photography, and is remarkably cer- tain in its results. It is practised very exten- sively, and very large pictures have been taken by it. The writer has practised it ; but with all its many advantages, his own experience is, that for finely-drawn, harmonious, and delicate effects in landscape, it is very inferior to the collodion process; and though as regards both economy and convenience the latter is far inferior to the former, still it is to be preferred in all cases where the effects in question are desirable in the picture. The next part of the talbotype process is common to all the three varieties of this art, constituting, as it does, the means of obtaining copies of the pictures impressed in the camera. Whether these are taken on paper or on glass, this process remains the same in every respect. LIGHT. 277 It may be conveniently termed the “ printing process for it is as really a kind of printing as that of ordinary type-printing, only that the ink, so to speak, is laid on the paper, not on the matrix, and its colour is given by the sun. Perhaps the most satisfactory plan to the reader, in describing this process, which is the easiest and simplest of all, will be to state exactly the method followed by the writer in preparing the picture which appears in the present work. So large a number of pictures were necessary for the first edition of this work, that it became important to devise the very simplest and best mode of producing them which the nature of these prints would permit of. It has already been noticed, that when nitrate of silver is mixed with a chloride of sodium (or common salt), it is immediately decomposed, and a chlo- ride of silver formed. This substance blackens, as we have before seen, very quickly when ex- posed to the sunshine, changing from a delicate cream- white to an intense purple-black. Now, all that we require to produce paper from positive prints, is to have the surface of the sheet covered with a fine layer of this substance, and then dried. And the way to effect this is very simple. We have merely to brush over the surface of the paper a solution of common salt and then dry it, and when dry to apply in the same way a solution 278 LIGHT. of nitrate of silver, and the decomposition will immediately take place on the surface of the paper, leaving a chloride of silver upon it, with a little excess of nitrate. Such, at least, would be the case for successful practice. But a great part of the trouble of this opera- tion is saved by purchasing at the photographic stationer’s paper already brushed over with the salt. The writer followed this plan, and pro- cured a large quantity of paper already half- prepared, and then proceeded in the following manner : — In half a gallon of pure distilled water a sufficient quantity of nitrate of silver was dissolved, to give to every ounce at least sixty grains of this substance. All that was now necessary to prepare the paper to receive positive impressions was simply to pour this liquid into a very large, clean dish, and lay the paper on its surface for five or six minutes ; then it was gently lifted up and hung up to dry, and when dry was fit for use. This paper will only keep good a few hours in hot weather, but in cool weather it is fit for use for two or three days. The paper being thus prepared, it remains for us to describe the method of obtaining the copy of the negative picture. We have now a sur- face so prepared that it will very quickly blacken all over if exposed to light ; but it will be obvious that if one part be shaded, and the other exposed, LIGHT. 27 » the part covered will remain white, and the whiteness would correspond in outline to that of the body shading it. If a leaf, for example, be placed on a prepared surface of paper, and ex- posed to the sunlight, the paper will become black all round the leaf, but underneath the blackening effect will be according to the amount of opacity of its various parts, and an impression will be produced corresponding to the various markings of the leaf. It is necessary, however, to obtain a correct copy of the leaf, that it should be in close contact with the prepared surface, so as to prevent any light affecting the paper under the edges of the leaf ; and for this purpose is employed a contrivance called a reversing or pressure frame. Many forms of this instrument have been devised, but, being all of one prin- ciple, they may be described as consisting of one or two pieces of thick plate-glass, placed in a vrooden frame, with a contrivance at the back, either by a wedge or sows, to press together the two plates, or else a flat board covered with two or three folds of flannel, which can also be pressed against the front glass. To copy objects that are flat, such as prints, drawings, &c., it is usual to proceed as follows : — Place a sheet of the prepared paper on one of the pieces of plate-glass, taking care that the prepared side, which is easily distinguished by 280 LIGHT. a mark previously made for that purpose, is upwards. The print or drawing should then be placed on the paper, printed side downwards, and a small particle of wafer or gum applied at one of the corners, so as to attach it to the pre- pared paper; the other piece of plate-glass is now to be placed so as to press them into close contact. The whole may then be exposed to the light, the drawing to be copied being upwards. And the same applies to the employment of a paper negative, or a collodion or albumen picture, the prepared side being always placed upon the prepared side of the positive paper. The time required to produce an impression depends, in a great measure, upon the nature of the negative ; a paper negative, even when waxed, and so made more transparent, requiring a longer time than a collodion or albumen nega- tive. From five minutes in bright sunlight, to half an hour in common light, are the ordinary limits; but experience will give the best in- formation on this and other points allied to it in this art. When the print is sufficiently intense, it is taken out of the pressure frame and finished in the following way. Two or three ounces of hyposulphite of soda are dissolved in one pint of distilled water ; and a solution of chloride of gold, in water sufficient to give half a grain of LIGHT. 281 gold to each ounce of the total quantity, is added a few drops at a time. Then a solution of nitrate of silver, about forty grains to the total quantity, is added also by a few drops at a time. The fixing liquid is now complete ; and in order to finish the picture, and at the same time impart to it a pleasing tone and colour, it is merely necessary to immerse it in a bath of this liquid for a few hours, and then very co- piously wash it with water. On being dried, it is finished. The use of the gold in this bath is to assist in giving a peculiar colour to the print, which it would not otherwise possess ; for, on being placed in the bath, at first it becomes of a very ugly and disagreeable brick-red colour. But if we have the patience to wait for an hour, we shall see this colour disappear, and a pleasing tint gradually comes over the whole picture. This bath is, therefore, appropriately called a “fixing and colouring bath,” the picture being rendered permanent by the removal of the free nitrate of silver, and the colour having been communicated to it by a process of decomposition of a peculiar kind which goes on in this bath constantly. The absolute permanency of pictures printed in this way has been made a subject of much inquiry. There seems no reason to believe that compounds of silver are less stable than those 282 LIGHT. of otlier metals ; and, therefore, there is no solid ground for doubting that a photograph is really imperishable if it be properly washed and mounted. A very complete system of washing the photographs, employed in this work, was devised by the writer, and has given very satis- factory results. It is unnecessary here to allude to the precise details ; but they resemble those adopted in washing calicoes by the Lancashire print-works, than which a more thorough and complete system of washing a fabric has never been devised. Many waters and prolonged im- mersion in them are necessary for the complete removal of the hyposulphite of soda of the photograph. It is then hung up to dry, and mounted in the ordinary way with very strong and perfectly fresh paste. It is always easy to an experienced eye to tell the prints taken from paper negatives by their sandy and spotted appearance. But those from collodion or albu- men excel the most elaborate engravings in delicacy and fineness of detail. To these pro- cesses we shall now direct our attention. It may be useful, however, to add, that by covering the surface of the paper on which the positive print is received with a thin layer of albumen, or white of egg, a very fine glossy surface is given to it, which some persons greatly admire. Both this and all other kinds of paper for photography LIGHT. 283 can be purchased ready for use at the stationers who devote their attention to this branch of their business. The next process for obtaining negatives is one by which most beautiful pictures are ob- tained ; but it is one which requires much care in practising. The writer not having made himself expert in this process, prefers giving the directions of others whose success has been very extraordinary and constant. The directions given are the following: — Break three or four eggs into a dish, keep- ing out the yolks, and also the non-transparent parts of the whites ; add to the white of each egg ten drops of a saturated solution of iodide of potassium, and also one drachm of pure water ; beat the whole well up into a firm froth, and allow it to stand one night, when the liquid part will be found to have fallen to the bottom. It is then ready for laying on the glass plate, which is done as follows : — Having cleaned the glass well, first with a weak solution of caustic potass, and finished with pure water, and dried with a silk hand- kerchief, the glass is fastened into a sort of brass-wire forceps at the two opposite corners ; it is then held in one hand, and the egg poured on the # clean side ; allow it to spread over as equally as possible. It is then slanted 284 LIGHT. quickly up, and turned before a clean clear fire by a worsted thread which is attached to the wire forceps, exactly on the same prin- ciple as a roast-jack : it is turned at a moderate rate — not too quick, or it throws it all off from the centre; and if too slow, does not make it equal — at a moderate fire. About four minutes is sufficient to dry it, but it must be taken away when it begins to crack at the edges ; it will then crack equally over of itself. With a little practice nothing can be more simple than this mode, or more certain; — in this state it will keep any length of time. Previous to taking a picture, we dip it face downwards in a flat glass dish containing a solution of nitrate of silver, seventy grains to the ounce of water, as well as the usual propor- tion of acetic acid (one and a half drachms). It is then washed in a dish of pure water, by dipping three or four times, and keeping the w T ater always running off the same way to pre- vent streaks ; it is now placed in the camera, and, if it is a light-coloured building or dis- tant view, it will require from three to four minutes’ exposure with an ordinary lens; but if the view be dark or green, it will take much longer. When taken out of the camera, it is washed over with a solution of gallic acid, and allowed LIGHT. 285 to develop well ; when a little of the nitrate of silver is mixed with fresh gallic acid and poured on the picture (and spread with fine cotton, ns the gallic acid at first should be), which will strengthen it to the proper degree. It is now fixed with hyposulphite of soda, which can be poured on a corner and spread with clean cotton ; to finish it entirely, it is only now to be washed in clean water, which can be done by pouring a little along the edge of it, and allowing it to run down the face in one direction ; it is best to allow it to dry by itself. If the glass negatives are taken proper care of, the number of impressions that can be taken from them are unlimited; several hundreds of impressions have been taken from some of these negatives. In the process, as it is practised by Messrs. Negretti and Zambra, of Hatton Garden, and also by Mr. Mayall, the plates after being covered with albumen are merely placed in a box to dry, which greatly simplifies the process. Our chief objections to this process are the great risk of getting plates covered with spots, the difficulty of getting good middle tints in the printed copy, and the extreme slowness of the process in taking the picture. On the other hand, it is stated that the albumen process is 286 LIGHT. superior to collodion, in economy, in the dura- bility of the picture, and the very great certainty of obtaining excellent pictures by its means. In the writer’s judgment, however, there is no question as to the essential superiority of the collodion process, in this country at least, where the light is variable and the atmosphere often so opaque as to render all attempts to get a good picture futile, unless a, very rapid and sensitive process be adopted. For the bright and clear skies of Italy and France albumen is very suitable, and it is most extensively used in those countries. A method discovered by a French photographer, M. Taupenot, which consists mainly in pouring albumen over col- lodion on glass plates, seems to combine the advantages of both processes, and is very sen- sitive to light. The collodion process is the invention of Mr. Archer, and is probably one of the most important and valuable discoveries ever made in the photographic art. Collodion is a solution in ether and alcohol of the peculiar substance called gun-cotton, the highly explosive pro- perties of which attracted such notice some years ago. Gun-cotton and its solution are both the in- ventions of M. Schonbein, a German chemist of celebrity, and these substances rank among the most interesting known to the chemical student. LIGHT. 287 The writer lias the greatest confidence in recommending the following instructions for the practice of this process. They were origi- nally read before the Photographic Society of London by the Count de ]\Iontizon, and they have never been excelled either in sim- plicity, or in the accuracy of the results ob- tained. With some slight and unimportant differences, this process is the one pursued by the writer with almost invariable success, and it is also that pursued by the best opera- tives in the photographic establishments of the metropolis. “ To make Collodion . — Put into a clean basin 10 drachms of sulphuric acid of ordinary strength, oz. of nitrate of potass, and 40 grains of clean carded cotton. “ With glass rods stir the cotton well for about six minutes, till it is thoroughly saturated ; then pour on it plenty of common water, changing the water seven or eight times, and finally washing twice with distilled or rain water. The more thoroughly it is washed, the more completely will it afterwards dissolve in the ether. Wring it dry in a clean cloth, press it between folds of blotting-paper, and, having pulled out the fibre with the hands, dry it near the fire. “ This is very important, and also it is very 288 LIGHT. necessary to make the gun-cotton in small quantities, not more than 40 grains, or else it will not imbibe the liquid. “ Instead of cotton, paper may be used. The Swedish filtering paper is well suited for the purpose. All this part of the process should be done under the chimney, to allow the gaseous fumes to escape. “ To dissolve the Cotton . — When the cotton is quite dry, to each oz. of good sulphuric ether add 8 grs. of prepared cotton or paper, using a large bottle, so that the clear liquid may be poured off when wanted. If the cotton is well prepared, it dissolves completely. Much depends upon the quality of the ether, which ought to be good, but not too strong. “ To iodize Collodion . — I have tried many methods of iodizing collodion. Those which have given the most successful results are the following : — “ 1st. In 1 oz. of collodion put a little iodide of silver and about 3 or 4 grains of iodide of potassium, and then shake it well up. The collodion becomes very turbid, but on being left for some hours it gradually clears up, beginning at the bottom. When it is quite clear, pour off the liquid into another bottle. “ 2d. To 1 oz. of collodion add 2 grains of iodide of ammonium. This will give very LIGHT. 289 beautiful gradation in the half-tones, but not so vigorous a picture as the first. “ 3d. In 8 drachms of pure alcohol dissolve perfectly 8 grains of iodide of ammonium or iodide of potassium, and \ grain of iodide of silver ; then add 24 drachms of collodion. The iodide of silver ought to be freshly made, or the resulting negative will be of inferior quality. The iodide of ammonium, too, ought to be newly made. This collodion is one of the most sensitive, but the half-tones produced by it are inferior. “ Method of Operating , — I employ nothing but water to clean the glass plate with, using plenty of it, and rubbing the glass with the hand till the water flows freely over the surface. It must be well dried and rubbed clean with a linen cloth which has been well washed without the use of soap. When the collodion comes away from the glass, it is almost always in con- sequence of the existence of grease or dirt or of a little moisture upon the surface. “ Pour the collodion upon the glass in the usual way, and almost immediately immerse it in the bath of nitrate silver, 30 grs. to the ounce of water, lifting it in and out of the solution to allow the ether to escape. Wlien it assumes a bluish opal hue, it is ready for use. By adding a little alcohol to the solution, one pait U 290 LIGHT. of alcohol to ten parts of water, and one part of nitrate of silver, the collodion is more speedily rendered sensitive, and the image produced is more vigorous. 66 It seems of some importance to immerse the glass in the nitrate bath, and to place it in the slide in the same direction as that in which the collodion was poured off the glass plate. “ After the appearance of the opal hue, if the bath be an old one, the plate may be left in it for some time without injury ; but if the bath be new, it must not be left longer than is necessary to excite, or the nitrate would attack the iodide ot silver. “ To obviate this, it is well in making a new bath to add 1 grain of iodide of silver to each ounce of the nitrate solution. “ It is unnecessary to filter the bath, as it is often altered in its nature by passing through paper containing injurious chemical constituents. A little blotting-paper drawn over the surface will remove any particles of dust that may be floating upon it. “ If the bath contain alcohol, it should, when not in use, be kept in a stoppered bottle. “ The time of exposure in the Camera varies ; but until you know the quality of your col- lodion, it is best to begin with a short exposure, as you may fancy a collodion good for nothing LIGHT. 25)1 which gives a bad picture after 30 seconds 7 exposure; whereas if you had tried it with 2 seconds', it might have produced a splendid proof. “ To develop the Picture . — Pyrogallic acid appears a better agent than protosulphate of iron. “ I find it better also not to place the negative upon a stand, but to hold it in my hand, taking a sufficient quantity of solution to cover the plate at once. I pour it by turns on to the surface and back into the glass measure, until the picture is completely developed. “ The solution of pyrogallic acid is the one usually employed. Mix 3 grains of pyrogallic and half a drachm of acetic acid to the ounce of water. For use it is diluted with an equal measure of water. I have quite abandoned the * addition of nitrate of silver to the developing solution, thinking that the negative is thereby rendered less clear, and more violent in its contrasts of light and shade. If added at all, it is only when the negative is feeble, and when it appears to have ceased developing. When, however, the plate has been kept for some time after being excited, it is well, before com- mencing to develop, to plunge it again into the nitrate bath for an instant. u Though it seems of small importance, yet u 2 292 LIGHT. the glass which contains the pyrogallic solution should be washed after each negative, and with distilled water. The solution of pyrogallic should be made when wanted, or at most two days before, or it loses a part of its developing power. “ At no time while the plate is in a sensitive state must any white light be allowed to enter the dark room ; but this is especially the case during the development of the image. “ The light of a candle or lamp, or fire of the grate, is very injurious, unless it is removed to a considerable distance, or falls at an acute angle upon the plate. In proof of this, I may state that I have taken in the camera a very distinct image of the flame of a candle in six seconds. “ By employing a yellow curtain the operator has a greater supply of light, in his dark room, and that of a kind that will not injure his negatives. “ For fixing the Picture . — Use a saturated solu- tion of hyposulphite of soda, and then wash the negative with plenty of water. Dry it, but not by the fire, and protect its surface by a varnish. The best varnish is that made from amber, according to the receipt given by Dr. Diamond. The amber is dissolved in chloroform, in the proportion of 2 drachms to 1 oz. of the spirit, LIGHT. 293 and should be left in it for two or three days. The varnish made by Mr. Horne from gum damma is also excellent.” One of the principal obstacles to the applica- tion ot the collodion process for landscapes, lias, until within the last few months, been the rapid loss of sensitiveness, and, indeed, the destruc- tion of the surface of the plate, by the crystal- lization of the nitrate of silver of the bath upon it. This was for a long time considered to be a fatal objection ; but very recently a simple, and, in the writer’s experience, a most successful method has been introduced by Mr. Shadbolt, which renders it possible to keep these plates in a sensitive state, ready for use at any moment, for as much as two or three weeks, or even longer. This process is as follows : — The plate is prepared as usual, without any variation from the ordinary method, until it is ready to be re- moved from the nitrate of silver bath. It is then taken out and washed in a second bath, of distilled water only, in which a minute portion of the nitrate of silver is contained. A solution of very pure honey is then poured over the plate several times, and it is set up to dry in a dark place. When dry, it is placed in the slide, and will be fit for use for many days if carefully excluded from the light. After the picture is taken, the plate must be well soaked 294 LIGHT. in a bath of distilled water, and it will then develop in the usual way. This plan has given the writer excellent pictures with very little trouble, and almost certain success. But it is one, the untold difficulties of which the ex- perimenter will best discover and conquer for himself. The collodion process is at present rapidly supplanting the daguerreotype for portraits. But the daguerreotype is still unsurpassed in its wondrous delicacy and in the gradation of the tones of the picture, and will probably never entirely lose ground in popular favour. We have hitherto alluded only to the pro- duction of collodion negatives ; and for this reason, that although direct positive pictures somewhat resembling daguerreotypes can be produced on collodion, we consider them so inferior, as to constitute rather a degradation of the art than otherwise. Occasionally, pleasing results are obtained, but for all practical pur- poses a good collodion negative is far to be preferred both for artistic effect, and for the possibility of obtaining many copies from it. The method for obtaining positive collodion pictures is to dilute the ordinary collodion one half with ether and alcohol, and to develop the picture with a solution of protosulphate of iron. The picture is finished in the usual LIGHT. 295 way, and the glass plate is backed up with black velvet. But we cannot recommend these pictures. We shall conclude our work with an account of the method of obtaining stereoscopic pictures ; the wonderful solidity and reality of which has given a new impulse to photographic art, and constitutes one of the most curious and instruc- tive of its departments. It is generally known that by means of this instrument the idea of solidity is given to the eye from pictures on flat surfaces. The prin- ciple upon which this instrument depends may be thus explained. When a house or a land- scape is looked at, it is found to possess a quality which no copy on a flat surface by the best artist can produce. This is solidity, depth, or distance, — the appearance of ob- jects standing immediately behind each other. We acquire this perception in the following manner : each eye separately receives a picture of the same object, the one picture being a little different in perspective from the other, in con- sequence of the difference in the relative position of the two eyes. One eye, in fact, sees a little more round one side of the object, while the other sees a little more round the other side ; and it is the combination of these two pictures by the faculty of sight that gives to objects their 296 LIGHT. solid appearance. This is shown in these figures. SEEN WITH LEFT EYE. SEEN WITH RIGHT EYE. Now, in order to obtain the same effect arti- ficially, the stereoscope is so arranged that two representations of the same object, the one slightly differing from the other, in perspective, are placed at the bottom of a little box, where an opening is made, through which they are illuminated. At the upper part of the box are two small eye pieces, adapted one for each of the observer’s eyes. Through these he looks at the pictures, and the appearance of solidity is received in a very remarkable manner. It was found very difficult to draw pictures with sufficient accuracy to give good stereoscopic views, since a slight error in perspective would, to a certain extent, vitiate the resulting im- LIGHT. 297 pression on the eye. But the photographic art supplied this want ; for, by taking two pictures with the camera first in the position of one eye, and then removed to a little distance to that of the other, this result is perfectly obtained without any risk of error. The figure shows the best form of this instrument. The most lifelike representations of objects, of persons, groups, and even scenery, are now taken by professional photographers, and it is difficult to believe, on inspecting them, that the real things or persons are not presented to the eye. Views of the Great Exhibition were taken which reproduced it in all its solidity to the eye. In January, 1852, Professor Wheatstone read a second paper at the Royal Society, and 298 LIGHT. exhibited an instrument which he calls a pseu- doscope, on account of its giving false percep- tions of all external objects. Some of the illusions were very extraordinary. Its effect may be briefly expressed as making whatever point is nearest seem farthest off, and vice versa; so that all objects seen through it seem as if they were turned inside out. A solid terrestrial globe is seen concave, like Wyld’s globe, with the map on the inside. The inside of a teacup appears a rounded projecting solid. A china vase, with embossed coloured flowers, appears as if it were cut in two ; and we saw the side with the flowers indented. A bust shows as a deep hollow mask. A framed picture hanging against the wall seems as if it were let into the wall ; and in general objects placed before a wall are seen behind it, as if the wall were a mirror. Other more complicated, and in some cases perplexing, illusions are produced by this instru- ment, which is very portable. Cameras are sold which are accurately ad- justed on a movable frame, for the purpose of taking stereoscopic pictures. In some of these arrangements one lens and camera are employed, but it is better to use two, and take the two pic- tures at once and not in succession, and this plan is followed by all professional photographers. It were a vain attempt to enumerate all the LIGHT. 299 applications to which this new art lias already been made subservient. F or copying landscapes, portraits, inscriptions, architecture, antique ob- jects, microscopic objects, seals, books, pictures, engravings, animals, birds, clouds, the physiog- nomy of disease, of insanity, of crime, botanical specimens, and a number of other things, it has proved itself a most useful handmaid to the arts and sciences. Perhaps, however, its appli- cation to astronomy is one of the most beautiful. Photographs of the moon, as before noticed, have been taken, and those examined by the writer render with wonderful accuracy the strange fea- tures of that luminary, impressed on the plate by its own light. These photographs have been taken by aid of a powerful telescope, kept con- tinually towards the orb by an apparatus called a heliostat, and thus permitting its light to shine sufficiently long on one spot of the prepared plate to give a good and clear impression. What other and more wonderful discoveries may yet remain to be disclosed about the nature, sources, effects, and applications of light we are scarcely able even to conjecture. We already know sufficient to justify our regarding this as one of the most mysterious yet powerful agencies permitted by Divine Providence to take its part in the operations of nature. And the poverty and narrow limits of our highest 300 LIGHT. knowledge concerning light may be useful in teaching the human mind its own feebleness ; and still more so if it should guide us for the best wisdom to Him, whom the Scripture has declared to be “ The Father of Lights, with whom is no variableness, nor shadow oi turning.” THE END. R. OLAY, PRINTER, BREAD STREET HILL. Spec-ioJl ’JI'A) 7698 THE GETTY CtNTcK library