he Photography of Coloured Objects * : - Second Edition ■'"v Eastman Kodak Co. Rochester, N.Y. 1916 V The Photography of Coloured Objects * Second Edition Eastman Kodak Co. Rochester, N.Y. 1 9 1 6 First Edition, 1909. Reprinted, 1913. Second Edition, 1916. PREFACE r F N HE first edition of “ The Photography of Coloured 1 Objects ” was written by Dodtor C. E. Kenneth Mees, who stated it was an attempt to put clearly the theory underlying the photography of coloured objects and the application of that theory to those branches of practice which are of the most immediate importance. This revision includes most ot the matter contained in the previous edition, together with an incorporation of much of that contained in the Wratten book on “ Orthochromatic Filters ” which was also written by Dr. Mees. While purely scientific nomenclature and phraseology are not employed, no attempt has been made to be en- tirely u practical,” since the application of an ounce of accurate knowledge may be worth a ton of unreasoning practice. No pretence is made of being unbiassed, though it is hoped that there is no conscious bias. The Wratten products are freely discussed, but the loss of generaliza- tion caused by this procedure will be compensated by the advantage to be gained from definite information. The large number of friends who have kindly assisted in the compilation of this volume makes it impossible to acknowledge all by name, and it would perhaps be invidious to mention only a few. We hope that all will understand that although they are not named we are none the less grateful to them for their various sugges- tions, and that we shall also be grateful for suggestions for the improvement of future editions. EASTMAN KODAK CO. Rochester, N.Y. ■ CONTENTS CHAP. PAGE Preface ........ 3 I. The Nature of Colour ..... 7 II. The Sensitiveness to Coloured Light of the Eye and of Photographic Plates . . 19 III. Orthochromatic Filters .... 24 IV. The Efficiency of Orthochromatic Filters 31 V. The Multiplying Factor of any Sharp-Cut Filter ....... 39 VI. The Rendering of Colour Contrasts . . 47 VII. Portraiture . . . . . . .61 VIII. Landscape Photography .... 69 IX. The Photography of Coloured Objects for Reproduction ...... 77 X. Three-Colour Photography .... 82 XI. The Optical Properties of Filters . . 97 XII. The Fitting of Filters . . . .105 XIII. The Care of Filters . . . . .112 Index . . . . . . . 1 1 5 5 THE PHOTOGRAPHY OF COLOURED OBJECTS CHAPTER I THE NATURE OF COLOUR \ T the commencement of this book, which is essentially concerned with the analysis and photography of colour, it will be well for us to get a definite idea as to what is meant by “colour,” and with what physical phenomena colour is associated. The nature of colour is involved in the conception we obtain as to the nature of light. The nature of light has long been a source of speculation, and it was generally held that perception of light depended on the reception by the eye of small discrete particles shot off from the source of light ; just as at one time it was held that the perception of sound depended upon the impact upon the ear drum of small particles shot oft' from the sources of the sound. This theory of light has the advantage that it immediately explains reflection ; just as an indiarubber ball bounces from a smooth wall, while it will be shot in almost any direction by a heap of stones, so these small particles would rebound from a polished surface, while a rough surface would merely scatter them. This theory of the nature of light appeared adequate until it was found that it was possible, by dividing a beam of light and slightly lengthening the path of one of the halves, and then re-uniting them, to produce periods of darkness similar in nature to the nodes produced in an organ-pipe, where the interference of waves of sound is taking place. It could not be imagined that a reinforcement of one stream of particles by another stream of particles in the same dire&ion could produce / PHOTOGRAPHY OF COLOURED OBJECTS an absence of particles, while the analogy with sound suggested that, just as sound was known to consist of waves in the air, so light also consisted of waves. Light cannot consist of waves in the air, partly because we know that it travels through interstellar space, where we imagine that there is no air, but also because the velocity of light, nearly 200,000 miles per second, is so great that it is impossible that it could consist of a wave in any material substance with which we are acquainted. It is, however, supposed that there must exist, spread through all space and all matter, a substance which is termed the ether, and that light consists of waves in this ether. Now, just as in sound we have wave notes of high frequency, that is, with many waves per second falling upon the ear, which form the high-pitched or shrill notes, and also notes of low frequency, where only a few waves per second fall upon the ear, forming the bass notes — so with light we may have different frequencies of vibration, some falling upon the eye at very short intervals, while other waves are of only half or even less fre- quency. Since the velocity of light is the same for waves of different frequencies, it is clear that the waves of high frequency will be of shorter wave length than those of low frequency, the length of a light wave being the distance from the crest of one wave to the crest of the next. The wave length of the light, like the velocity, varies with the medium in which the light is travelling. For instance, when light is travelling through glass, it will only have about two-thirds of the wave length of the light travelling in the air. But it is convenient to consider simply the wave length of light as the length of the wave in free ether, or for pra&ical purposes, in air. White light consists of vibrations of many degrees of frequency, i.e. y it consists of waves of various lengths; and a mixture of waves of all lengths in certain proportions forms what we term white light. If instead of allowing this hetero- geneous mixture of waves to fall upon the eye, we omit waves of some frequencies from those entering the eye, then the brain will receive a sensation of colour. Thus colour is associated with wave length. White light being made up of waves of different lengths may be regarded as being made up of light of various colours, and by different devices may be split up into these colours. We can analyse white light or discover the composition of any light with the spectroscope, an instrument by means of 8 THE NATURE OF COLOUR RED GREEN which a small portion of the light to be examined is passed through a prism, or transmitted by, or reflected from a diffrac- tion grating. The result is that the light is split up into a band WAVE LENGTHS of different colours, which is known as the spedtrum. If the light analysed is white these colours merge into one another without any break, but there will be a break or breaks (ab- sorption bands) if the light ex- amined is coloured. Figure I shows the relative length of the waves corresponding with light of various colours, the diagram being drawn to scale. Since different length of waves correspond with different col- ours, a scale mav be made in which thedifferent wave-length numbers represented corre- BLUE spond in position with the dif- ferent colours of rhespedtrum. The following diagram gives a simple arrangement of the nor- mal spedtrum, the numbers re- p IG , presenting the length of the waves in Angstrom Units (A.U.), which are ten-millionths of millimetres, and the colours being placed against them : BLUE VIOLET 1 BLUE GREEN 1 GREEN T ORANCf rauiv 1 RED 4000 5000 6000 7000 Fig. 2 It will be seen that the visible spedtrum extends from 4,000 to 7,000, and is equally divided into regions which may be broadly termed : Blue-violet Green Red 4.000 to 5,000 5.000 „ 6,000 6.000 ,, 7,000 9 PHOTOGRAPHY OF COLOURED OBJECTS Light-filters, that is transparent media absorbing certain waves and transmitting others, can be constructed which will absorb some particular region of the speCtrum, and they are generally called after the colour they transmit; thus if we make a filter which only lets through the portion of the speCtrum between 4,000 and 5,000, then we should call that filter a blue- violet filter, a filter letting through from 5,00c to 6,000 would be a green filter, and a filter letting through from 6,000 to 7,000 would be red in colour. Thus from the spe&rum we already derive the idea that light can be divided into three colours which we may call the primary colours, red, green, and blue- violet. Remembering this conception of light, let us consider why we term a given filter red. It will appear red because it only lets through red light, but white light consisting of blue-violet, green, and red is falling upon it, so that clearly it is red because it stops or absorbs the blue-violet and green light. Similarly, a piece of red paper is red because it reflects red light, but it has falling upon it white light consisting of blue- violet, green, and red, so that it must absorb the blue-violet and green light, not reflecting them, but only reflecting the red light. We are therefore justified in saying that anything which absorbs blue-violet light and green light together will be red. It is this aspeCt of colour, that objeCts are coloured because they absorb, which must be clearly and definitely understood if the best results are to be obtained in the photography of coloured objeCfs. Unfortunately, however, the conception of colour as an absorption is not common, though it is the most useful one, and it will be necessary somewhat to elaborate this idea in order to prevent misconceptions arising. We should form the habit of considering a red objeCt, not as one that refleCts red, but as one that absorbs blue-violet and green. The importance of this definition is that it defines “red” without reference to the colour of the incident light, l ake a scarlet book and examine it by a light containing no red; such for instance as the mercury vapour lamp, in which red is almost entirely wanting. The book will no longer refleCt red light because there is no red light for it to refleCl, but it will still absorb the blue-violet and green light of the lamp, and will look black ; it has not, of course, changed its nature, and we should still be justified in saying that it is red if we define red as we have done above. In the same way a yellow objeCt is not one which refleCls yellow light (there is very little yellow light indeed in the 10 THE NATURE OF COLOUR spedlrum, and if an objedt refledted only yellow light it would be so dark as to be almost black), but a yellow colour is due to blue absorption. It reflects the other two components of white light, green and red, so that we should be justified in saying that yellow light consists of green light plus red light, but for our purpose let us consider yellow simply as a lack of blue; yellow is minus blue, so that if you have a beam of yellow light and add blue light to it, you will get white light. Now what is green? Well, since white light consists of blue light, green light, and red light, green is clearly white light minus red and minus blue; and a green body is one which BLUE GREEN RED t|BLUEt| YE LI GREEN -OW RED ||blue|| |green| RED ■MSI ||BLUE% GREEN fc|RED Fig. 3 absorbs both red and blue. The difference between a green objedt and a yellow objedt being that the yellow objedt absorbs blue only, whereas the green object also absorbs most of the red light which the yellow objedt refledts. We can now make clear what is meant by complementary colours. As is shown in the diagram (fig. 3), white light con- sists of blue light, green light, and red light. The next sedtion under this shows the blue blotted out, leaving the mixture of green and red — that is, yellow. We should say, then, that yellow is complementary to the blue-violet. In the same way, in the next diagram all blue and green are blotted out, leaving only red, so that red is complementary to blue-green. In the bottom PHOTOGRAPHY OF COLOURED OBJECTS diagram all blue and red are blotted out, leaving only green ; green is complementary to this blue-red mixture, which is usually known as magenta. In general, then, the light absorbed by an object may be said to be complementary to that reflected by it. So far we have only considered intense colours. We have imagined that a red objedl absorbs the whole of the blue-violet and the green light, that is to say that its absorption was com- plete. But most things have only partial absorption — the absorp- tion is incomplete. Partial absorption can be of two forms: it can be gradual, or it can be sharp; thus, if when taking a photo- graph of a spedlrum there is put in front of the spedtroscope Fig. 5. Gentian Violet Absorption Spectrum a solution of erythrosine then that erythrosine will absorb a clean patch of green from the spedtrum between 4,800 and 5,500, as is shown in the photograph (fig. 4). But if you put in front of the spedtroscope slit a cell containing gentian violet you will get a very gradual diminution of intensity between about 4,700 and 6,200, with the least light transmitted about 5,800 (fig. 5). Thus different dyes and different substances give different classes of absorption, the two kinds being roughly subdivided into (1) sharp absorptions, and (2) gradual absorptions. 12 THE NATURE OF COLOUR Let us examine the effect of a single sharp absorption band in different parts of the s-pedtrum. First, consider a sharp absorp- tion band situated in the red about 6,500 and producing a total absence of red in this part. The remaining colour consists of all the blue-violet and all the green, with some of the red. The actual visual effedt of the mixed colour is what one might term a “sky-blue.” Imagine this band to shift so as to absorb the CHART SHOWING RESIDUALS POSITION OF ABSORPTION BANDS RED GREEN BLUE Fig. 6 orange; absorbing between 5,800 and 6,200, the colour will be a light violet-blue, because there is a great deal of red being transmitted and less green. If the band shifts into the yellowish green from 5,600 and 6,000 it will absorb a great deal of the green and none of the red, and the colour will become bluish purple; as it shifts lower in the green towards the blue this purple becomes a reddish purple, so that when the band is situated at from 5,600 to 5,200 we have what is generally known as magenta in colour. As the band shifts towards the blue, the blue fades out of the magenta, green taking its place. When *3 PHOTOGRAPHY OF COLOURED OBJECTS the band is from 4,700 to 5,200 the colour is a sort of orange, and as the band moves into the blue-violet the orange becomes a yellow, and finally a lemon-yellow. So that if we imagine a single band to pass down the spedtrum, we get a change from light sky-blue through purple, magenta, orange, and yellow, to lemon-yellow (fig. 6). Now it will be seen that there is one class of colour which does not enter at all into this series, namely, the greens. There is really no visual suggestion of green in any colour formed by using a daylight spedtrum and absorbing one narrow band only. In order to get a green we must have an absorption both in the red and in the blue. If we absorb the extreme blue and also the extreme red, we shall at once get a green, and as these two bands vary with regard to each other, we shall obtain various shades of greens. 'Thus, if the blue absorption band is weak, and the red absorption band is very strong, we get blue-greens ; if the red absorption weak, and the blue strong, yellow-greens. Green is almost the only common colour due to two absorp- tion bands, and other colours which on analysis prove to have two absorption bands generally tend to be mere variants in hue of some colours which we have already discussed under the heading of single absorption bands. A brown colour is fairly common, and the bands of a brown are of a gradual absorption type generally extending through the blue-green with a trans- mission band in the violet — that is to say, a brown is really a degraded orange, and is a variant on the colour described as orange, resulting from a single absorption band in the blue- green. It i^ clear, therefore, that if a thing is coloured sky-blue it means that it is absorbing the deep red, a violet-blue object absorbs the orange, a purple the yellow-green, a magenta the central green, an orange the blue, a yellow the blue-violet, and a lemon-yellow only the extreme violet. If a sky-blue objedt be looked at through a piece of yellow glass it will be found to look bright green in colour, so that a green colour is produced by the absorption both of the red and of the blue, the blue objedt absorbing the red light and the yellow glass the blue light. Natural colours do not generally show sharp absorption bands, though the absorption bands produced by the stains used in microscopy are mostly fairly sharp. The same rule holds true, however; if a magenta objedt in nature does not signify a clean sharp absorption band in the green it still means that that objedt absorbs far more of the green than of any other colour, and, as 14 THE NATURE OF COLOUR regards photography, we can apply the rules deduced from theoretical residuals to natural colours. These rules are as follows : i. If a colour is to be rendered as black as possible then it must be photographed by light which is completely absorbed by Fig. 7. Theoretical and Actual Violet the colour; that is, by light of the wave-lengths comprised within its absorption band. 2. The second rule deals with the case where contrast is required, not against the background but within the object itself. The proper procedure in this case is to photograph the object by the light which it transmits. In fig. 7 we see a sharp absorption band depidted, which 4,000 5.000 6,000 7.000 Fig. 8. Theoretical and Actual Green would give rise to a violet-blue colour. An actual violet-blue, however, will have an absorption band of the type shown by the shading in the opposite direction on the diagram. Similarly, fig. 8 shows an ideal green, having sharp red and violet absorption and an adtual green with its gradual ending and absorption of the green itself. The sharpness of absorption bands is of great importance in rcspedt of the luminosity of the colours produced by them. *5 PHOTOGRAPHY OF COLOURED OBJECTS The side of an absorption band, which is toward the red end of the spedtrum, generally has a sharp edge, as shown in fig. 7, while that which is toward the blue end has a gradual edge, a considerable amount of absorption remaining even in the trans- mitted portions of the spedtrum. As a result, colours which are bounded by the sharp edges — that is, reds, oranges, and yellows — are bright colours, while colours which are bounded by the gradual edges — blue-greens, blues, and violets — are dark colours. A green will, as a general rule, have a sharp edge at its blue limit and a gradual edge at its red limit, and will consequently be of intermediate brightness. If we divide the spedtrum at 5,000 and at 6,000 so that we get three portions, 4,000 to 5,000 which we may term blue- violet, 5,000 to 6,000 which we may term green, and 6,000 to 7,000 which we may term red, then examination of the luminosity curve, given in Chapter II, fig. 10, will show that about of the whole light should be “ green,” about ^should be u red,” and about should be “blue.” But inasmuch as a bright red objedt will refledt nearly all the incident “ red ” light, while a bright green objedt will only refledt about ^ of the “green” light, and a bright blue objedt J of the “ blue” light; a red objedt will be the brightest, a green objedt less bright, and a blue objedt very dark indeed. A yellow, having only a single sharp absorption edge, is very bright. A yellow objedt usually refledts even more red light than a red objedt, and much more green light than a green objedt. Dr. Mees made a number of measurements of the absorption by various filters and colours of the light which they are sup- posed to transmit or refledt, with the following results: Pure Dye Filters Transmitting. Region for which Trans- Transparency was parency, measured. per cent. 5,900 to red end (tricolour red) 5,900 » » 4,800 to 6,000 (tricolour green) . 4,800 to 6,000 „ 4,000 to 5,100 (tricolour blue) 4,000 to 5,100 „ 4,000 to 4,700 (D) (methyl violet) 5.600 to red end (E) . 5.600 „ Same filter 75 6,100 to red end 78 Same filter 32 4 . 9 00 to 5,8oo 35-5 Same filter 1 1*5 4,000 to 4,800 1 6-5 4,000 to 4,600 15 Same filter 69 5,900 to red end 85 THE NATURE OF COLOUR Region for which Trans- Transmitting. Transparency was parency, measured. per cent. 5,100 to red end (G) Same filter 79 5,ioo „ 5,600 to red end 89 4,000 to 5,400 (H) . Same filter 14 4,000 to 5,400 „ . 4,600 to 5,100 16 4,600 to red end (K2) . Same filter 7 2 ‘5 4,600 „ • k 3 85 K3 • k 3 81 K 3 . 5,ioo to red end 86 Light naphthol green (about 4,500 to 6,500) .... . 4,900 to 5,700 40 Dark naphthol green . . 4,900 to 5,700 i 4*5 Xylen red (4,000 to 5,100), purest blue obtainable . 4,000 to 4,700 41 The chief points of interest are the luminosity of the yellows (K2, K3, G); orange (E); and red (A); the darkening in the greens, and even more in the blues and violets. Printing Inks Bright scarlet 5,900 to red end 83*5 V) 55 . 6,100 „ 88 Bright light blue . . 4,000 to 5,100 42 Dyed Wools These were kindly given by Dr. E. Koenig, of Hochst a/M, who stated that he considered them to be very good pure colours. Colour. Region of Percentagt Measurement. Reflected. Dark purple . . 4,000 to 4,800 9*5 Bright „ . 4,000 to 4,800 22 » » * . . 6,500 to red end 42 Dark blue . . . 4,000 to 4,700 l 9 Light „ . . . . 4,000 to 5,100 18 Dark blue-green . . 5,000 to 5,800 1 1 Light „ . 4,900 to 5,400 25 Dark yellow-green . 5,000 to 5,800 17-5 Bright „ . 4,800 to 6,100 . . 5,000 to 5,800 26-5 » » 39 Yellow . . 5,100 to red end 67 17 B PHOTOGRAPHY OF COLOURED OBJECTS Colour. Orange Scarlet Bright scarlet Deep red Region of Percentage Measurement. Refle&ed. 5,600 57*5 5,900 70 5 , 9 °° 61 6,100 68 6,100 7 1 6,100 49 6,500 77 18 CHAPTER II THE SENSITIVENESS TO COLOURED LIGHT OF THE EYE AND OF PHOTOGRAPHIC PLATES W E have seen that the eye distinguishes light of different wave lengths by the produdfion of an appearance of colour ; thus a ray of light containing waves of a length of 4,600 of our units would be called violet, while if the waves were of the length of 6,500 the resultant impression on the eye would be said to be deep red. But the sensitiveness of the eye is not the same for waves of different lengths, and if we attempt to represent in monochrome the band of coloured light called the spedlrum as it appears to the eve, it will look something like ^ A A COLOUR Invisible Limit ot Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 9. The Luminosity value of the Spectrum as it appears to the Eye fig. 9, the yellow-green light appearing brightest and the yellow, orange, and red light on one side, green, blue-green, and blue on the other appearing progressively darker until the violet appears very dark and the visible spe&rum ends. The eye cannot perceive at all waves below 4,000 units, i.e. y what is known as ultra-violet light; neither can it perceive rays which are above 7,000 units, so that to these we must regard the eye as insensitive. The eye is very little sensitive to the extreme violet rays between 4,000 and 4,500. The blue affects it more and appears, as we say, bright. Between 5,000 and 6,000 the green appears as the brightest part of the spectrum ; above 6,000 we have the bright reds, but the intensity rapidly " 19 PHOTOGRAPHY OF COLOURED OBJECTS falls off as the waves get longer, until beyond 7,000 we see practically nothing. We may also draw a curve showing the sensitiveness of the eye to the spedlrum. It will be noted that this curve has a maximum at about wave length 5,900, but this only holds for intense light. As the intensity of the light diminishes, not merely does the eye see less, but the relative sensi- tiveness of the colours changes somewhat, shifting towards the blue. This is what is known as “ Purkinje’s Phenomenon.” The explanation offered for it by Professor Schaum is suffici- ently interesting and little known to be worth repetition. It is known that the retina consists of rods and cones, of which the cones are considered to be colour-sensitive, and the rods colour- blind. In the part of the retina exactly opposite the centre of the pupil there is a small depression which contains no rods, but only cones, and here it is found that the Purkinje phenomenon 4.000 5,000 sooc* 7000 Fig. 10. is non-existent, so that the intensity maximum remains con- stant. So that we may conclude that the colour-sensitive cones alone display no Purkinje phenomenon, and that the phenomenon is due to the association of these cones with the colour-blind rods. It is found that the sensitiveness curve for this region containing only cones is identical with the curve of sensitiveness for great intensities of light, so that this is the curve of the cones. On the other hand, since the rods are much more sensi- tive to feeble intensities of light than the cones, as is shown by the fa£t that the sense of light remains after colour can no longer be distinguished, the sensitiveness curve of the rods will corre- spond with thecurve for minimum intensity ; so that for minimum intensity the sensitiveness curve is due to the rods alone, and as the intensity grows, the curve is more and more influenced by the cones, until with maximum intensity the curve of sensitive- ness is almost entirely determined by the cones. It is for the reason that in very weak lights the eye has a 20 SENSITIVENESS OF THE EYE AND PLATE maximum sensitiveness to a particular colour, namely, a green, that the safelights supplied for use in the dark-room when Wratten panchromatic plates are handled are of this green colour. The plate is sensitive to all colours, but an amount of green light can be used, if discretion is shown, that is sufficient to see by without too much danger of fogging the plate, whereas if a red were to be chosen, so much more would have to be used for it to make objedts visible that the plate would inevitably be fogged. Just as the eye is unequally sensitive to light of different colours, so a photographic plate is unequally sensitive to light of different colours. If we take an ordinary photographic plate and measure its sensitiveness, we shall find that it differs very markedly from that of the eye. The eye can see waves of no shorter length than 4,000 units; a photographic plate can see very much shorter waves, and can detedt light which is quite invisible to the eye, this light being usually called ultra-violet, because it is beyond the violet. Also the maximum of sensitive- ness of an ordinary plate is in the violet, and all the red, orange, and nearly all the green light is invisible to it. That is to say, the ordinary plate perceives objedts only by the blue and violet light which they refledl, and this is a grave fault in the plate when regarded as an instrument for perceiving and recording coloured objedts, because the record which it makes of coloured objedts differs entirely from that which the eye makes. It was found by Vogel in 1873 that, by treating plates with dyes, they could be given, besides their usual sensitiveness, a secondary sensitiveness in approximately the region of the spectrum which those dyes absorb. Thus if a plate is treated with a solution of erythrosine which absorbs the yellowish green, it will be sensitive to the yellow-green, besides being sensitive to the blue and violet. Plates which have been treated in this way are those which are known as u orthochromatic,” the word implying that they can render objedts in their true colour values. The ordinary commercial orthochromatic plate, which is usually made by putting some eosine or erythrosine into the emulsion, has a sensitiveness curve of the type shown in fig. 1 1 (b), and it will be seen at once, on comparing this with the sensitiveness curve of the eye, that, although the plate is certainly better in consequence of this treatment with erythro- sine, it cannot be described as at all comparable in sensitiveness with the eye. It has an enormous excess of sensitiveness in the blue and violet, it has the sensitiveness to the ultra-violet which the eye has not at all, it then has very little sensitiveness 21 PHOTOGRAPHY OF COLOURED OBJECTS indeed to the blue-green, a small maximum of sensitiveness in the yellow-green, and an absence of sensitiveness to the red. It may be assumed that if we take the blue to include the whole spedrum up to 5,000, the green to be the spe&rum from 5,000 to 6,000, and the red from 6,000 upwards, that the sensitive- ness of the ordinary orthochromatic plate is distributed in the 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 (B) Ordinary Ortho or Isochromatic Plate 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 Fig. 11. Different Types of Plate Sensitiveness Curves to Daylight ratio of 40 parts in the blue, one part in the green, and none in the red. If we assume for the sake of argument that the eye sees the three parts of the spectrum as of equal intensity, then the orthochromatic plate, besides the fa< 5 t that it is not sensitive to the red, has only -fo of the sensitiveness in the green that it would require to be equal in sensitiveness to the eye. If, however, instead of sensitising a plate in the wav we have described, we bathe the finished plate in a solution of certain of 22 SENSITIVENESS OF THE EYE AND PLATE the dyes called isocyanines, we can prepare a plate which is very much more sensitive both to the green and to the red. Messrs. Wratten and Wainwright, Ltd., were the first to succeed in preparing a plate commercially in this manner, the plate being sensitive to both green and red, and this plate they called the “Wratten Panchromatic Plate.” The plate is sensitive to the whole visible spe&rum ; although it has a considerable excess of sensitiveness in the blue, this excess is very much less than in the case of the ordinary orthochromatic plates, and there are no absences of sensitiveness throughout the whole spectrum. The distribution of sensitiveness in this plate is also shown in fig. io, and it may be said that of its sensitiveness is in the blue, in the green, and ^ in the red ; so that the sensitiveness to blue is seven times too great compared with the rest of the spe&rum, while the sensitiveness to green and red together is §- of that required to have the same sensitiveness as the eye. In order to attain the same relative sensitiveness as the eye, it is necessary with an ordinary orthochromatic plate or with the panchromatic plate, to use absorbing colour filters which shall diminish the excess of blue light, and it is the considera- tion of these colour filters and of the effect which they will have on the total sensitiveness of the plate, which must now be con- sidered. 23 CHAPTER III ORTHOCHROMATIC FILTERS T HE need for orthochromatic filters in photography is still insufficiently realized by many workers. For so many years photographers have been accustomed to the incorrect rendering of coloured objedts in monochrome which is given by ordinary photographic plates that a kind of photographic convention has been set up in their minds, so that a picture of a landscape in which grass is rendered as a dark patch, and a blue sky is almost white paper, is accepted without any feeling of its in- correctness; the more experienced a photographer, indeed, the more fixed this convention becomes, so that photographs taken under conditions which corredtly reproduce the luminosities of the subjedt may sometimes appear over-corredted to a worker who has become accustomed to a “ photographic rendering,” and in whose mind the reproduction of a scarlet as dead black would raise no question whatever. Recently, however, many photographers have become more critical in this respeCt, and it is beginning to be recognized that, so far as possible, photographs should corredtly translate into monochrome the luminosity values of colour as seen by the eye, and for this purpose corredtly adjusted orthochromatic filters and plates are a necessity. In order that we may understand the need for an ortho- chromatic filter let us take a simple example of a coloured objedt such as that presented by a yellow daffodil. If we photo- graph side by side a daffodil and a narcissus on an ordinary photographic plate we shall find that although to the eye the yellow daffodil appears almost as bright as the white narcissus, yet in the print (fig. 12) the daffodil appears much darker, the difference being especially marked in the more deeply coloured trumpet of the flower. Clearly the light which is refledted from the daffodil is deficient in some essential constituent which has much adtion on the photographic plate, although its loss does not make the flower seem much darker to the eye. If we examine the light refledted from the flower by means of a 24 ORTHOCHROMATIC FILTERS spectroscope it will at first sight appear that we have the same spectrum as we got at first from white light, but on closer in- spection we shall see that while the green, orange, and red regions are fully present, the blue light is dimmed and the violet light is almost completely absent. We thus see that the reason why the daffodil looks different from a white flower, appears “yellow” in faCt, is that it fails to refleCt all the con- stituents of white light, and absorbs the violet and blue con- Fig. i2. Daffodil and Narcissus on Ordinary Plate stituents. These violet and blue constituents of white light are thus shown to be those which have a strong aCIion upon a photo- graphic plate, although to the eye they appear dark. The frontispiece of this book shows a chart which is intended to represent the spe&rum colour both in hue and also approxim- ately in brightness and the strong aCfion of the violet and blue rays, and the feeble aCtion of the green, orange, and red rays is shown in fig. 13, which is a photograph of the frontispiece taken upon an ordinary fast plate. In this photograph the 25 PHOTOGRAPHY OF COLOURED OBJECTS violet, blue, and blue-green patches, which are darkest to the eye, are reproduced light, while the rest of the chart appears dark. If we take a photograph of the spe&rum of white light upon Fig. 13. Colour Chart on Ordinary Plate this plate we get the result shown in fig. 14. It will be seen that the plate sees the coloured rays in a very different manner from the eye. The red, orange, and green colours which are bright COLOUR Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 14. The Luminosity Value of the Spectrum as it appears to an Ordinary Plate to the eye appear quite dark to the plate, while the deep violet light which is very dark to the eye is the brightest colour to the plate, and in addition the plate will demonstrate that there are rays beyond the violet which are quite invisible to the eye. 26 ORTHOCHROMATIC FILTERS COLOUR TO EYE These ultra-violet rays are very strong in daylight and play an important part in the economy of the universe, being the Fig. 15. Daffodil and Narcissus on Panchromatic Plate (unscreened) chief cause of most of the effects which are produced by sun- light, such as the tanning of the skin, the fading of coloured A Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of Ultra-Violet Visibility Green Green Red Visibility Fig. 16. The Spectrum photographed on a Panchromatic Plate cloths, or the development of plants. They comprise, indeed, those constituents of white light which produces the “ chemical ” " 27 PHOTOGRAPHY OF COLOURED OBJECTS effedts of light, and naturally they play a great part in the essentially chemical adtion of the exposure of a photographic plate. The insensitiveness of a plate to the colours which are bright to the eye, and sensitiveness to those which are dark to the eye are of much importance in photography; in landscape photo- graphs, for instance, the grass, which because it absorbs the violet and blue rays and also some of the red rays, appears Fig. 17. Taken on a Wratten Panchromatic Plate with K3 Filter green, is always reproduced too dark, and white clouds are lost against the blue sky, although to the eye they appear much brighter, because the light from the blue sky is deficient in red rays, and these rays being bright to the eye their absence pro- duces a strong efFedt. To the plate, however, which is blind to the red rays, their presence or absence is indifferent, and con- sequently the blue skv and white clouds appear of nearly the same intensity. If we photograph the daffodil and narcissus on a Wratten panchromatic plate we shall obtain a result which approximates 28 ORTHOCHROMATIC FILTERS much more closely to the truth than did that which we got with the ordinary plate, fig. 12. It will be noticed that in order to photograph the flowers thev were placed in a vase; this was a white vase with a blue landscape painted upon it, and although the daffodils appear bright in the photograph, as they do to the eye, yet the blue design on the vase is almost invisible, although to the eye it stands out most distindly. A comparison of fig. 16 with fig. 9 will show the reason for this; the panchromatic plate is sensitive to green, orange, and red light, just as the eye is, but it is still very much too sensitive Fig. 18. Colour Chart on Wratten Panchromatic Plate with K3 Filter to the blue and violet light, and to the invisible ultra-violet rays. Owing to the great intensity of these blue, violet, and ultra- violet rays in daylight, most of the photographic action, even on a panchromatic plate, is produced by them, so that the advant- age gained by sensitising the plate to the green and red rays largely is lost by their effed being drowned by the violet and ultra-violet rays. With artificial light (excepting mercury vapour and enclosed arc) the more adinic rays are weak, and a panchromatic plate gives at once manifestly better results than an ordinary plate. In daylight we can secure a similar result if we modify the light reaching the plate by passing it through a filter or screen 29" PHOTOGRAPHY OF COLOURED OBJECTS (usually attached to the lens), which removes all the ultra-violet light and as much of the blue and violet light as is necessary. Such a filter or screen is called an a orthochromatic ” or M isochro- matic” filter. Fig. i 7 shows a photograph of the daffodils and narcissi in the vase taken through such a filter on a W ratten panchromatic plate, and it will be seen that not only are the flowers rendered corredtly in their relative tone values, but also the design on the vase is clearly defined, as it appears to the eye. In the same way fig. 18 shows a photograph of the frontis- piece taken through a corre&ly adjusted (K3) filter on a panchromatic plate and represents the colours of the chart in their relative luminosities as they appear to the eye. 30 CHAPTER IV THE EFFICIENCY OF ORTHOCHROMATIC FILTERS W E have seen that orthochromatic filters are designed to remove the ultra-violet and so much of the violet light as is necessary to compensate for the extra sensitiveness of the plate to those rays. Now in* removing this light the orthochromatic filter increases the necessary exposure, because if we remove those rays to which the plate is most sensitive we must compensate for it by exposing the plate fora longertime to the adfion of the remain- ing rays, and the amount of this increased exposure will clearly be dependent both on the proportion of the violet and the blue rays which are removed by the orthochromatic filter, and also upon the sensitiveness of the plate for the remaining rays (green, orange, and red), which are not removed by the filter. The number of times by which the exposure must be in- creased for a given filter with a given plate is called the multi- plying fattor of the filter, and this depends upon the plate with which it is used. It is meaningless to refer to filters as “ two times ” or “ four times ” filters. Now, since it is always desirable that we should be able to give as short an exposure as possible, what is required in a filter is that it should produce the greatest possible effedt with the least possible increase of exposure, so that a filter will be con- sidered most efficient when it produces the maximum result with the minimum multiplying factor. The ideal filter will, therefore, absorb all the ultra-violet light, and as much as is needful of the violet and blue light, but will transmit all the orange and red light which falls upon it. If a filter transmits any of the ultra-violet light, which it should absorb, or absorbs any of the green, orange, or red light, which it should transmit, then it will be more or less inefficient from that cause. An inefficient filter will, therefore, have a high multiplying factor compared with the correction which it will give, while, on the other hand, a low multiplying factor for 3 1 PHOTOGRAPHY OF COLOURED OBJECTS a filter may be due simply to insufficient correction. An efficient filter will have a low multiplying factor but will also give good correction. Some of the earlier filters were made of yellowish brown glass, and a few such filters are still issued. Such filters are very inefficient, producing only a small degree of correction because they transmit a good deal of the ultra-violet and violet light, while at the same time they require a very considerable increase of exposure because they absorb much of the green light and even some of the orange and red rays, which should be com- pletely transmitted. Much the same objections apply to the Fig. 19. Diagram painted in Chinese White on White Card filters which are sometimes met with made ot green glass. These usually absorb the ultra-violet and violet light in a fairly satisfactory way, but the very strong absorption which they possess for red light causes an unnecessarily great increase in exposure if they are used with panchromatic plates, while, as they absorb a considerable amount of the green light they can- not be considered efficient even if ordinary orthochromatic plates, insensitive to the red, be used. For orthochromatic work all filters which are not a clear yellow should be disregarded ; if the filter is laid down on a sheet of white paper it should appear a bright or pale yellow, according to its depth, but it should not appear brown, and if it 3 2 EFFICIENCY OF ORTHOCHROMATIC FILTERS does appear brown, then it will be unsatisfactory in correction, and will require an unnecessarily long exposure. Compared with a modern u K ” filter, a brown glass filter is as inefficient for photographic purposes as a speCtacle lens compared with a modern anastigmat. Nor is it sufficient for an orthochromatic filter to be yellow, for a yellow filter may still be inefficient, and the great criterion as to the efficiency of an orthochromatic filter which is clear yellow in colour, and which therefore absorbs only a minimum amount of the rays which it should transmit, is that the filter should completely absorb the ultra-violet light. We have seen that a photographic plate is extremely sensitive to ultra-violet light. Now some substances which are quite without colour to the eye strongly absorb ultra-violet light ; Chinese white, for instance, which is often used by artists for the high lights in drawings, and which appears quite white to the eye, absorbs ultra-violet light, so that when the drawings are photographed by an arc lamp upon wet collodion plates, which are chiefly sensitive to the ultra-violet, the Chinese white appears a dirty grey. Fig. 19 shows such a photograph of a diagram painted in Chinese white on a white card, the diagram being almost invisible to the eye but photographing as shown. Moreover, ultra-violet light is far more easily scattered by traces of mist in the atmosphere than visible light is, so much so, that when Professor R. W. Wood took photographs by means of the ultra-violet light only, using a special apparatus, he found that if one could see the rays which he was using, even the clear atmosphere of the United States would appear to be continually filled with mist; so that the well-known photo- graphic haze which so often spoils the distance in photographs taken on ordinary plates is due to the ultra-violet light, and our orthochromatic filter must be adjusted to cut out all of the ultra-violet light and just so much of the violet light as is neces- sary to produce exactly the effect of “ atmosphere ” which is seen by the eye. If too much violet light is removed by the filter all effect of atmosphere will be lost ; but this efFedf, known as “over-correction,” will be discussed later. Now some yellow dyes, while removing violet light quite satisfactorily, transmit a great deal of ultra-violet light, and it is indeed possible to use one such dye to produce an anti-ortho- chromatic filter; that is, a filter which will exaggerate instead of diminishing the false tone rendering to which photographic plates are prone. Until a few years ago the dyes which were mostly used for 33 c PHOTOGRAPHY OF COLOURED OBJECTS the making of orthochromatic filters, while they gave clear yellow films and were stable to light, were unsatisfa&ory in that, except when very strong, they transmitted more or less ultra-violet light, and only the introdudlion of new dyes a few years ago made it possible for the first time to prepare ortho- chromatic filters of almost ideal efficiency, combined with great stability to light. Such filters are prepared by us under the registered name of“K” filters. To illustrate the advance which the introduction of these filters marked there is shown in figs. 20 and 21 two photographs of the spectrum produced by the light of an electric arc burning between iron poles, the first taken through the K2 filter, the second through one of the best of the earlier filters, which is of almost the same depth to the eye and which requires about the same increase of exposure. It will be seen that while the green INVISIBLE ULTRA-VIOLET [VIOLET BLUE. GREEN RED . LIMIT OF VISI 6 ILITV Fig. 20. (a) K Filter Fig. 21. (b) Old Filter and red portions of the speCIrum are as bright through the K2 filter as through the older one, the violet portion is much fainter and the ultra-violet is altogether absent, while the old filter is shown to transmit a very considerable amount of ultra- violet light. Since the use of a filter is to compensate for the excess sensitiveness of even an orthochromatic or panchromatic plate to the violet and ultra-violet rays, it follows that plates of differ- ent degrees of sensitiveness will require filters of different kinds to produce the same effect as is seen by the eye. In the first place it is clear that no matter what filter be used, an ortho- chromatic plate which is not sensitive to red can never render tone values as they appear, from the very fadt that the plate is red blind and that, except in a few unfortunate cases, the eye is not. The most perfect filter, in fa£t, with such a plate can only give a result similar to that seen by a “colour-blind ” per- son. But even so it is not a matter of indifference what filter is 34 EFFICIENCY OF ORTHOCHROMATIC FILTERS used with a non-red-sensitive orthochromatic plate. If the filter be too strong the photograph will appear over-corrected. This over-corredtion will show itself chiefly in the manner previously referred to, that is, the atmosphere in the distance will be lost ; but also other unpleasant effects may be observed. In landscape, the sky may appear too dark (this is often also the effedt of under-exposure) and light grass may appear almost COLOUR Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 22. Absorption of Sharp cut Filter white; while in flower photography, yellow flowers maybe in- distinguishable from white ones. These defects are produced by a filter which too completely removes the violet and blue light, as depicted in the diagram (fig. 22), instead of simply diminishing them to the required extent, as shown in diagram (%. 23). Provided, however, that a filter is satisfactory in this respedt and A A COLOUR Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 23. Absorption of Correct Filter does not produce over-correction, while at the same time it com- pletely removes ultra-violet light, little is gained by adjusting a filter to a special orthochromatic plate, and the Ki, Ki^, and K2 filters, which have almost ideal efficiency and are free from any tendency to produce over-correction, will give results as satisfactory as can possibly be obtained on plates which are not sensitive to red. The K3 filter, however, is of rather a different type, and brings us to the consideration of panchromatic plates. 35 PHOTOGRAPHY OF COLOURED OBJECTS When panchromatic plates are used, since they are sensitive to light of all colours, it is possible to use a filter which will produce upon the plates a tone-rendering identical with that perceived by the normal eye, but in order to do this the filter must be carefully adjusted to the plate for which it is intended. The older plates issued as panchromatic had very little sensi- tiveness to red and it was consequently necessary, in order to get correct rendering, to use with those plates filters which absorbed not only ultra-violet, violet, and blue light, but also green and even yellow-green light, thus allowing the undimin- ished red rays to produce their just share of aCtion upon the plate. Similarly it is conceivable that a plate having an excess of sensitiveness to red and but little sensitiveness to green, might require a green filter to absorb the red rays as well as the violet and ultra-violet rays in order to allow the green to aCt more COLOUR Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 24. Photograph of the Spectrum on an Orthochromatic Plate fully. All such filters as these require a very great increase of exposure, and for this reason they came but little into use. But fortunately the W ratten panchromatic plate is sensitive in the right proportion to green, orange, and red light, and all that is necessary to produce with this plate an exactly correct render- ing of tone values is a clear yellow filter, removing the ultra- violet, and absorbing the violet and blue to a slightly greater extent than the K2 filter, thus requiring an exposure of not more than five times that needed for the unscreened plates. Such a filter is the K3 filter, which, however, is not to be re- commended for use with other plates than the panchromatic plates, because, as a reference to fig. 24 will show, ordinary orthochromatic plates have a band of insensitiveness to the blue and blue-green rays, and the K3 filter, by absorbing these rays to some extent, accentuates this defeat. While the K3 filter is required to produce full correction upon the Wratten panchromatic plate, so that the photographic ren- 36 EFFICIENCY OF ORTHOCHROMATIC FILTERS dering of tone values is the same as that perceived by the eye, yet very satisfactory results are obtainable by the use of lighter filters. From the point of view of correction it is of as great importance that the plate should be strongly sensitive to the green, orange, and red light as that the filter should be efficient and of sufficient depth, so that a W ratten panchromatic plate used without a filter at all will, in some cases, give results superior to the much less colour-sensitive orthochromatic plate used with a filter increasing the exposure four or five times. Moreover, the correcting aCtion of such weak filters increases with the colour-sensitiveness of the plate, while the more colour-sensitive the plate the lower the multiplying faCtor of the filter. Conse- quently, for satisfactory orthochromatic work the first essential is that the plate shall be as colour-sensitive as possible, and then the choice of filter must be governed largely by the exposure which can be given, the K3 being used where full correction is desired, and where the duration of the exposure is of but little importance. For all-round work, however, the K2 filter will be found the most useful. When used with the Wratten panchromatic plate it gives a degree of correction slightly less than that obtained with the K3, but it requires only two-thirds of the exposure needed for the deeper screen. Where short exposure is of greater importance than full cor- rection the K.i J or the Ki filter should be employed ; the latter requires only half the exposure needed for the K2 and notably improves the colour rendering as compared with that given by the unscreened plate. This filter is also largely used for hand camera work, and the advantage obtained, even with such a weak filter, is very manifest in the results. In some classes of landscape work it is desirable to produce over-correCtion ; in surveying or tele-photographic work, for instance, where the utmost clearness and detail are desired rather than a piCtorial rendering, it is necessary to remove all haze and atmosphere, and for this purpose, a strong yellow filter, such as the Wratten “ G ” filter, is best. Owing to the depth of this filter, however, it can only satis- factorily be used with panchromatic plates, because with less sensitive plates its multiplying faCtor is very great, and the exposure, which in any case is often considerable in tele-photo- graphy, becomes impracticable. When selecting filters for telephoto work a K2 filter should be obtained as well as a “ G,” because in many cases the lighter filter is all that is required, and it has the advantage of requiring a shorter exposure. 37 PHOTOGRAPHY OF COLOURED OBJECTS ^For the photography of cloud forms against a blue sky a red filter may be used with great advantage, and the u A” filter is very suitable for this work. The results obtained show, of course, a greatly exaggerated contrast, but if the form of the clouds is all that is required such an exaggeration is not a dis- advantage, though we should not recommend the use of so deep a filter in pidtorial work. 3 * CHAPTER V THE MULTIPLYING FACTOR OF ANY SHARP-CUT FILTER S UPPOSE that we have a filter which has a perfedtly sharp absorption — that is to say, which cuts a clean sedtion out of the spedtrum, passing only light between two definite wave lengths, and without any absorption of that light — then, if we wish to find the multiplying fadtor of this filter, we must con- sider it in relation to the sensitiveness curve of the plate. It will be convenient first to consider a filter which does not transmit light below 5,000 A.U., which absorbs the whole of the ultra-violet and blue-violet, but does not absorb any green 4,00 0 5,000 6,000 tooo Fig. 25. Sharp-cut Filter on Erythrosine and Panchromatic Plates or any red. This filter will be a bright yellow in colour, yellow being, as we have seen, made up of green light and red light — that is to say, yellow being simply an absorption of blue. Con- sider the effedl of this on an orthochromatic plate which has ^ of its sensitiveness in the blue and ^ in the green. The yellow screen will remove all the blue light, i.e. y of the adtive light, and it will increase the required exposure 40 times, so that it is what we term a 40 times screen. Now consider the same screen to be used with the W ratten panchromatic plate. With this plate £ of the whole sensitive- ness is in the blue, J- in the red and green. The screen will then remove J- of the adtive light, leaving only -J- to adt; it will 39 PHOTOGRAPHY OF COLOURED OBJECTS increase the exposure 8 times. This example shows at once the intimate relation between the plate and the multiplying factor of a filter. Take now a filter cutting the spedtrum sharply at 5,500. This screen will be bright orange in colour. It transmits all the yellow-green, orange, and red light. It absorbs the blue- violet and blue-green light, /.*., adopting our convention as to the division of the spedtrum — it transmits the red and half the green, and absorbs the blue and half the green. The efFedt of this on the ordinary orthochromatic plate is to remove the blue sensitiveness, of the whole sensitiveness of the plate ; but inasmuch as this plate is not sensitive to the blue-green, and the yellow-green region of sensitiveness which represents the other Jg- of the sensitiveness of the plate is transmitted by the filter undiminished, the filter will only increase the exposure 4,000 5.000 6.000 7000 Fig. 26. Sharp-cut Orange Filter on Erythrosine and Pan- chromatic Plates 40 times, being the same increase as is shown by the former screen. On the panchromatic plate, however, the matter is different, of the sensitiveness of the plate is in the blue, and is removed by the filter, ^ is in the green, and half of this is removed by the filter; so that the sensitiveness left is y 1 ^, due to the un- diminished red-sensitiveness, and being half of the green sensitiveness — the total residual sensitiveness, therefore, being fV of the original sensitiveness, and this filter will, on the Wratten panchromatic plate, increase the necessary exposure 1 0§ times. Again, consider a filter cutting the spedtrum at 6,000 — that is, transmitting all the red, but absorbing all the blue and all the green. The ordinary orthochromatic plate has no appreciable sensitiveness in the red, and therefore could not be used in pradtice with such a screen. The Wratten panchromatic has 40 THE FACTOR OF A SHARP-CUT FILTER y 1 ^ of its total sensitiveness in the red, and consequent!) this red filter will, on that plate, be a 16 times screen. Let us now examine into the multiplying factor of the filter which will give correct reproduction of red, green, and blue, a* seen by the eye. We have assumed in all these figures that, in order to get correct reproduction, the sensitiveness for red, green,, and blue should be equal ; that is, we have chosen our units with that condition in mind. On the orthochromatic plate we have no red sensitiveness, but the nearest approximation to cor- reCt rendering that we are able to obtain will be given if the green and blue are of equal intensities, we require a sensitive- ness in the blue equal to the sensitiveness in the green. The sensitiveness of this plate in the green is ^ of its total sensitive- ness, so that we must use a screen which will give us of its total sensitiveness, y 1 ^ being in the green, and in the blue. 4000 5,000 COOC T,000 Fig. 27. Orthochromatic Filter on Erythrosine and Pan- chromatic Plates That is, it must cut off 38 of the 39 parts of blue sensitiveness which the plate has, and the screen will increase the exposure 20 times. With the Wratten panchromatic plate we have T V of the sensitiveness in the red, and y 1 ^ in the green, consequently we must have ^ in the blue; that is, the total sensitiveness will be and the increase of exposure required by the filter will be 5^ times. This filter will reduce the 4 sensitiveness of the blue to y 1 ^, it will remove of the blue sensitiveness. Two points must be noted here : First that the panchromatic plate will require very much less exposure to correCf it fullv than will the orthochromatic plate,, and secondly that, not only is less exposure required, but that a lighter screen is necessary; that is to say, in the one case we had to remove all but of the blue, but in the other j 1 ^ of the blue was left, and consequently a screen which would give the 4 1 PHOTOGRAPHY OF COLOURED OBJECTS maximum correction obtainable on an ordinary orthochromatic plate will over-correct the panchromatic plate. When working by artificial light (except enclosed arc lamps) the proportion of “ colour ” sensitiveness rises, and that of “ violet ” sensitiveness falls, and these alterations greatly affeCt the multiplying factors of filters, as well as the relative sensitive- ness of orthochromatic and panchromatic plates as compared with ordinary plates. With incandescent gas almost full correction upon the Wratten panchromatic plate is obtained by the use of the Ki filter. A point of some interest, which is occasionally referred to in the photographic press, is the multiplying factor of two filters used, the one on the top of the other. It is often put as follows: Suppose we have two filters — a three times filter and a five times filter — how much will they increase exposure if used together? The increase can be found neither by adding nor multiplying the separate faCtors of each filter, but the answer must depend entirely on the nature of the filters, and somewhat on the plate. For instance, one might be a deep violet filter and the other a strong yellow, in which case it would be possible for them together to let through no light at all! On the other hand, with a panchromatic plate, if one were a K3 filter and the other a filter in depth between the Ki and the K2, the effedl would be negligible, and the multiplying faCtor (and cor- rection) of the combined filter would be the same as that of the first one (K3) used alone. If the plate is to be taken as an ordinary orthochromatic plate and the filters are clear yellow filters, one being a Ki filter and the other a filter between Ki and K2, the combined multiplying faCtor would be about 7. If the filters were brown glass filters, of the type formerly used, the combined faCtor would probably be about 10. While this paragraph may serve to inform some who have puzzled over the question, it is not to be taken as recommend- ing the use of two filters together. Such a procedure is not at all desirable, especially on optical grounds. Contrast Filters These filters differ from orthochromatic filters in that it is not desired to obtain in them a gradually increasing absorption as shown in fig. 23, but as sharp a transition as possible between the region of absorption and that of transmission. A red contrast filter (such as the w A ” filter), for instance, 42 THE FACTOR OF A SHARP-CUT FILTER when examined in a speCtroscope will be seen to give a spectrum like fig. 28 in which the absorption is complete up to the point where the yellow-green passes rapidly through yellow into orange, and at this point the absorption falls suddenly to almost nothing, practically all the orange and red light being trans- mitted. To find the faCtors of these contrast filters we require to know only the relative sensitiveness of the plate to the part of the light transmitted compared with its sensitiveness to the part of the light absorbed. Assuming that a panchromatic plate has about r *g- of its total sensitiveness in the red and orange por- tions of the speCtrum, about another being in the green, yellow-green, and blue-green portions, while the remaining f is in the blue, violet, and ultra-violet regions, the factor of the “ A ” filter is consequently 16, since it transmits only the COLOUR Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of TO EYE Ultra-Violet Visibility Green Green Red Visibility Fig. 28. Spectrum transmitted by “A" Filter red and orange rays, to which the plate has ^ of its sensi- tiveness. A useful strong yellow contrast filter is the “G” filter. This filter absorbs all the ultra-violet, violet, blue, and blue- green light, transmitting the remainder. On the Wratten panchromatic plate it has a multiplying factor of 8, with daylight. Other contrast filters have approximately the following faCtors on the Wratten panchromatic plate: Filter. Colour. tJ Multiplying Fa&or. A. Red. Tricolour work, Mahogany Fur- niture, Cloud photography 16 B. Green. Tricolour work . 16 C Blue. Tricolour work . 6 E. Orange. T wo-colour work, Contrast filter 12 F. Strong Red. Copying Blue Prints, Screenplate Analysis .... 43 30 “P” Filter “L” Filter Fig. 29. Colour Chart reproduced through various Wratten Contrast Filters i i 4 + THE FACTOR OF A SHARP-CUT FILTER Filter. Colour. Use. Multiplying Fa&oi G. Yellow. Telephotography, Furniture, General Contrast . 8 L. Violet. Screenplate Analysis . b N. Strong Green. Screenplate Analysis . .24 P. Blue-Green. Two-colour work,Cop\ ingType- writing • ’ • 15 R. Deep Red. Contrast . . 80 Aesculine. Colourless. Photography or drawings containing Chinese White . on Process Plate Infra-red filter, after Professor R. W. Wood on the SpeCtrum Panchromatic plate 3,000 The results obtained by photographing the frontispiece through these contrast filters are shown in fig. 29. Orthochromatic Plate con- taining its own filter 7°° 6,400 Fig. 32. W. L. of Light 51 PHOTOGRAPHY OF COLOURED OBJECTS block; being blocked up in detail, this gives the maximum degree of contrast (fig. 32). Photographing at 5,700, on the border of the absorption band, we get a considerably lessened contrast, which for this particular sedtion will give us the best result. There is plenty of detail in the section, while at the same time the contrast is sufficient for reproduction purposes. Photographing at 6,400 in the red, and in the light which is completely transmitted by the section, the photograph has no contrast, is very flat, and results are useless. So that for the maximum contrast we must photograph in the absorption band. For example, it is sometimes necessary to copy a print of which the paper has become yellow with age. An ordinary plate is sensitive only to the ultra-violet, violet, and blue rays, which are more or less absorbed by the yellow paper, so that if a negative is made of such a print on an ordinary plate the repro- duction of the yellow paper will appear dark, or, in any case, dirty. If a colour-sensitive plate is used with a yellow contrast filter the yellow stain will have no efteCt and will fail to photo- graph. It should be noted that the yellow filter for such a pur- pose should not be an orthochromatic filter if the best results are required, but a much stronger filter, such as the W ratten “ G ” filter, because an orthochromatic filter is adjusted to photograph objeCts in their relative luminosities as seen by the eye, and if the yellow stain is visible to the eye it will also photograph through an orthochromatic filter. If the stain be examined through the strong “ G ” filter there will reach the eye no light which is not yellow, and so the stain will not appear different from the white ground. Fig. 33 shows two photographs of a platinotype print which had been splashed with yellow dye so as to leave a yellow stain. In the upper photograph, taken on an ordinary plate, the stain appears quite black, while in the lower one, for which a pan- chromatic plate has been used with a “ G ” filter, the stain has entirely disappeared, only a trace, which cannot be reproduced, being visible in the negative. Another difficulty is sometimes met with in copying prints due to the faCt that the prints are of a brown colour, such as is given by sepia platinotype, carbon, or toned bromide prints. This brown colour has a very much stronger absorption for the violet light, to which the plate is sensitive, than for the yellow- green and orange light, which represents the maximum sensitive- ness of the eye, and consequently such prints when photo- graphed on a 11011-colour sensitive plate give negatives having far too much contrast, and with blocked up shadows, and it will 52 On Ordinary Piate On Panchromatic Plate with “G” Filter Fig. 33. Yellow-stained Platinotype Print 53 PHOTOGRAPHY OF COLOURED OBJECTS generally be found that no increase of exposure will reproduce satisfactorily such photographs. The obvious course is to photo- graph them as the eye sees them, that is by means of a fully correcting filter and a panchromatic plate. A difficult task without the proper plate and filter is the photography of engineers’ and architects’ blue prints. Ordinary orthochromatic plates with yellow filters do not give the best results with such a subjeCt because a great deal of the yellow- green light to which such plates are sensitive is reflected by the blue, and in order to obtain really first-rate results the M A ” or u F” filter should be used with “Process Panchromatic” plates, thus photographing the print by red light which is completely absorbed by the blue colour. With such a plate and filter the results from a blue print are in every way as satisfactory as could be obtained from a black and white print in the ordinary way (fig. 34). Suppose, to take another example, we have a sheet of type- writing, with corrections in red ink; the violet typewriting absorbs the whole of the orange and green, the red ink absorbs only the green. If we photograph through the green filter, u B,” of the tricolour set, we shall get both the typewriting and the red ink completely black, and therefore the greatest contrast which can be obtained (fig. 35). If, on the other hand, we photograph through the red “ A ” filter, the typewriting will ap- pear plainly visible, but the red ink will show so little contrast that it easily can be intensified out of existence, and we can make a reproduction of the sheet showing the typewriting only (%• 3 6 )- Most commercial work, such as catalogue illustrations of carpets, wall papers, linoleums, china, marble, etc., can best be accomplished by the aid of the K3 filter on the panchromatic plate, but occasionally a red or green filter will be found ex- tremely useful. For general commercial photography the follow- ing set of filters will be found to cover most requirements: Ki, K2, K3, tricolour set (red, green, and blue), strong red (“ F ”), and strong yellow ( M G ”). For one branch of commercial photography, furniture work, the tricolour red “A” and the yellow “G” filters are in- valuable; the “A” filter used with the panchromatic plate giving a splendid rendering of the grain of red mahogany such as can be obtained in no other way. For satinwood and inlaid work the “ G ” filter is required, so that for furniture photo- graphy a set comprising the K3, “ G,” and “A” filters should be obtained. 54 ( b ) Blue print photographed on Panchromatic Plate through “A” Filter Fig. 34 55 PHOTOGRAPHY OF COLOURED OBJECTS Subjects for which a correctly orthochromatic rendering is particularly desirable are postage stamps and reproductions of coloured advertisements, such as posters. In copying maps a K3 filter must be used if the map con- The typewriting is in violet ink. The script, including correction, is in red ink. 6c In photographing typewriting, a green screen must^uaed: if there are any red ink corrections, the green screen will record these also 4) a filter has been used in which the astigmatism is very severe. If a filter shows astigmatism the set of black lines running in one direction will have a focus in a different plane to the set of lines running at right angles to them, so that one set must always be out of focus ; in the case of bad astigmatism it is pos- sible that one set of lines will disappear entirely when the other set are focussed sharply, so that in Fig. 53 ( a ) shows one set of lines in focus, and ( b ) those at right angles to them. We require of filters in B glass that at full aperture the image shall be free from astigmatism and that the definition shall be such that filters up to two inches in diameter shall not visibly degrade the definition of a lens of six inches focus used at F 4.5. Filters between two inches and three inches in diameter must give good definition on a lens of ten inches focus used at F 4.5, 101 PHOTOGRAPHY OF COLOURED OBJECTS while filters above three inches in diameter must give good de- finition on a lens of sixteen inches focus used at F 8. For very long focus, or telephoto, lenses only flats will give satisfactory results, and for the semi-telephoto lenses, now being Fig. 51. Filter-Test Object Fig. 52. Test Object through Astigmatic Filter introduced, it is also desirable that filters should be cemented in glass of the highest quality. On stopping down the definition given by the filter should improve as the aperture is diminished. In addition to giving satisfactory definition it is necessarv (*) ( 6 ) Fig. 53 that a filter should not alter the focus when it is used upon a lens. A camera should always be focussed with the filter in position, but the use of focussing scales upon many small cameras renders it necessary that filters for hand camera use should not affeCt the focal plane of the lens. *A filter can alter the distance 102 1 OPTICAL PROPERTIES OF FILTERS between the lens and its local plane in two ways, it may ad as a weak supplementary lens, or it may, if behind the lens, pro- duce an effed due to its thickness. A u K ” filter is tested not to affed the focal plane of a six inch lens by a greater amount than i 50 inch when used on the front of the lens. If a filter is used behind the lens, the lens must be moved back by an amount equal to about one-third of the thickness of the filter, but this rule assumes that the filter Fig. 54. Set of Tricolour Filters with Fourth Printer, CEMENTED IN FLATS does not ad in any way as a lens, and it is probable that some filters, which have been noted as not corresponding to the rule, really aded to some extent as lenses. Besides the test for definition, which we have described, a set of filters, such as tricolour filters, which are to work together, must be tested for another optical requirement. 'They must give images of the same size, so that they will register when printed one upon another. It is obvious that if the filters are used behind the lens, or 103 PHOTOGRAPHY OF COLOURED OBJECTS even if they are used in front of the lens and the object be near the lens (as in ordinary picture copying or process work), that the filters must all be of the same thickness, and that the shorter the focal length of the lens the greater the error in register (for the same size of image) which a difference in thickness will introduce. For some time it was thought that equal thickness was the main necessity for register, but when the construction of the large Wratten filter testing instrument showed that filters vary also in their effeCt upon the focal length of the lens, it appeared that a complete theoretical and practical investigation of the optical conditions governing register was desirable. In order to measure the accuracy of registration, an instru- ment was constructed in which a lens of about ten inches focal length formed images eight inches apart of two sharply defined sets of lines, and the exaCt distance between these sets of lines could be measured by means of two microscopes mounted on carriages aCtuated by micrometer screws. Filters can be placed in front of the lens and the effeCt of these filters upon the size of the images can then be measured very accurately. The tests upon this instrument have completely confirmed the theoretical sizes calculated from the known laws of geo- metrical optics, and the information which has been obtained is of considerable use in ensuring that sets of tricolour filters will give satisfactory register, while tricolour filters cemented in flats ( u A ” glass filters) can be guaranteed to show perfect register under even the most trying conditions. All filters issued by the Eastman Kodak Company are tested on the two instruments described for definition, accuracy of focus, and, in the case of tricolour filters, for register. CHAPTER XII THE FITTING OF FILTERS F ILTERS can be fitted either before or behind the lens, or just in front of the plate in a repeating back or special dark slide. This last position has the advantage that glass of lower optical quality can be used, but much larger filters are required and any speck or mark upon the filter shows in every negative, so that for orthochromatic filters a lens fitting is certainly to be preferred. Such filters should preferably be fitted in front of the lens, as in this position no appreciable shift of focus is introduced For Flats For Filters in “ B” Glass Fig. 55. Screw Cells by the thickness of the glass, and the filters are also more readily accessible. Film filters may be conveniently placed between the lens components, but this cannot be done with cemented filters, because the introduction of a piece of glass would seriously affeCt the corrections of many lenses ; even gelatine film should not be put inside a Cooke lens, which, owing to its construction, is sensitive to small alterations in the air spaces. The neatest method of fitting filters on the front of a lens is the mounting of the filter in a screw cell (see fig. 55), which is then screwed into the thread cut inside the front lens cell, if the lens has such a screw thread. This cell, having the same 105 PHOTOGRAPHY OF COLOURED OBJECTS diameter as the lens barrel, enables the same lens cap to be used, or if a roller blind shutter or lens hood is used it will fit on to the screw cell in the same way as on to the lens. In order that a filter may be fitted in this way, either the lens or the front combination must be sent when ordering such a fitting, as the cell cannot be made without actual fitting to the screw thread. Screw cells which are made for filters consisting of a single glass, such as those sometimes issued by camera makers, contain- ing a brown glass filter, cannot be used for holding cemented filters, but must be replaced by a deeper cell, designed to hold the thicker filter. They a£t, however, as admirable patterns for the preparation of the new cell, and if such a cell can be sent there is no need to part with the lens. On no account should For Flats For Filters in “ B 5 Glass Fig. 56. Slip-on Cells cemented filters be held in cells by burnishing the edge of the rim over, as the pressure is very likely to strain the filter. A filter cell should always be designed so that the filter is held securely but without pressure, and if the filter is fastened in place by a screw ring this ring should have a shoulder and and should be turned down, so that when it is screwed home the filter can easily be turned round by holding it between the thumb and finger, but will not give any side shake. Another method of fitting, which is rather less expensive, is to have the filter mounted in a light metal cell, which is slipped on to the lens like a lens cap (see fig. 56). This method of fitting has the advantage that the filter can be readily removed or changed. For this form of fitting it is only necessary to send the outside measurement of the lens, but it is necessary that this measure- ment should be made very exactly. If a pair of sliding callipers cannot be obtained a strip of hard writing paper should be 106 THE FITTING OF FILTERS wrapped round the lens so that the ends overlap, and then the two pieces of paper, where they just overlap, should be cut through in position with a sharp knife (see fig. 57). An attempt to cut a strip of paper which will just go round the lens is un- likely to result in a measurement sufficiently accurate to ensure a well-fitting cell. When slip-011 metal cells are used there is some danger of pulling the cell off if a lens cap be used over it, but such a cap is unnecessary if a between-lens shutter be used. With a Roller Blind Shutter it is better to fit the cell on the back of the lens, Fig. 57. Cutting a Slip of Paper to fit a Lens unless a screw cell can be used, thus leaving the front free for the shutter. Filters may also be fitted into velvet coloured plugs to go inside lens hoods, so that the same lens cap can be used, or a Roller Blind Shutter will fit over the lens without difficulty. Roller Blind Shutters can be adapted to take filters by fastening wooden or metal grooves to the front of them, and using square unmounted filters, or a similar fitting can be slipped on to the lens; the filters can be easily changed, and this method of fitting will be found quite satisfactory in practice, fig. 58. In order that the same filter may be used on different lenses adjustable holders are supplied. These are made to contain circular filters, and fit on lenses having diameters from } to 1 J 107 PHOTOGRAPHY OF COLOURED OBJECTS inch, £ to if inch, and f- to 2f inches. Adjustable holders are also made for square filters to go on lenses of diameter from -J to if inch and from if to 2f inches (figs. 59, 60, and 61). With filters cemented in flats it is necessary that the filter should be wider than the front lens component, because other- Fig. 58. Slip-on Cell to take Square Filters wise the filter, being of considerable thickness, wilPtend to cut down the angular aperture of the lens. The additional width necessitated can be found approximately in the following manner (see fig. 62): On a piece of paper draw a line equal to the focal length of the lens, at right angles to this draw a^line equal Fig. 59. Adjustable Filter Holder with Square Filter Fig. 60. Adjustable Filter Holder with Circular Filter to half the diagonal of the plate with which the lens is used. Now extend the first line for a distance equal to the distance from the diaphragm to the edge of the hood of the lens plus three-quarters of an inch, and at right angles to this draw another line. If now we join our starting point to the end of the line representing the diagonal of the plate, and produce this 108 THE FITTING OF FILTERS until it cuts the last line drawn, the length of that line which it cuts off represents half the necessary width of the filter. Fig. 6i. Eastman Adjustable Filter Holder Since filters in flats must be wider than the lens with which they are to be used, they should be fitted in special out-built Fig. 62 cells, which preferably screw into the mount of the front lens component, though these also may slip on to the lens barrel (see figs.^55 and 56). 109 PHOTOGRAPHY OF COLOURED OBJECTS In order that there should be no mistake when ordering filters in “ A ” glass (flats) the following particulars should be given : 1. The name of the lens. 2. The focal length. 3. Maximum working aperture. 4. Length and diameter of lens barrel, over all. 5. Size of plate used. 6 . If used for other than ordinary infinity work give average extension of camera. 7. If screw cells are required it is necessary to send front combination, and if slip-011 cells are required it is better to send it so that a good fit may be ensured over the hood. Fig. 63. Tricolour Slide-past Fitting The most satisfaCtorv way of fitting a set of filters in flats to two or more lenses is to have screw cells to fit the largest lens and then to obtain adapters which can be screwed on to the smaller lens and carry the screw cell. The above mentioned adjustable holders can also be made for filters in “ A ” glass, but they are not to be recommended for these heavy filters. Sets of tricolour filters, if small, are best fitted in a repeating back, and used with a dark slide carrying long plates; the three negatives being taken side by side on the same plate. Where larger sizes are required, or where it is not desired to use plates of special sizes, the filters may be fitted in a frame which can slide behind the lens through an outside protecting cover screwed to the lens panel, thus changing the filters somewhat in the 1 10 THE FITTING OF FILTERS manner in which lantern slides are changed by a lantern slide carrier. Although this method is rather slow in changing — as the dark slide must be changed as well as the filter — it is in other respeCts a satisfactory fitting (fig. 63). It is also possible to make a holder, either of metal or of wood, to fit on to the lens, into which the three filters can be slipped in turn; this holder can also be used for any other filters in addition to the tricolour set. It is not advisable to use screw cells for tricolour filters, as the changing of the cell is almost sure to shake the camera, and involves much loss of time. For process cameras special holders for illters are advisable, and one holder in which each filter is put in place separately is probably the most satisfactory. In such work great care is necessary that each filter is put in the same place, and exadtly square to the lens. One way up may be better than another, and all Wratten three-colour filters are marked to show the best way to insert the filter into the holder. 1 1 1 CHAPTER XIII THE CARE OF FILTERS I N its simplest form (gelatine film) a filter requires a consider- able amount of care in handling. The safest way is to place it between the combinations of a lens and leave it there. If it be used in front of or behind the lens in any form of carrier it should be removed after use and placed, in clean paper, between the leaves of a book, where it will keep flat and dry. Moisture tends to cloud gelatine film filters. The fingers are almost in- variably moist and, to a certain extent, greasy, hence in handling gelatine films care should be exercised to hold them by the ex- treme corner, if the filters be square, or, better still, by the edges only. If it be necessary to cut the film it should be placed between two clean pieces of fairly stiff paper, note-paper for instance, and cut with a sharp pair of scissors. A knife can also be used if the film is firmly held between two pieces of glass and trimmed round, taking care to cut and not simply to pull with the knife, as the film very easily splinters and cracks. A film filter may be dusted with a piece of soft silk, or per- haps a better plan is to use the edge of a piece of very fine tissue paper. The latter will not scratch if used carefully, and does not leave any fluff on the film. Cemented filters should be treated with care equal to that accorded to lenses, they should be kept in their cases and on no account allowed to get damp or dirty. Filters are clean when first sent out by the makers, and with reasonable care in handling they should never become so dirty as to require other cleaning than can be given by breathing upon them, and polishing with a piece of silk or tissue paper. A filter should never be washed with water, under any circumstances, because if water comes into contact with the gelatine at the edges of the filter it will cause it to swell and so separate the glasses, causing air to run in between the gelatine and the glass. Even if the swelling does not cause air to run in in this manner, the filter will be strained and the definition spoiled. THE CARE OF FILTERS If for any reason the filter gets so dirty that it cannot he cleaned bv simple rubbing, after breathing on it, a piece of fine tissue paper should be damped with methylated spirit and gently rubbed over the surface of the filter. Care must be taken that the tissue paper is not wet enough for the spirit to run out and spread over the edge of the filter, as methylated spirit is a solvent of the balsam with which filters are cemented, and will soften it so that air may run in. Before attempting to clean a filter at all it is well to make sure that both the surface of the glass and the material are entirely free from grit, which will scratch the glass. If the glass becomes badly scratched the only thing to do is to recement the filter with the scratch inside, which can be done i(U the case of Hats, although this involves a considerable delay while the recemented filter is drying. In addition to moisture, undue heat is dangerous to filters, as it softens the balsam and causes the gelatine to contract, so that filters should always be protected from heat, as far as possible. The dves used for most filters are quite stable to light, a table showing the stability to light of all the Wratten filters being published in u Wratten Light Filters.” The “ K ” filters are particularly stable, and no fear of fading need be felt. Filters, however, should always be kept in their cases when not required for use; tricolour filters, especially, should be put back into the cases as they are taken from the camera in turn, they are then quite safe and can always be found when required. The Aesculine filters, used for the removal of ultra-violet light in photographing drawings which contain Chinese white, must be protected from light, to which they are unstable, going brown when exposed to it for any length of time. INDEX “ A ” FILTER, 38. Aberrations in relation to focal length of lens, 97. Abney, Sir W. A., 92. 'Absorption, 10, 39. of dyed wools, 17-18. of erythrosine, 12. of Gentian violet, 12. measurements of, 16-17. of natural colours, 14. partial, 12. of printing inks, 17. sharp and gradual, 13. ultra violet, 32. A&inometer, use of in high alti- tudes, 72. Aesculine filter, the, 113. Alpine photography, 69, 75. Angstrom ^inits, 9. Anti-orthochromatic filter, 33. Artificial light, photography with, 29, 42, 67. Astigmatism in filters, ioi. Atmosphere in photographs, 33, 37. Autochrome plate, the, 87. Cloud photography, 38, 72, 76. Colour, associated with wave length, 8. -box of Clerk Maxwell, 82. complementary, n. chart, 25, 30. chart, photographs of, 26,29, 44- contrast, rendering of, 47. contrast and tone contrast com- pared, 47. definition of, 10, means absorption, 10. nature of, 7. perception, variation with in- tensity, 20. Colours, with equal luminosity, ren- dering of, 47. under mercury vapour, 10. Commercial photography, 54. Complementary colours, 1 1. Contrast filters, 42. possible in halt-tone work, 78. Copying sepia and brown prints, 5 2 * Curves of plate sensitiveness, 21,22. Bathing plates to sensitize, 22-23. Blue filter, the, 91. Blue prints, to copy, 54. Brown glass filters, 32. Bull, Mr. A. J., 89, 95. Callier, Andre, 69. on plates for landscape photo- graphy, 72. Catalogue illustrations, 54. “Chemical” or ultra-violet rays, 27-28. Chinese white, absorption of ultra- violet, 33. Chromo-lithography, 77. Clerk Maxwell, 82. Clothing in portraiture, 66. Daffodil, photography of, 24. Daylight photography, 29-30. Defers in colour rendering due to inks, 94. Distance, photography of, 33, 74. Ducos du Hauron, 86. Dyed wool absorptions, 17-18. Effect of exposing under three tri- colour filters. 49. Eosine, 21, 50. Erythrosine, 21, 39. Estate photography, 56. Exposure, 37. in portraiture, 66. with filters, 31. Eye, perception limits of, 19. PHOTOGRAPHY OF COLOURED OBJECTS Eye, function of the rods and cones, zo. safelights considered in relation to, 21. FaCtors for Wratten contrast filters, 43 - for contrast filters, 43. Filter holders, 1 1 1. adjustable, 110. Filters, Aesculine, 113. anti-orthochromatic, 33. astigmatism in, 101. brown glass, 32. care of, 1 1 2. cells for, 106. cemented, 112. contrast, 42. criterion of efficiency of, 33. drying after cementing, 100. effect of strain on, 100. effeCt on focus of, 103. efficiency of, 31. errors in, 98. tests for, 97. film, 99, 1 12. fitting of, 105. for microscopical work, 98. in flats, information necessary when ordering, 109. green glass, 32. how to determine size required, 108. ideal, 31. “ K ,” 33, 34. measurements for fitting, 107. orthochromatic or isochromatic, 30. orthochromatic, need for, 24. optical properties of, 97. necessary in subtraCtive work, 90. in portraiture, 66. registration with, 104. sharp-cut, 39. stability of, 1 1 3. three-colour, 48. two used together, 42. varieties of, 98. Flashlight, 68. F lat and foggy negatives, cause of, 7 5. Flesh tints, 61. Flower photography, 25, 35. Frontispiece, photographs of, 44. Furniture photography, 58. I 16 “ G filter, 37, 43. Gentian violet, absorption of, 12. Gradation of negatives in three- colour work, 92. Green colour, 1 1 . Green filter, the, 90. Green glass filters, 32. Greens, reproduction of, 95. theoretical and aCtual, 1 5. Hair in portraiture, 64. Half-watt lamps, 68. Haze and atmosphere, 37. “ High light ” negatives, 78. Houses and buildings, photography of, 56. Hue in colour reproduction, 83. Incandescent gas, 42. Infra-red, photography in, 76. Joly, Prof., 86, 87. Jones, Mr. Chapman, 92. “K” filters, 33, 34, 35. efficiency of, 34. Koenig, Dr. E., 17, 89. Landscape photography, 35, 37, 69. Lens hood, need for, 74. Light, analysis of, 8-9. filters, 10. frequencies, 8. nature of, 7. and sound, analogy between, 7. theories of, 7. velocity of, 8. waves, 8. Lumiere Autochrome plate, 87. Luminosities, correCt rendering of, 24. Luminosity curve, 16. of speCtrum on ordinary and col- our sensitive plates, 27. Luminosity of colours, 16. “ M ” filters, 45. “ M ” plates, 45. Mahogany, to photograph, 58. Maxwell, Clerk, 82. Measurements for filter fitting, 107. Mees, Dr. C. E. K., Preface, 16, 88, 91. Mercury vapour lamp, 10, 63. INDEX Misconceptions of tone values of colours, 48. Mist, effeCt of in photography, 73. its causes, 73. scattering of, 73-7+. scattering of light by, 33. Multiplying fa ft or for filters, 31. Newton, Mr. A. J., 89. Night photography, 68. Orthochromatic filters, efficiency of, 3 1 * Orthochromatic plates, 40. Over-correftion, 33, 35. Photochromoscope, the, 84. Photographs, tinting, 66. Photography, landscape, 35. of flowers, 24-25, 35. by night, 68. Photomicrography,” 58. Pifture copying, 48. Plate sensitiveness, curves of, 21. Plates, orthochromatic, 40. “ red-blind,” 34. Portraiture, 61. artificial light for, 67. clothing in, 66. exposure in, 66. filters in, 66. hair in, 64. in ordinary rooms, 67. with mercury vapour lamps, 68. through red filter, 64. Postage stamps, to photograph, 56. Posters, to photograph, 56. Principles in three-colour photo- graphy, 50. Printing ink absorptions, 17. inks, ideal and real, 93. colours for subtraftive process, 88 . colours, seleftion of, 93. Projeftion, three colour, 84. Purkinje’s phenomenon, 20. Red and black, to reproduce, 80. Red silver prints, to photograph, 60. Relief processes, 78. Reproduction, photography of col- oured objefts for, 77. “ Reproduction Work with Dry Plates,” 78-79. Retouching, how to avoid, 61. Rules for photographing colour, 15. Safelights considered in relation to eye sensitiveness, 21. Screen plate colour photography, 86. Sensation curves, three-colour, 82. Sensitiveness conferred by dyeing, 21. distribution of in plates, 23. of eye and photographic plate compared, 19. of plates to artificial lights, 67. of plates to various colours, 21. ratio of to different colours, 22. Sensitizing plates by bathing, 22- . 23 *. Sepia prints, to copy, 52. Sheppard, Dr. S. E., 91. Skin texture, to photograph, 63. Sky in landscape, 69. Speftroscope, the, 9. Speftrum, the, 9. represented in monochrome, 19. reproduction of, 83. Stains, to photograph, 52. Stenger, Prof. E., 92. Straining a filter, effeft of, 100. Subtraftive process of colour-photo- graphy, 88. Technical photography, 54. Telephotography, 37, 74. Tinting photographs, 66. Three-colour half-tone by direft method, 78. photography, 82. projection, filters for, 84-85. Tone contrast, 47. Tricolour filters, 48. Two-colour reproduction, 80. Typewriting, to photograph, 54. Ultra-violet, 21. or “chemical rays,” 27-28, 29, 3 *» 33 » 34 > 35 > 3 6 , 39 - Violet, theoretical and aftual, 15. Violets, reproduction of, 96. Vogel’s discovery, 21. 7 PHOTOGRAPHY OF COLOURED OBJECTS Wall, Mr. E. J., 89. Wood, Prof. R. W., 33, 76. Wratten copyboard chart, 93. ink control, 96. ink tester, 96. green filter, 91. panchromatic plate, 23, 28, 30, 36, 39, 40, 41, 47. Wratten process panchromatic plate, 54 - Wrinkles, exaggeration of in ordin- ary portraiture, 63. Yellow, its composition, 1 1. Yellowed prints, 52. 3 3125 00002 9278