ACHROMATIC MICROSCOPES, OAKST.HDSF I r } Y ‘V / f r • -?P' +■ Digitized by the Internet Archive in 2017 with funding from University of liiinois Urbana-Champaign Aiternates https://archive.org/detaiis/iliustratedscien00jwgr_0 ILLUSTRATED SCIENTIFIC AND DESCRIPTIVE CATALOGUE OF ACHEOMATIC MICEOSCOPES, MANUFACTURED BY J. & W. GRUNOW & CO., NEW HAVEN, CONN. 30 CE^NTTS. NEW HAVEN: T. J. STAFFORD, PRINTER. 1857. Entered, According to Act of Congress, in the year 1857, By J. & W. GRUNOW & CO., In tlie Clerk’s Office of the District Court of Connecticut. PREFACE. In presenting to the public this Illustrated and Descriptive Catalogue, it is our intention to furnish, especially to those living at a distance, the means of becoming acquainted with the various forms and qualities of the microscopes and micro- scopical apparatus which we manufacture. Having been engaged almost exclusively, for a series of years, in the manufacture of this important instrument, we have devoted great care and attention to improving the struc- ture of the mechanical parts, as well as to perfecting the qual- ity and efficiency of our object-glasses, and the optical parts in general. The extensive patronage with which we have been favored affords sufficient evidence that in our efforts to deserve the confidence of the scientific public we have not been without success. But while it has been our object to bring to perfection the branch of practical optics to which we have devoted ourselves, we have been constantly adding to our facilities for manufac- turing the various apparatus connected with the microscope. By the introduction of machinery especially adapted to our purposes, and by a judicious division of labor, we have en- deavored to secure the most finished workmanship in every part, at such prices as shall place the most perfect instruments ^within the reach of all the lovers of science and students of nature. IV PREFACE. We are daily receiving inquiries from persons who desire information and advice in regard to the selection of micro- scopes adapted to their particular purposes. It frequently happens that persons making these inquiries have had but little opportunity to become acquainted with the theory and uses of microscopes made in the most recent and improved style. Properly to reply to all these inquiries would absorb too much of our time, which we desire to devote exclusively to the more delicate and important labor of our art. We have therefore decided to publish in our catalogue such infor- mation as shall enable every one to understand the philosophy and structure of the most improved microscopes, and to jndge of the qualities of different microscopes that may be offered to their patronage. So rapidly has the microscope been improved within a very recent period, that instruments which were made but a few years since have become of little value, compared with the improved achromatic miscroscopes which are now made by the best opticians of England and America. This rapid advance of improvement has caused a large stock of microscopes of inferior quality to be thrown into the market at very low prices, and those who are not aware of the great su- periority of the achromatic microscope, as at present con- structed, are induced to purchase cheap, but inferior^ instru- ments. With this species of trade it is not our purpose to com- pete. But to those who desire to obtain microscopes of the best construction, combining the most recent scientific improve- ments and fitted for the prosecution of the highest order of sci- entific inquiries, we offer our microscopes with confidence that wherever their merits are known they will give ample satisfac- tion to purchasers. J. & W. Grunow & Co. Kew Haven, Conn., Oct. 1, 1857. CONTENTS ACHROMATIC MICROSCOPES. CHAPTEE 1. THEORY OF THE MICROSCOPE. SECTION PAGE 1. Introduction, 1 2. Simple Lenses, 1 3. Foci of Lenses, 1 4. Aberration, 2 5. Spherical Aberration, 2 6. Amount of Spherical Aberration, 3 7. Proportionate Curvature, 4 8. Negative Aberration, 4 9. Aberration of Sphericity : Curvature of Image, 4 10. Chromatic Aberration, 5 11. First method of diminishing Chromatic Aberration, 6 12. Second “ “ “ “ “ 6 13. Achromatism, 6 14. Combined Lenses, 7 15. Angular Aperture, 8 16. Oblique Illumination, 9 17. Lister’s Discoveries, 9 18. Essential requisites of good Object-Glasses, 9 19. Mr. Lister’s two Preliminary Propositions, 10 20. Principles discovered by Mr. Lister, 10 21. Aplanatic Object-Glasses, 11 22. Superiority of English and American Object-Glasses, 12 23. Compound Achromatic Microscope, 12 24. Positive Eye-piece, 12 25. Negative Eye-piece, 12 26. General view of Compound Achromatic Microscopes, 13 27. Enlarged section of Achromatic Microscope, 15 28. Action of Negative Eye-piece, 16 29. Advantage of Over-correcting Object-Glass, 17 30. Negative Eye-piece nearly Achromatic, 17 31. Use of the term Achromatic Objective, 18 32. Aberration produced by Glass Cover, 19 33. Objective Corrected for Glass Cover, 20 34. Eye-pieces of different Powers, 21 vi CONTENTS. SECTION PAGE 35. Draw-tube, 21 36. Names applied to Object-Glasses, 21 37. Foci of Higher Powers inconveniently near to Objective, 22 CHAPTEE IL MECHANICAL POETION OF THE MICROSCOPE. 38. Modern Improvements, 23 39. Base, 23 40. Arrangement for Inclining the Instrument, 23 41. Stage, 24 42. Adjustment of Focus, 24 43. Simplicity and Facility of Adjustment, 24 44. Description of Microscope Stands, 24 45. No. 1, Educational Microscope, 25 46. No. 2, Student’s Microscope, 27 47. No. 3, Student’s Microscope, 28 48. No, 4, Student’s Larger Microscope, 29 49. Stage movable by Back and Screw, 30 50. No, 5, Another form of Student’s Microscope, 31 51. Chevalier’s Prismatic Body, 32 52. Prof, Bailey’s Indicator Stage, 33 53. No, 6, Portable Microscope, 35 54. No. 7, Large Microscope, 37 55. Chemical or Inverted Microscopes, 38 56. No. 8, Simple form of Inverted Microscope, 39 57. More complete Inverted Microscope, 40 58. Obiect-r. White’s Micrometer. At the suggestion of Dr. White, of this city, we have recently made micrometers of an- other very simple form, which have given great satisfaction. Fig. 21 represents one of these micrometers. Fig. 21. A semi-circular piece of glass, A C B, has micrometer lines ruled as shown in the figure. This is cemented to the dia- phragm of the negative eye-piece, and occupies very nearly one-half of the field. The edge of the glass appears as a dark line A B, across the field, while A D B, occupying a very little more than one-half the field, is entirely unobstructed, as though no micrometer were used. This allows the eye-piece contain- ing this micrometer to be used for ordinary observations. By CATALOGUE OF ACHROMATIC MICROSCOPES. 4 50 ACCESSORY APPARATUS. moving the object, or rotating the eye-piece, the lines can easily be brought to measure any object in the field. The object may be brought up to the ends of the lines without being covered by the micrometer glass. With this micrometer, in the eye-piece used for ordinary ob- servation, hundreds of objects may be measured, whose magni- tude would pass unnoted if it were necessary to change the eye- piece to effect the measurement. These advantages, combined with the very low price for which it is furnished, will, we think, cause this micrometer to be regarded with very general favor, and observers who have more expensive micrometers, will find it convenient to possess this also. The lines in this micrometer are ruled to or 5^0 of an inch, to suit purchasers, but the value of the measurements made by them must be calculated for each object-glass, in the same manner as with other eye- piece micrometers. 69. Prof. J. li. Smitli’s Ooiiioineter and Micrometer. Fig. 22. Prof. Smith has invented a Goniometer for measuring the angles of crystals under the microscope. It is also combined with a micrometer. The following description of the instru- ment and the method of using it, are taken from the Appendix to Carpenter on the Microscope^ edited by F. G. Smith, M. D., American edition. Philadelphia : Blanchard & Lea. 1856. E, Fig. 22, is the upper end of the draw-tube of the micro- scope with the ring Ic soldered to it. Over this ring screws another ring F, which serves as a support and a centre to the graduated circle D, which freely, but without shaking, revolves upon the same. Into the bore of the ring F fits by its lower conical end A, tlie tube G, which is held in it by the screw-ring c>, that prevents its being taken out. Into the tube G, which also has a free re- J. & W. GRUNOW & CO’S ILLUSTRATED GONIOMETER AND MICROMETER. 51 volving movement, fits rig- 23. the positive eye-piece d being the field-lens, and 8 the eye-lens. The slits h h, on oppo- site sides of G, (the ref- erences in Figs. 22 and 23 are the same,) allow the micrometer with its mounting B B, to be in- troduced into G, and permit the graduated lines to be brought into the field of the eye-piece. C is an index, attached to G by the screw c, it may be taken off when the apparatus is not used as a goniometer. 70. Method of using the Ooiiiometer. Bring the object into focus, near the centre of the field of the micrometer, applying the finger to the knob K, revolve the micrometer till the lines of its graduation are parallel to one side of the angle to be measured. Bevolve then, separately, the graduated circle till zero is brought to agree with the point of the index C. Then revolve again the micrometer by the knob K, until the graduation lines are parallel to the other side of the angle to be measured, when the index C will show the value of this angle. The micrometer lines are about iiich apart, but their value, when used for measurements with the different object-glasses and eye-pieces, must be ascertained by a stage micrometer and recorded in a table. 71. Method of finding the value of lines in any Eye- piece Micrometer. For this purpose we must employ a stage micrometer, having lines ruled at some known distance, and this instrument should be of the very best quality, as the accu- racy of all our measurements wdth the eye-piece micrometer depend on the accuracy of the instrument with which their values are determined. CATALOGUE OF ACHROMATIC MICROSCOPES. 0, OF [LL' UB« 52 ACCESSORY APPARATUS. To illustrate the method of making these calculations, we give the process and results in a single case, using the ■§■ inch objective and negative eye-piece [N’o. 2 , in which is inserted a glass micrometer, with lines ruled about 2 Jo inch; (the exact value of these lines is of no conse(][uence, as their value, as used for micrometer measurements, depends on the magnify- ing power of the glasses used.) We select a stage micrometer with lines ruled at of an inch, with no covering over the lines. We place this micrometer on the stage with the ruled lines upward. Setting the adjustment of the object-glass at the mark ^ uncovered^ and carefully adjusting the focus, we find one space on the stage micrometer covers very nearly nineteen spaces in the eye-piece micrometer ; we therefore increase the magnifying power, by extending the draw-tube, till a conven- ient number, as twenty spaces, are covered by one space on the stage, when we find that the draw-tube is extended xVo inch. As now twenty spaces in the eye-piece micrometer are equal to 5 Jo an inch on the stage, each division in the eye-piece measures toJw of an inch on the stage, when the object is uncovered, and the draw-tube is extended of an inch. We find this estimate exceedingly convenient for use, and accord- ingly record the conditions and estimated measurement for future reference. But as our measurement will often, perhaps generally, be made upon objects covered with thin glass, we now place a cover of thin glass over the micrometer lines on the stage, and repeat our calculations. We correct the object-glass for thickness of glass cover, by turning the graduated collar till we obtain perfect definition of the lines on the stage micrometer, and we find, in this case, that the adjustment of the object- glass has been turned forward twelve and one-half degrees of its graduated scale ; counting the spaces in the eye-piece micrometer, covered by one space on the stage, we find, not twenty as before, but twenty-one and a fraction; we therefore diminish the magnifying power by pushing back the draw-tube till twenty spaces in the eye-piece again exactly fill one division J. & W. GRUNOW & GO’S ILLUSTRATED METHOD OF USING THE MICROMETER. 53 in the stage micrometer, and we find the draw-tube extended only to inches. Thus, in covering the object with thin glass, and turning forward the adjustment of the object-glass twelve and one-half degrees, we have found it necessary to push in the draw-tube jVo inch, or yf □ inch for every degree the adjustment of the object-glass has been turned forward. The same method is to be pursued in estimating the value of the micrometer lines for every object-glass with which it is to be used. The values here given apply only to the particular instrument and glasses used in this calculation, for slight differ- ences in glasses of the same name, and different lengths of the compound body, however slight, cause the measurements of the micrometer to vary. Hence every microscope should have its own table of measurements for its micrometer. 72. Method of using the Micrometer. Suppose we are examining a delicate Diatomacea with the instrument which we have used in making the preceding calculations, and we wish to measure the breadth of the rows of hexagonal mark- ings which we find covering the object. We examine the adjustment of the object-glass, and find that it stands at eleven degrees, then yf ^ inch for each degree would give y^^ of an inch, which the present adjustment requires to be subtracted from yW» the distance the draw-tube is to be extended for an uncovered object, which leaves yo”^ of an inch as the distance the draw-tube is to be extended to have our micrometer lines measure ten-thousandths of an inch ; adjusting the draw-tube to this calculation, we see that two spaces in the micrometer cover nine rows of the delicate hexagonal markings on our object, {navicula angulata^) therefore the breadth of each row is Tsioo of an inch. Suppose any object measured is equal to a certain number of spaces and a small part of an additional space, one-half, or even a quarter of a space between two micrometer lines can gen- erally be estimated with tolerable accuracy, so that the meas- urements made by the glass micrometers attached to a nega- tive eye-piece, may be relied upon for measurements as small as one twenty thousandth, or even one forty thousandth of an CATALOGUE OF ACHROMATIC MICROSCOPES. 54 ACCESSORY APPARATUS. inch ; which is a degree of accuracy sufficient for all ordinary observations. Those who desire more accurate measurements will, of course, procure the more expensive instruments. For more detailed descriptions of the method of using microme- ters, we would refer our readers to the valuable works of Quecket and Carpenter on the microscope. 73. Frainihofer’s Stage-§crew-Micro]iieter. The labor of making the necessary calculations to determine the value of measurements made by micrometers placed in the eye-piece of the microscope, and the amount of care required to secure accurate results, have rendered it desirable to obtain a microm- eter at once minutely accurate, simple in its use, and requiring no calculation of the value of its measurements. The instrument most nearly fulfilling these requirements is Fraunhofer’s Stage-Screw-Micrometer. The value of the measurements made by this micrometer is always independent of the magnifying power of the microscope. The instrument indicates, directly, and with great accuracy, the absolute dimensions of the object measured. This micrometer is placed upon the stage and consists essen- tially of two plates, one of which, carrying the object, is moved upon the other by a micrometer-screw, having one hundred threads to an inch, with a graduated head and vernier by which motions of the plates and object upon it are accu- rately measured to the one hundred thousandth of an inch. Fig. 24 shows this instrument, the upper part of the figure being a view from above, while the lower part of the figure gives a side view of the same. B B is the body of the instrument which is fastened to the stage of the microscope by the short cylinder C, which is made elastic so as to hold the instrument securely in its place when it is inserted into the circular opening of the stage. A A is a revolving plate with spring clips to support the object to be measured. This plate can be revolved so as to bring the object in a position to measure it in any direction. This circu- lar plate is supported on another plate in the body of the instrument, which is moved by a fine micrometer screw, with exactly one hundred threads to the American standard inch. .J. & W. GRUNOW & GO’S ILLUSTRATED Fraunhofer’s stage-screw-micrometer. 55 Fig. 24. On the end of this screw is a milled head m, with a head having one hundred divisions ; by means of the vernier ti, each division of the scale g is divided into ten parts, each of which is equivalent to one thousandth part of a turn of the screw. The graduated head g can be turned without moving the milled head m or the screw to which it is attached, by which means the graduation can be set at zero in any position of the screw and object. At -y is a scale showing the number of turns given to the screw, each equal to one hundredth of an inch, therefore the divisions of g are ten thousandths, and the divi- sions shown by the vernier are hundred thousandths of an inch, whatever may be the magnifying power of the microscope by which the object is viewed. Accompanying this micrometer is a negative eye-piece with a cobweb drawn across the field of view, in the focus of the eye-lens, the magnitude of an object being determined CATALOGUE OF ACHROMATIC MICROSCOPES. 56 ACCESSOKY APPARATUS. by the number of turns and parts of a turn given to the screw to cause the image of the object to pass entirely across the cobweb. For using this micrometer, the stage (if movable) is made fast by a clamp. The object-slide is placed upon the plate A A, which is rotated till the object is in a proper position to be measured. One edge of the object is brought to coincide exactly with the cobweb in the field of view, and the reading of the scale v, graduated head y, and vernier n, are accurately noted, or if the object is a small one, the graduated head y can at once be set to zero of its scale. The milled head is then turned till the object has passed entirely across the line in the eye -piece, and the readings of the scale, graduated head, and vernier are again examined. The difference between the first and second readings is the exact measurement of the object. The great accuracy of this instrument, and the facility with which it can be used on any microscope, no preliminary calcu- lations being required, will commend it to the favor of all who desire the most perfect micrometer yet invented. INSTRUMENTS FOR DRAWING WITH THE MICROSCOPE. 74. Wollaston’s Camera Ciicida is the instrument com- monly used by artists for sketching from nature. Tliis instru- ment is fitted to the eye-piece of the microscope, and enables the observer to sketch upon paper, placed on the table, the magni- fied image of any object seen in the microscope. It consists of a four sided prism of glass, having its faces and angles so arranged that light entering the first face of the prism is totally refiected by the second and third faces, and emerges perpen- dicular to the fourth face of the prism, and at right angles with its original direction. When using this camera^ the microscope is placed in a hori- zontal position, and the object appears projected upon the paper placed on the table to receive it, and it may be traced by a pencil which the eye sees at the same time as the object. 75. Wacliet’s Camera I^tieida. This instrument consists of a triangular prism, having its three faces and angles equal. J. & W. GRUNOW & GO’S ILLUSTRATED CAMERA LUCIDA. 57 Fig. 25 sliow8 this camera, mounted in rig. 25 . a cap, which fits the top of the eye-piece. If the microscope is inclined at an angle of thirty degrees with the horizon, this camera may be used in the Same manner as Wollaston^ s Camera Lucida^ described in the previous section. With this camera the microscope is sometimes placed in a . perpendicular posittion, and the drawing made on a table which is inclined. 76. Soemmeriiig’§ Steel Speculum. This is a plane speculum of polished steel, smaller than the ordinary pupil of the eye, commonly set at an angle of 45°, and mounted as shown in Fig. 26, and attached to the eye-piece in the same manner as the camera. This speculum is so mounted that its angle of inclination may be changed to project the image on any part of the paper where it is most convenient. With this instrument, the reflected image of the object, and the pencil, both appear together on the paper, and the microscope may be placed at any angle which is found most convenient. 77. Using the Camera. In using the camera lucida, or speculum, some care is required to see both the object and the pencil at the same time. Sometimes the light on the paper is to6 strong, and it requires to be shaded by a book or other object. In the evening, on the other hand, the pencil and paper require additional illumination. If the object and pencil are not both visible on the paper at the same time, the diffi- culty will generally be overcome by moving the eye about till the proper position is found, and -when once obtained the eye should be kept steadily in that position till the drawing is com- pleted. To have different drawings on a uniform scale, it is neces- sary to have the microscope always inclined at the same angle, that is, to have the camera at a uniform distance above the paper. Nine inches will generally be found the proper dis- Fig. 26 . CATALOGUE OF ACHROMATIC MICROSCOPES. 58 ACCESSORY APPARATUS. tance for easy and accurate drawing. The size of the picture will vary with the distance of the paper from the instrument. Experience is of course required to give steadiness to the hand and to secure accurate drawings, but it is often so desira- ble to make accurate drawings of objects seen with the micro- scope, that every one ought to practice till he can draw with facility. It makes little difference which of the three instru- ments described is used, a person draws best with the instru- ment to which he has become accustomed. 78. Camera I^acida applied to Micrometry. If the microscope is placed at an angle convenient for using the camera, and care is taken to adjust it always in the same posi- tion, the lines on a stage micrometer may be projected on paper, by means of the camera, and form an accurate scale for measuring any object drawn with the same power, and with the instrument in the same position. Supposing the divisions of a stage micrometer, whose real values are 2 J 0 an inch, are projected with such a magnifying power as to be at the distance of one inch from each other on the paper, it is obvious that if any object is delineated on the paper with the same power, every inch of the drawing corresponds to 2^5 of an inch on the object, J of an inch would equal one thousandth of an inch in the object, and so on. We may there- fore draw parallel lines on the paper, subdividing the spaces formed by projecting the micrometer lines, and the scale thus formed will serve for measuring any object we may examine. A similar scale may be prepared with each object-glass, and by viewing any object through the camera, with the scale placed below, we determine at once its magnitude. When sufficient magnifying power is used, the instrument properly adjusted, and the scale thus made is minutely divided, great accuracy may be obtained. 79. Movable Hiaphrag^m-Platc. “ ISTo microscope stage (says Dr. Carpenter) should ever be without a diaphragm- plate, fitted to its under surface, for the sake of restricting the amount of light reflected from the mirror, and of limiting the angle at which its rays impinge on the object.” This appa- ratus, shown at Fig. 27, is attached to the under side of the J. & W. GRUNOW & GO’S ILLUSTRATED bull’s-eye condenser. 59 stage by a bayonet joint. The dia- phragm having a variety of open- ings of different form and size, turns upon a pivot so situated that each , opening is successively brought into I the axis of the microscope. The® space between the largest and small- est openings is greater than between any other two, and is designed to exclude all transmitted light and give a dark background when viewing opaque objects. A bent spring is attached to the fixed part, and rubs against the edge of the movable plate, which is provided with notches, so arranged that when either of the holes is brought into its proper position the end of the spring drops into the notch. In the plain form of diaphragm all the openings are circular? but in the more expensive kinds, openings of a variety of forms are employed, some excluding the central rays, others are crescent shaped, or semi-circular, admitting only light from one side. When the eye becomes fatigued by too strong light, or by the intense glare or yellow rays of artificial light, relief is afforded by inserting in the body of the diaphragm a piece of gray, neutral tint, or light blue glass, by which the light can be modified to any extent required. 80. Bull’§-Eye Condenser. This is a large plano-convex lens, mounted on a brass stand and pillar. The lens is attached by a cradle joint to a revolving arm, which is supported by a sliding tube, so cut as to act as a spring and retain the arm and lens steady at any elevation and in any position. For illumina- ting opaque objects, the bull’s-eye is so adjusted above the stage, with its fiat side towards the object, as to bring the light to a focus upon the object on the stage. In using artificial light, the large bull’s-eye is to be placed with its plane surface towards the light, and so adjusted that the beam of light transmitted shall be about the size of the mirror, or of the smaller bull’s-eye, when the rays will be nearly parallel, or slightly converging. The mirror, or smaller bull’s-eye, may then be used to bring the light to a focus upon the obj ect. If the CATALOGUE OF ACHROMATIC MICROSCOPES. 60 ACCESSOEY APPARATUS. smaller bull’s-eye is used to bring the light to a focus, its flat surface should be turned towards the object. By following these directions, very little spherical aberration is produced by the form of the bull’s-eye, and the illumination is rendered more efiective than by any other position of the lenses. 81- Smaller Biill’s-Eye Condenser. This instru- ment is mounted as shown in Fig. 29. It is very conven- ient for illuminating opaque objects to be viewed with low powers. The method of using it will be understood from the preceding section. J. & W. GRUNOW & CO’S ILLUSTRATED ACHROMATIC CONDENSER. 61 82. Aclironiatic Condenser. For low powers the concave mirror, placed below the stage, furnishes all the light which is required, but for developing the best effect of the higher pow- ers, as the J, J and j 2 i^^^h objectives, it is sometimes necessary to have the object illuminated with achromatic light highly concentrated. If the object-glass has an angular aperture of from ninety to one hundred and sixty degrees, and the pencil of light by which the object is illuminated is condensed at an angle of only fifty degrees, (and especially if it is also affected with spherical and chromatic aberration,) it is evident that unless the object itself disperses the light, there will be no light to be taken up by the marginal part of the object-glass. It is found that generally objects do thus disperse the light to a limited extent, but that to secure the fullest advantage from object-glasses of large angular aperture, it is necessary to illumi- nate the object with a pencil of achromatic light, condensed at an angle bearing a considerable proportion to the aperture of the object-glass. This object is secured by passing the illuminating pencil through an achromatic combination of lenses, of large aper- ture, placed beneath the stage. Fig. 30. The achromatic condenser is so mounted that its focus can be easily adjusted to the exact position of the object. The appa- ratus in which it is mounted, is shown in Fig. 30, and is attached to the under side of the stage by a bayonet catch shown in the upper part of the figure. An inner cylinder, CATALOGUE OF ACHROMATIC MICROSCOPES. 62 ACCESSORY APPARATUS. which carries the condenser, is moved by rack and pinion by turning the milled head, which serves to adjust the focus. The achromatic condenser fits to the inner tube by a bayonet joint, and when it is removed, a diaphragm, or N'achefs Oblique Prism^ may be inserted in the same brass work. The achromatic condenser furnishes a pencil of light which is, 1st, free from color ; 2nd, free from spherical aberration ; 3rd, condensed at a very large angle. These qualities render it exceedingly valuable for displaying delicate objects viewed with the higher powers. To use this condenser, first place an object on the stage, illuminating it by means of the concave mirror, and with a low power bring the object into exact focus; now remove the object without altering the focus of the object-glass, attach the achromatic condenser and illuminate it by means of the plane mirror; now by turning the milled head, so adjust the con- denser that the image of window bars or trees may be seen in the microscope ; then turn the mirror so as to reflect light from a white cloud, or other strong light, through the condenser ; then place the object on the stage and attach one of the higher powers to the microscope, and when the object is brought into focus it will be seen most beautifully illuminated, and delicate structures, scarcely distinguishable by other methods of illumi- nation, will be clearly defined. 83. ]Vactiet’§ Prii^m for OMique Illumiiiatiou. This prism is so contrived as to illuminate the object with a pencil of light from one side of the axis, the form of the prism being previously constructed so as to give to the light any angle of inclination required. Let M O, Fig. 31, be the axis of the microscope, h c d q are the four sides of ITachet’s prism, 'M? O is the angle at which the pencil of light is re- quired to fall upon the object O. To the side d q^ is cemented a lens, having such a focus as to condense the light, transmitted by the prism, upon the object O, on the stage of the micro- scope. This prism is set in a piece of tube C C, a diaphragm placed below cuts off all the light whicli would not pass through the prism and be brought to a focus at O. When this prism is used a beam of light L, is reflected J. & W. GRUNOW & GO’S ILLUSTRATED LIBERKUHN SPECULUM. 63 upwards by the plane mirror M, so as to enter the prism. It then suffers total reflection in the direction w and is again reflected by the opposite force of the prism in the direction v O ; as it emerges from the con- vex surface of the lens c? it is condensed and brought to a focus upon the object O, which is in the focus of the object- glass immediately above it. This prism is so mounted be- low the stage of the micro- scope that it can be revolved to give light from any direc- tion, or illuminate successive- ly every side of an object. This method of illumination brings out many delicate markings, and reveals pecu- liarities of structure not otherwise appreciable. See Section 16. When the microscope is furnished with an achromatic con- denser, bTachet’s prism can be inserted in the same brass work. 84. liieberkubn Speculum. This instrument, shown in Fig. 32, consists of a small concave metallic reflector L L, attached to a short tube which is fitted to the lower part of the object-glass A. The polished surface presents that degree of concavity which is adapted to bring to a focus, upon the object, the beam of parallel light reflected from the plane mirror below the stage. These little reflectors are called Lieberkuhns, from the name of their inventor. The rays of light reflected from the mirror B, Fig. 32, pass through that part of the slide S S, not covered by the object, and being again reflected by the Lieberkuhn L L, are brought to a focus upon the object, which is then seen by reflected light. If the object is transparent, the small stop or darh well D, gives a black ground behind the object, which is then seen CATALOGUE OF ACHROMATIC MICROSCOPES. 64 : ACCESSORY APPARATUS. brilliantly illuminated upon a dark field. The dark well is made in the form of a cup, in order that the bottom may not be sufficiently illuminated to form a light ground to the object, which might happen if the disk were employed. Three sizes of these dark wells are usually supplied, the largest being always used with the lower powers. The Lieberkuhn gives a full and uniform illumination on every side of the object, while the bull’s-eye gives only oblique light from one direction. The Lieberkuhn is also available to illuminate opaque objects viewed with object-glasses of such short focus as to preclude the use of the bull’s-eye. 85. The Erector is inserted in the lower end of the draw- tube of the microscope. Its use is to reverse the position of the image, (which is inverted in the compound microscope.) so that it shall appear in the true position of the object. It is used with low powers for dissecting and other manipulations, where the hands require to be guided by looking through the microscope. 86. Among the recent valuable additions to the microscope, should be mentioned the Ortlioscopic Eye-piece, invented by Mr. Charles Kellner, optician of Wetzlar. It is adapted to micro- scopes, and also to telescopes of all kinds, the dialy tic included. It gives a large field of view, free from curvature or distortion of any kind, perspectively correct, with sharpness of definition throughout its whole extent, without the blue ring which en- circles the borders of the field in the ordinary negative eye-piece. Fig. 32. J. & W. GRUNOW & GO’S ILLUSTRATED ANIMALCULE CAGE WITH SCKEW. 65 87. Compressor. It is often required to tear up delicate portions of tissue upon the field of the microscope, or to fioat them out, as it were, from the general substance under exam- ination, by pressing down the thin glass which covers them ; many parts of plants are also better seen when slightly com- pressed. Fig. 33. The compressor, Fig. 33, enables us to apply any amount of graduated pressure upon the thin glass which covers the object. The lever bears at one end a fiat brass ring, which moves on a universal joint, and which can be elevated or depressed by turning the screw at the other end of the lever. When the screw is loosened the lever can be turned around on the pivot which secures it to the plate ; the ring being thus turned away, the object on the glass plate, which covers the opening in the compressor, can be changed. If desired, an ordinary slide can be placed upon the plate of the compressor, and the cover pressed down upon the obj ect on the slide. Generally a plate of glass is cemented over the opening in the compressor, and a circle of thin glass is cemented to the movable ring attached to the lever. These glasses can be easily removed, if desired, or their places supplied by new ones, if they chance to be broken. 88. Atiimalciile Cage witli Screw. The animalcule cage shown at Fig. 31, consists of ^4. a brass plate, or slide, carry- ing a short cylinder, which supports a circular plate of glass, over which fits a cap bearing a circle of thin glass / a screw collar retains the cap in place, and, when screwed down upon it, produces moderate CATALOGUE OF ACHROMATIC MICROSCOPES. 5 66 -ACCESSOEY APPAEATUS. pressure sufficient to retain any animalcules in place while they are examined. This instrument may also be used in some cases instead of the “ compressor. 89: Tlie Simple Animalcule Cage differs from the prece- ding only in having the cap retained in place by a cylindrical ring, so cut as to act as a spring. The use of this instrument is the same as the preceding, though the amount of pressure obtained by it is somewhat less. 90. Stage Forceps. The very convenient instrument shown in Fig. 35, can be attached to the stage and made fast by turn- ing the screw with a milled head. These forceps slide like a pencil in a cylindrical support, while the jointed arm and pivot allow of motion in any Fig. 35. direction. Minute insects or other objects can be held by this instrument in any position required. One end of the instru- ment carries a needle which can be used for the same i^urpose. Hand Forceps, both of brass and of steel, to be used with the microscope, are furnished to order. 91. Frog Plate. For viewing the circulation of the blood, the most convenient subject is the common frog. The capillary circulation in the thin transparent web of the foot, or in the tongue, affords the most interesting exhibition the microscopist can enjoy. Some care is required in so arranging the little an- imal as to avoid giving him pain, or even stopping the circula- tion., by undue pressure on some part of the circulatory system. The Feog Plate is an instrument devised to secure these ob- jects. An extra stage or brass plate, about two and a half by three inches, having a central opening, is attached to the ordi- nary stage of the microscope by a short piece of tube attached J. & W. GRUNOW k GO’S ILLUSTRATED MACHINE FOR CU'ITING CIRCLES OF THIN GLASS. 67 to its centre, and so cut as to act as a spring when inserted in the opening of the stage. An axis extends horizontally from one side of the brass plate above described, to which is attach- ed a plate of wood about three inches square, on which the frog is secured in a bag. This plate carrying the frog, can be rotated so as to give to the animal the most easy position for extending the foot, so as to allow the web to be spread out upon a glass plate over the opening in the stage of the microscope. Oil the opposite side of the brass plate are sliding sockets, in which are inserted pivots or keys like those which tighten the strings of a violin. To these pivots or keys are attached the threads which extend the toes of the frog. The threads may be tightened by turning the keys, or their position changed by moving the sliding sockets. This instrument enables one to view the circulation in the frog with great facility. 92. ITlacliiiie for Cutting Circles of Tliiii Glass. The base of this instrument, which is of mahogany, supports a strong bent arm of japanned cast iron. From the end of this arm, firmly attached to it, projects downward a cylindrical guide, two inches long, designed to steady the other parts of the ap- CATALOGUE OF ACHROMATIC MICROSCOPES. 68 ACCESSORY APPARATUS. paratus. Through this cylinder passes a steel stem, ending be- low ill a conical base of brass, in the bottom of which is insert- ed a piece of cork or india rubber to press upon the thin glass and hold it steady. At the top is a milled head «, under which is a helical spring, which supports the stem, and lifts it from the glass when not in use. Outside the cylindrical guide is a tube o, carrying a milled head and the socket n, to which is attached the bent arm c, carrying the waiting dia- mond d. The diamond is secured by a screw, and the arm which carries it can be extended at pleasure, and secured by the screw attached to the socket. The brass cone at the base of the stem supports the tube c>, but the tube can be raised on the cylindrical guide, so that the diamond presses upon the glass only by the weight of the arm , that line is termed the axis of double refraction.^ on all sides of which double refraction takes place. This line is fixed only in direction ; every other line parallel to a J, is equally a line of no double refraction. The other parts of the crystal can be split off by natural cleavage so as to leave any line parallel to a b, the perpendicular or major axis of the remaining crystal. Figure 40 shows the appearance of lines A B, C D, E F, G H, and a circle drawn around their common intersection, seen through a fiat prism of Iceland spar, about an inch and a Fig. 40. quarter thick. A B is parallel to the principal section of the prism ex- pressed hj a G b d, Fig. 39. The circle and lines are all seen in their true position when viewed in a direction perpendicular to the face of the prism, but just above the lines, and parallel to them, are seen with equal bright- ness, the dotted lines c d, efgh, and about their common intersection a second circle, the double image of the first. The pencil of light is divided into the ordinary and extra- ordinary rays. The extraordinary image of a line, seen through a double image prism, is always parallel to the ordi- nary image. The extraordinary image of the circle shows clearly that every point is displaced in a plane parallel to A B, or the principal section joining the obtuse lateral edges of the prism. Every line drawn parallel to A B, will appear single, J. & W. GRUNOW & GO’S ILLUSTRATED nicol’s single image prism. 77 and every other line will appear double, when viewed re- spectively in a direction perpendicular to a face of the prism. 101. Polarization produced by double refraction. If one face of a prism of Iceland spar is covered by an opaque substance, (as paper or tin foil,) in which a small hole has been pierced, and this hole is viewed from the opposite face of the prism, held before a beam of light, two illuminated discs will be seen. As the spar is revolved before the eye, one image {fhe ordinary image) remains stationary, while the other {or extraordinary image) appears to revolve around the first. Examined by an analyzing plate of tourmaline, both of these images are found to be perfectly polarized. Tliis is at once evident when the analyzer is rotated in front of the stationary prism, the two images alternately disappear and appear again at every 90° of the revolution of the tourmaline plate, one arriving at its maximum brightness when the other disap- pears, and vice versa, the maximum brightness of both images being equal. If now, in place of the tourmaline, a second prism of Iceland spar is used as an analyzer, and is held with its principal sec- tion parallel to that of the first prism, both images will still be seen, but separated twice as far as when one prism only is used. If, now, however, the second prism be revolved 90®, 180°, or 270®, only one image remains. But in all other positions of the second prism, each of the images produced by the first prism is doubled, so that four images will be seen at the same time. The ordinary and extraordinary rays, on issuing from a doubly refracting prism, are parallel to each other, and it is clear from the preceding observations, that they are polarized in planes at right angles to each other. A substance having such proper- ties as we find in Iceland spar, must be of great value as a means of analysis and polarization of light. 102. Nicol’s Single linage Prism. This beautiful con- trivance, devised by Mr. Nicol of Edinburg, is of the greatest value to the microscopist of all known means of polarization and analysis, since it furnishes a perfectly colorless pencil of polarized light, and when of sufficient size allows of brilliant CATALOGUE OF ACHROMATIC MICROSCOPES. 78 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. illumination. This prism is constructed from an elongated rhombohedron of Iceland spar in the following manner : The natural faceP of the prism, Fig. 41, which makes, with the ob- tuse lateral edge K, an angle of about 71°, is ground away so as to form a new face, making an angle of 68° with the edge K, and a right angle with the plane of princi- pal section joining the obtuse lateral edges K and IP, which is the same as the section a G I Fig. 39. From the obtuse solid angle E, the prism is sawn through in the direction E F, making a right angle with the new terminal face, and also with the plane of principal section. From F another terminal face is constructed at right angles with the section E F and with the principal section, and making an angle of 68° with the edge K'.* All the new faces are now carefully polished, and the two parts of the prism are cemented together, in their former position, with Canada balsam. The lateral faces of this compound prism are all painted black, leaving only the terminal faces for the transmission of light. Figure 42 represents a section of Nicol's prism through the obtuse lateral edges, and shows the course of the two polarized rays into which com- mon light is divided by this prism. A ray of com- mon light, a h, entering this prism, is refracted into the ordinary ray h c, and the extraordinary ray h d. The index of refraction of Iceland spar, for the ordinary ray, being 1.6543, and that of balsam only 1.536, the ordinary ray suffers total reflection at the surface of the balsam, and cannot pass into the lower part of the prism, unless the incident * In tlie manufacture of Nicol’s prisms, the inclination of the terminal faces is sometimes varied to suit particular purposes. Fig. 42. Fig. 41. J. & W. GRUNOAV & CO’S ILLUSTRATED THEORY OF POLARIZED LIGHT. 79 ray diverges very widely from the axis of the prism. The extraordinary ray has a refractive power so low that it is not reflected by the balsam, unless it is very nearly parallel with its surface, but passes freely through it into the lower part of the prism and emerges in the direction g A, parallel to the incident ray.* Nicol’s prisms are capable of transmitting pencils of light, perfectly polarized, varying from 20° to 27'^. The prices of these prisms vary with the size and purity of the crystals. 103 . Common and Polarized liiglit Contrasted. A Ray of Common Lights 1. Is capable of reflection at oblique angles of incidence, in every position of the reflector. 2. Penetrates a plate of tourmaline (cut parallel to the axis of the erystal) in every position of the plate. 3. Penetrates a bundle of parallel glass plates, in every position of the bundle. 4. Suffers double refraction by Iceland spar, in every direction except that parallel to the major axis of the crystal. A Ray of Polarized Lights 1. Is capable of reflection at oblique angles of incidence, in certain positions only of the reflector. 2. Penetrates a plate of tourmaline (cut parallel to the axis of the crystal) in certain positions of the plate, but in others it is wholly intercepted. 3. Penetrates a bundle of parallel glass plates in certain positions of the bundle, but not in others. 4. Does not suffer double refraction by Iceland spar in every direction, not parallel to the major axis of the crystal. In some other positions it suffers single refraction only. THEORY OF POLARIZED LIGHT. lOT. CiidHlatory Theory. In attempting to account for the phenomena of polarized light, the most satisfactory expla- nations are furnished by the undulatory theory of light, first proposed by Huyghens and more fully investigated by Dr. Thomas Young. According to this theory, the particles of luminous bodies * The refractive index of Iceland spar, for the extraordinary ray, varies by a somewhat complicated law, ranging from 1.483 to 1.654, but for such pencils of light as can be transmitted by Nicol’s prism, it varies only between 1.5 and 1.56. The extraordinary ray in this prism never suffers total reflection by the balsam, unless it approaches within about 10° of its surface. CATALOGUE OF ACHROMATIC MICROSCOPES. 80 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. are constantly in a state of alternate contraction and expansion, and are capable of communicating tlieir own motion to a very subtle ether pervading universal space, (and even solid bodies themselves,) through which vibrations are propagated like waves through water, but with immensely greater velocity. 105. Illustrations of wave motion. Let a rope, made fast at one end, be stretched in a horizontal direction, while the other end is held in the hand ; if this end is moved quickly upwards and downwards at regular intervals, an undulatory motion will be propagated through every part of the rope, by a series of tremors or waves passing along from one end to the other. The vibratory motions will all take place in a perpen- dicular plane, the motion of each particle being at right angles to the general direction of the rope. This vibration of the rope may represent the vibrations of one of the polarized rays into which common light is separated by double refraction. Let another similar rope be agitated in the same manner from right to left, the particles of this latter rope will vibrate in a plane perpendicular to the vibrations of the other rope, and will serve to illustrate the vibrations of the other ray of light, polarized at right angles to the first, by the action of the doubly refracting medium. If we suppose a single rope agitated successively in an infi- nite number of planes, varying through every degree of the circle, the diflferently inclined vibrations, following each other at infinitely short (but successive) intervals, while the vibra- tions would take place in every possible plane, the successive waves, by which the vibrations would be propagated, would advance like the coils of a spiral, or the threads of a screw. Let us suppose this rope, whose waves are propagated in a spiral direction, gradually restrained by the approach of two plane resisting surfaces, parallel to each other, the spiral motion would be gradually obliterated, and vibration would be con- tinued only in a single plane, as was supposed in the case of the first rope. This may serve to illustrate, somewhat, the action of the tourmaline, which transmits light polarized or vibrating only in a single plane. J. & W. GRUNOW & GO’S ILLUSTRATED POLARIZATION ILLUSTRATED BY RESULTANNT MOTIONS. 81 106. Polarization illustrated by resultant motions. Every student of meclianics knows, that two forces at right angles to each other may combine and form a resultant force represented in direction and intensity by the diagonal of a parallelogram, the sides of which represent the direction and in- tensity of the original forces ; and that a single force, repre- sented in direction and intensity by the diagonal of a parallel- ogram, may be resolved into two forces at right angles to each other, which will be represented in direction and intensity by the sides of the parallelogram. Applying these principles to illustrate the polarization of light, let O, Fig. 43, represent the centre or axis of a ray of common light passing in a direction per- pendicular to the plane of the paper. Let A B, G H, D C, F E and I J, represent transverse sections of the planes in every direction in which the ray of light causes the luminiferous medium to vibrate, we can always se- lect two planes, as A B, C D, at right angles to each other, which shall correspond with the planes of polarization in which the light vibrates after double refraction. The vibrations in all the other planes, in which ordinary light is supposed to vibrate, may be resolved into vibrations in the planes A B and C D. Thus the vibration O G will be equivalent to two vibrations represented by O a and O d ; OF will be equivalent to O 5 and O d' j O H will be equivalent to O V and O cy O E will be equivalent to O aJ and O g\ and so on. AV can thus resolve the vibrations in any number of planes into others in the planes A B and C D. Vibrations O I, very nearly coinciding with one of the planes C D, will give a resultant intensity in the direction of that CATALOGUE OF ACHROMATIC MICROSCOPES. 6 82 POL SEIZED LIGHT AND ITS APPLICATION TO THE MICKOSCOPE. polarizing plane, almost equal to the original intensity, and but a feeble intensity in the other polarizing plane. But there will also be vibrations very nearly coinciding with each of the polari- zing planes, so that the sum of the resulting vibrations in each polarizing plane, will be exactly equal. The rays polarized in each plane will be represented in inten- sity by the sum of all the resultants in that plane. Figure 44 is designed to represent a transverse section of a ray of common light, vibrating in an infinite number of planes, and at right angles to the direction of the ray. Fig. 44. Fig. 45. Fig. 46. Figure 45 is designed to represent this vibrating ray, resolved into vibrations in two directions at right angles to each other, as when a ray of common light undergoes double refraction and polarization by passing through Iceland spar. Figure 46 shows another and more common method of repre- senting the two rays, produced by double refraction, polarized in the planes A B and C D, the fine lines in A B and C D, in- dicating the condensation and combination of resultant vibra- tions, entirely separated from the vibrations in the other plane of polarization. 107. Polarizing effect of Iceland Spar. Thus tlie polari- zing action of Iceland spar, and all doubly refracting substan- ces, is to separate a ray of common light, whose vibrations are in every plane passing through the direction of the ray, into two parallel polarized rays, whose vibrations are in planes at right angles to each other. The preceding illustrations may aid in understanding that if a 'polarized ray falls upon another polarizing medium, in J. & W. GRUNOW & GO’S ILLUSTRATED PARTIAL POLARIZATION. 83 such a position that its vibrations are oblique to the polarizing planes or axes of the medium, they will be resolved into vibra- tions in hoth those axes or polarizing planes, and two new polarized rays will result, each of which might be again sub- divided in the same manner by another polarizing prism placed in a position oblique to the new axes. 108. Familiar illustrations. A ray of common light is sometimes compared to a cylindrical rod, whereas the polarized rays are like two flat parallel rulers, one of which is laid hori- zontally on its broad suface, and the other horizontally on its edge. The alternate transmission and obstruction of one of the flattened beams, by the tourmaline, is similar to the facility with which a card, or flat ruler, may be passed between the wires of a cage if presented edgewise, and the impossibility of its passing in a transverse position. We may also suppose a refracting substance, with a reflect- ing surface, made up of parallel fibres. Such a surface would allow the passage of all the rays in common light which vibrate in a plane parallel to the direction of its fibres and would reflect the rest ; while polarized light, if vibrating in a plane parallel to the fibres, would be wholly transmitted, but if vibra- ting in a plane at right angles to the fibres, it would be wholly reflected. 109. Partial Polarization. Having already explained in the previous section how light is polarized, 1st, by reflection ; 2nd, by refraction ; 3rd, by transmission through bundles of thin plates ; and 4th, by double refraction ; it now remains to state that a great variety of substances, and in different condi- tions, ^xodiWQQ partial polarization of light reflected from their surfaces or transmitted through them. It is well known that no substance either transmits^ or reflects^ all the light that falls upon it ; even the most transparent sub- stances reflect a small portion of light, the proportion reflected and transmitted varying with every angle of incidence. While plate glass polarizes very nearly all the light that falls upon it at an angle of 57°, the intensity of the reflected ray is equal to only one-half the intensity of the incident ray, the other half of the light is transmitted through the glass, and is, at the CATALOGUE OF ACHROMATIC MICROSCOPES. 84 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. same time, polarized by refraction in a plane at right angles with the plane of the reflected ray. If the light falls npon the glass at any other angle, some portion of it will be polarized by reflection, and an equal amount will be polarized by refraction, but each of the rays thus polarized will be mingled with other light that is reflected or transmitted without polarization. If instead of transmitting light through a bundle of plates of thin glass, we transmit it through only a single plate, and at any angle, that plate will polarize a part of the light ; if two plates are used, still more light will be polarized, and when a sufficient number of plates are used, all the light will be polarized. Many substances, not perfectly homogeneous throughout, have veins, laminae, or isolated spots, capable of producing either partial or complete polarization of the light which passes through them. In Iceland spar the two images produced by double refrac- tion are equal in intensity, and are both completely polarized ; but a great variety of substances give, by refraction, a secon- dary image so faint and so little separated from the ordinary image, that its existence is not generally recognized unless examined by some test for polarized light. There are many substances, both animal and vegetable, that possess this power of partial polarization, and which, when viewed by polarized light, exhibit an arrangement of structure totally un apprecia- ble by ordinary light. Even glass which has been cooled more rapidly in one part than another, or in which unequal pressure has been exerted in any manner, (as by bending or heating on one side,) is capable of polarizing some of the light transmitted through it. Many crystals found in animal fluids or vegetable tissues, pos- sess the power of partially polarizing light. Hence, polarized light becomes the most delicate test known for discovering differences of density or structure, in animal, vegetable, or min- eral substances, and is of great importance as a method of illu- mination in microscopic investigations. J. & W. GRUNOW & GO’S ILLUSTRATED POLARIZED LIGHT APPLIED TO THE MICROSCOPE. 85 POLARIZED LIGHT APPLIED TO THE MICROSCOPE. 110. Polarizing apparatus. The apparatus employed for microscopic investigations with polarized light, consist of a Nicol’s prism placed below the stage and called jpolarizeT and an analyzer^ which is usually also a Nicol’s prism, set in a brass tube and inserted in the body of the microscope imme- diately behind the object-glass. The 'polarizer is so mounted that it can be revolved, allowing the polarizing planes of the two prisms to be made to coincide or to make with each other any required angle. These two in- struments together constitute the Polariscojpe, Fig. 47. Fig. 48. MOUNTING OF POLARIZING PRISM. MOUNTING OF ANALYZING PRISM. Figure 47 shows the mounting of the polarizer which is attached to the stage by a bayonet joint. The polarizing prism is revolved by turning the milled head at the bottom of the instrument. Figure 48 shows the mounting of the analyzer, which is in- serted in the lower end of the compound body, and attached by a bayonet-joint in the usual position of the object-glass, but passing upward into the body, the object-glass being attached to the lower end of the analyzer. Figure 49 shows a section of the polarizer and the form of mounting. Above and below the ISTicol’s prism are circles of thin glass to protect the delicate faces of the prism from injury. CATALOGUE OF ACHROMATIC MICROSCOPES. 86 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. Figure 50 shows a section of the analyzer consisting of a Nicol’s prism and its mounting, and protected like the polarizer by circles of thin glass. Fig. 60 . SECTION OF POLARIZER. SECTION OP ANALYZER. The mountings of the jpolariscojpe are made of different sizes, to suit both large and small microscopes, and the prices vary accordingly, depending principally upon the size and quality of the i^icol’s prisms employed. The largest and best Nicol’s prisms transmit the most light and are suited for the more delicate investigations. Some opticians place the analyzer above the eye-piece, but this arrangement diminishes the extent of the field. If the analyzing prism is small, and is placed just above the object- glass, it stops out a portion of the light, but if it is of sufficient size it transmits all the light from the object-glass, and does not in any manner limit the field of view. For these reasons we prefer mounting the analyzer in the body of the microscope just above the object-glass. It may also be adapted to the lower end of the draw-tube, which arrangement allows it to be rotated by the milled ring of the tube itself. 111. A Touriiialiac plate fitted to the Eye-piece makes an excellent analyzer, when it can be obtained of suitable size and purity of color. Such tourmaline plates are compara- tively rare and costly. J. & W. GRUNOW & GO’S ILLUSTRATED COLORED POLARIZATION. 87 112. IIerai>atBiite or Artificial Tourii(ialiiac§. These are crystals of disulphate of iodine and quinine^ and when of sufficient size can be used both as polarizers and analyzers. They are called Herapathite from the name of their discoverer, Dr. Herapath, but large crystals have not yet been produced in sufficient abundance to allow of their general use. 113. Valite of the Polariscope isi Micro§copic iaivesti- gatioifi§. When the polariscope is attached to the microscope, the polarizer being below the stage and the analyzer above it, any object can be subjected to examination with polarized light. Objects having parts of their structure more dense than others, will present greater contrast of light and shade than by ordinary light, and thus the most delicate structures, as capil- lary blood-vessels, nerves, cell-walls, &c., will be well defined where they could not be otherwise distinguished. Sections of horn, teeth, bones, quills, shells, and many vegetable tissues, exhibit their delicate structure under the influence of polarized light. Revolving the polarizer causes the relative brightness of difierent structures to vary, and is essential in developing the greatest effect of polarized light in distinguishing delicate structures. Those portions of an object, possessing the power of partial or complete polarization, act like particles or veins of tourmaline to obstruct the passage of polarized light in certain positions, while the other parts of the object appear as lumin- ous as by ordinary light. So, also, when the position of the polarizer and analyzer is such as to cut off all the light from ordinary objects, the delicate structures that possess the property of polarizing light, depolarize the light already polarized and allow it to be transmitted, showing points or veins brilliantly illuminated amid other parts of the object which appear dark. It should therefore be laid down as a rule in microscopic in- vestigation, says Prof. Queckett, “ That every new variety of tissue should be subjected to the action of polarized light.” 114. Colored Polarization. All substances, whether ani- mal, vegetable, or mineral, which, by the unequal arrangement of their particles possess the property of double refraction, when placed between the polarizing and analyzing prisms exhibit colors^ varying according to the otherwise unapprecia- CATALOGUE OF ACHROMATIC MICROSCOPES. 88 POLAKIZED LIGHT AND ITS APPLICATION TO THE MICEOSCOPE. ble difference of density of their various parts, and these differ- ences may thus be distinguished and traced out much more satisfactorily than by common light. Polarized light may be compared to a new sense given to the student of nature, by which he is enabled to see things wholly invisible by ordinary light. Where the doubly-refracting properties of the tissue are too feeble to produce sufficient difference of color, the effect may be increased by placing the object over a plate of selenite or mica, of such a thickness as to give to the light any shade of color required, 115. Tiie Cawse of Colors produced toy selenite or mica, when polarized light is transmitted through them, is easily understood by reference to the undulatory theory of light. In all doubly refracting substances, (of which selenite and mica are examples,) the ordinary and extraordinary rays move with different velocities, and consequently, when the two rays are again blended, unless the retardation amounts' to a certain num- ber of entire waves, the two rays will, by the interference of their waves, produce some change in the color of light. If the retardation equals any number of entire vibrations, the result will still be white light, the two rays conspiring to increase their mutual intensity. If one ray is retarded an odd number of half vibrations, they will mutually destroy each other and produce darkness, just as if two waves of the sea meet in such a state that the phase of elevation in one coincides with the phase of depression in another, the two will produce a level, or mutual obliteration results. Such a result in the case of light, would require the most exact adjustment of the thickness of the crystal, and would not often occur. The interference produced by selenite and mica, are, in general, similar to the results which would be obtained by placing one prismatic spectrum upon another, in a reverse position, but not exactly superimposed upon it. The amount of overlaping would determine the resultant color. 116. Meltood Varying llie CoSors. When a film of selenite, of uniform thickness, is placed between the polarizer J. & W. GRUNOW k CO’S ILLUSTRATED ISIETHOD OF VARYINO THE COLORS. 89 and analyzer, on rotating tlie film a brilliant color is perceived at every quadrant of a circle, but in intermediate positions it vanishes altogether. We observe, however, that the tint does not change, but only varies in intensity. If, now, the film of selenite is fixed and the polarizer is rotated, we also observe color at every quadrant of revolution wdiich disappears in intermediate positions, but the tint changes and becomes complementary at every quadrant, — the same tint reappearing at every half-revolution. Thus, when the film alone is revolved, one color only is seen, but when the polarizer is revolved two complementary colors are seen. The following is Sir David Brewster’s table of complemen- tary colors : Red, complementary. Bluish green. Orange, Blue, Yellow, Indigo, Green, Violet reddish. Blue, Orange red. Indigo, Orange yellow, Violet, a Yellow green. Black, White, White, Black. Films of selenite, varying between .00124 and .01818 of an inch in thickness, will give every variety of tint in the solar spectrum. If two films of selenite are placed over each other, with their crystallographic axes parallel, the color produced wdll be that which belongs to the sum of their thicknesses. But when the two films are placed with similar axes at right angles, the resulting tint is that which belongs to the difference of their thicknesses. A film of selenite or mica of such thickness as to produce a bright purple, or a light blue color, will be found to present the most agreeable contrast, and, as a single plate, prove most generally useful to the microscopist. Three films of selenite, which separately give three different colors, may each be mounted in Canada balsam, between slips of thin glass, and used singly, or in double, or triple combinations. As many as thirteen different tints may thus be obtained. CATALOGUE OF ACHROMATIC MICROSCOPES. 90 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 117. Seleaiite §tage. This instrument, which was invented by Mr. Darker, is shown in Fig. 51. It consists of a plate of brass, three or four inches long, 1^ inches broad, and J of an inch thick, having a piece of raised brass screwed to it, against which objects may rest when the body of the microscope is inclined. Fig. 51. In the centre of the brass plate there is a hole, one inch in diameter, into which is fitted a ring of the same metal, with a shoulder on its under side to receive certain cells, into which plates of selenite are fitted ; this ring can be revolved either to the right or the left of a central index or dart, by means of an endless screw S. P A J, P A |, P A represent three brass cells, into each of which are burnished two plates of thin glass^ having between them films of selenite of difierent thicknesses. The dart P A, denotes the direction of the positive axis of the selenite, and the figures •}, f , J, denote the parts of a vibration retarded by each disc, which, by their superposition and varia- tion in position, by means of the endless screw motion, produce all the colors of the spectrum. 118. Polarizer with RevoSviaig §eleiiiitc Carrier. In order to afford the greatest facility of revolving the selenite plate, and for convenience of using it, a revolving selenite carrier is attached to the polarizer, as shown in Fig. 52. The solid ring is attached to the stage in the usual way by a bayonet-joint. A cylinder, with a milled head c, is supported by the ring d, which revolves in the ring a. Upon d rests the selenite carrier 5, covered by the cap e, so that the selenite plate is revolved by turning the milled head c. Within the cylinder J. & W. GRUNOW & GO’S ILLUSTRATED DELICATE STRUCTURES, VIEWED BY COLORED POLARIZED LIGHT. 91 v/hich supports the selenite, another cylinder, carrying the Nicol’s prism P, is separately revolved by turning the milled b. The Nicol’s prism revolves more easily than the selenite carrier, so that the latter may remain stationary when the former is revolved. By turn- ing the milled head c, the polarizer and selenite are re- volved together. This has the same effect upon the light as if the analyzer were revolved. By means of this apparatus, the axes of the polarizing prism, selenite plate, and analyzer^ may be placed in any relative position desired, affording great facility for using polarized light colored of any tint required in microscopic investigations. We are happy to acknowledge our obligations to Dr. White of 'New Haven, for suggesting this improved arrangement for revolving the plate of selenite. 119. Delicate structures, viewed by colored polarized light, produce much more sensible changes upon the light than upon plain polarized lights — microscopic crystals are especially beautiful when viewed in this manner. The crystallization of various salts, viewed by polarized light, is a subject of great importance to the practical chemist and mineralogist. So minute a quantity as one thirteen millionth of a grain of potassa, when tested with bichloride of platinum, gives a dis- tinct and characteristic tint, sufficient to distinguish it from every other alkali, when viewed in the microscope by this kind of light. Many substances, known in organic chemistry, are more readily distinguished by polarized light than by any other means. The examination of microscopic structures, by polarized light, affords to the enterprising student a rich field of investi- gation, as yet but partially explored. As a method of investi- gating delicate structures it is of the highest value. The chem- CATALOGUE OF ACHROMATIC MICROSCOPES. 92 POLAEIZED LIGHT AND ITS APPLICATION TO THE MICEOSCOPE. ist may perform the most dexterous analysis ; the crystallogra- pher may examine crystals by the nicest determination of their forms and cleavage ; the anatomist or botanist may use the dis- secting knife and microscope with the most exquisite skill ; but there are still structures in the mineral, vegetable, and animal kingdoms, which defy all such modes of examination, and which yield only to the magical analysis of polarized light. A body, which is quite transparent to the eye, and which might be judged as monotonous in structure as it is in aspect, will yet exhibit, under polarized light, the most exquisite organization, and will display the result of new laws of combination which the imagination even could scarcely have conceived. 120. Iii§t off Otojects for tlie Polariscope. Sufficient having now been stated to give the reader a general view of the nature and use of polarized light and its application to the microscope, we shall conclude this subject by simply giving a TABLE OF THE MOEE INTEEESTING OBJECTS FOE THE MICEOSCOPE, WHICH AEE ESPECIALLY BEAUTIFUL WITH POLAEIZED LIGHT : ANIMAL STRUCTUKES. ^ Bone of Cuttle-fish, ^ Fibres of Sponge, Hoof of Ass, Hoof of Camel, Hoof of Sheep, Hoof of Horse, Hoof of Ox, Horn of African Rhinoceros, transverse section, longitudinal section, Horn of Indian Rhinoceros, Horn of Antelope, Horn of Ox, Horn of Sheep, Polyzoaries, y Quill of Porcupine, Quill of Echidna, Quill of Condor, Tendon, Human, Tendon, Ostrich, Gray Human Hair, Raw Silk, Scale of Eel, Scale of Sole, Skin, Elephant, Skin, Crocodile, Skin, Human, f- Skin, Rhinoceros, Skin, various Serpents, Spicules of Gorgonia, r* Whalebone, Palate of Whelk, ^ Palate of Limpet, Palate of Nassa, Palate of Paludina, Palate of Cyclostoma, Wing cases of Beetles, Scales of Fishes, Sections of Hairs, P* Sections of Teeth, ^ Nerves and Muscle. J. k W. GRUNOW & GO’S ILLUSTRATED LIST OF OBJECTS FOR THE POLARISCOPE. 93 VEGETABLE. y Starch, Potato, X Starch, Arrowroot, Starch, Custard -powder, X Starch, Indian-corn, Starch, Tous les Mois, Gun-cotton, y Hairs from leaf of Deutzia, i Hairs from leaf of Elaeagnus, Hairs from leaf of Olive, Hairs from leaf of Cactus, Raw Cotton, ^ Raw Flax, ^ Siliceous Cuticle of Bamboo, Siliceous Cuticle of Eqiiisetum, Siliceous Cuticle of Rice, Siliceous Cuticle of Wheat, Raphides, Spiral Cells and Vessels, Wood, longitudinal sections mount- ed in balsam. y Agate, Arragonite, X Asbestos, Adventurine, Granite, Marble, y Selenite films. MINERAL. Sea sand, Tremolite, Satin Spar, Sandstone^ Feldspar, Crystals (of Titanic Iron, ?) in some varieties of mica. CRYSTALS, VIZ: Acetate of Copper, Bichromate of Potash, y Borax, Boracic Acid, Borate of Ammonia, Borax and Phosphoric Acid, Carbonate of Lime, Chromate of Potash, Chlorate of Potash, Cholesterine, Nitric Acid, Epson Salts, Oxalic Acid, Oxalate of Ammonia, Oxalate of Chromium, Oxalate of Lime, Oxalate of Soda, Nitrate of Ammonia, Nitrate of Baryta, Nitrate of Lead, Nitrate of Potash, Nitrate of Soda, Phosphate of Soda, Salicine, % Sugar, Sugar of Milk, Sulphate of Cadmium, Sulphate of Copper, Sulphate of Magnesia, Sulphate of Nickel, Tartaric Acid, ' Tartrate of Lime, Uric (or Lithic) Acid, ^ Triple Phosphate. CATALOGUE OF ACHROMATIC MICROSCOPES. 94 PRACTICAL DIRECTIONS. CHAPTER V. PRACTICAL DIRECTIONS. 121. Care of tlie ]TIicro§cope. It is of the first importance that every part of the microscope should be kept perfectly clean and free from dust. For this purpose a good case is re- quired, into which the instrument can be easily placed wdien not in use. In order that no unnecessary time may be lost in packing and unpacking, the upright case is more convenient, as the instrument can be placed in it nearly in the same condi- tion as when arranged for use. Each object-glass, when not in use, should be carefully returned to the brass box provided for it, as it is thus less likely to be injured by dust or other means than when attached to the microscope. The case should be arranged with proper fittings to hold all the apparatus belonging to the microscope. Two or three drawers are required to hold slides, thin glass, dissecting needles, &c. If the microscope gets soiled it may be wiped with fine linen or cambric. The lenses of the eye-piece may be wiped wdth soft buckskin which has been thoroughly freed from dust, but care should be taken that this operation may be required as seldom as possible. The object-glasses will lose their fine polish if often wuped. Some microscopists are so careful of their object-glasses that they do not require cleaning for years. If an object-glass requires cleaning it may be brushed with a fine short carners-hair-pencil which is used for no other pur- pose, or it may be moistened by breathing on it and then gently pressed with soft buckskin. When fluids are used about the microscope, care should be taken to avoid wetting the object-glasses. If this accident happens, they must be wiped dry with the buckskin. In handling the object-glasses, care should be taken to avoid touching the surface of the lenses. The glasses should be examined when returned to the case to see if they have been soiled. Too great precaution cannot be taken to keep every- thing about the microscope scrupulously clean. J. & W. GRUNOW & GO’S ILLUSTRATED CHOICE OF A MICKOSCOPE. 95 122. Illumination. For whatever purpose the microscope may be used, it is important to secure a pure and adequate illu- mination. The young microscopist will find daylight more easily managed, and more pleasant for the eyes, than artificial light. Even experienced microscopists can work longer and easier by good daylight, though they may have learned to manage artificial light so as to make it more available than daylight in bad weather. The best light is that reflected from white clouds, but light from fleeting clouds is troublesome to the eyes and requires constant moving of the mirror. Light from a luminous atmos- phere, near the horizon, is better than that near the zenith. The direct light of the sun cannot ordinarily be used for illu- minating the microscope, but the light of the sun, reflected from white houses, or from a white plastered wall, gives a soft and beautiful illumination. If possible, the microscope should be placed at a little dis- tance from a window on the side of the house, opposite to where the sun is shining. If artificial light is used, the Ger- man student’s lamp, constructed on the bird fountain principle, which gives excellent illumination, will be found as convenient as any other. If gas light is used, a pane of light blue glass placed between the light and the microscope, will take off the intense glare which is otherwise apt to injure the eyes. All flickering lights are very unpleasant in microscopic investiga- tions. Observations will generally be conducted with greater ease, when the body of the microscope is inclined. 123. Ctioice of a Microscope. In choosing a microscope, reference should always be paid to the nature of the investiga- tions for which it is to be used. The most common error among inexperienced persons, is the idea that all objects can be satisfactorily examined with a high magnifying power. This is by no means correct, for it is often as desirable to ascertain the relations of different portions of an object, as to examine minutely any single part. Small crystals, insects, and parts of flowers, hairs, &c., which are readily obtained, are best studied with magnifying powers of from twenty to one hundred diame- ters. Young people in schools, academies, and private fami- lies, generally first examine this class of objects, and but few such persons can devote the requisite attention to prepare speci- mens with sufficient care to allow of their being examined with magnifying powers higher than three hundred and fifty diameters. The Educational Microscope, (page 25 of this Catalogue,) CATALOGUE OF ACHROMATIC MICROSCOPES. 96 PRACTICAL DIRECTIONS. has been prepared with especial reference to the greatest effi- ciency for this class of observers. It is also furnished at a very moderate price. Our advice is often solicited by persons who have a limited amount of money to expend for a microscope, and who desire to know what selection would be most useful for their several purposes. For those who design to devote but a limited amount of time to the use of the microscope, we would recommend one of the smaller microscopes, 'No. 1, 2, or 3, with the appara- tus usually attached to them as stated in our Price List. To those who desire to have at once an instrument suited to the more delicate investigations at the smallest cost, we would recommend the Student’s Microscope, Ho. 3, furnish- ed with First Class Ohjectives.^ to be purchased according to their means in the following order: For the study of botany and mineralogy, the 1 inch, J inch, ^ inch, 2 inch, J inch, inch objectives. For studying entomology, anatomy and pathology, the -J- inch, i inch, 1 inch, J inch, 2 inch, ^2 i^^^h object-glasses. The Student’s Larger Microscope, Ho. 4, furnished with 1 inch, -J- inch, and i inch objectives, Bull’s-eye Condenser, Camera Lucida, and Micrometer, forms a very complete instru- ment with which almost all vegetable and animal tissues and fluids can be examined satisfactorily. This combination is especially recommended to Medical Students, and all others who intend to devote considerable attention to microscopic in- vestigations, and desire to make the most economical invest- ment of limited means, and yet secure s, first class instrument. 124. equalities of Objcct-Crlasses. In considering the value of an object-glass, and its adaptation to any particular purpose, several distinct qualities are to be examined, viz: (1.) its defining power ^ or the power of giving sharpness of outline, especially on the borders of an object, or where dots or lines are examined ; (2.) its resolving power., by means of which closely-approximated markings are distinguished ; (3.) fiatness of field ; (4.) depth of definition., which refers to the distance above and below the focus, that parts of an object can be seen with tolerable accuracy. 1. Defining power ^ in addition to perfect workmanship, depends principally upon the perfection of the corrections, both for Spherical and for Chromatic aber- ration, The difficulty of securing this perfect correction increases with the angular aperture. Any inaccuracy in adjusting the object-glasses for thickness of glass cover, which every observer must arrange for himself, is more conspicuous in glasses of large angular aperture. For this reason, object-glasses of moderate J. & W. GRUNOW & CO’S ILLUSTRATED QUALITIES OF OBJECT-GLASSES. 97 aperture are more suitable to be used in schools and private families where many persons use the same microscope, and where, for want of time, in examining a variety of specimens, or from inexperience, the necessary attention cannot be devoted to adjusting the correction for thickness of glass cover. 2. Resolving 'power (correct definition being presupposed) may be said to stand in a direct relation to angular aperture, and consequently to the obliquity of the rays which can be received from the surface of an object. To measure the angular aperture of an object-glass^ place the microscope, with the body horizontal, on a thin board which turns on a pivot exactly under the focus of the object-glass ; set a lamp on a level with it a few yards distant; then having directed the body of the microscope so far on one side of the light that only half of the field is illuminated, leaving half of it dark, trace a line by the edge of the board on which the microscope stands ; now revolve the microscope horizontally to the other side of the light till only the opposite half of the field is illuminated ; the angle now formed by the edge of the board and the line pre- viously traced, is equal to the angular aperture of the object-glass. If the object- glass has a very large angular aperture, the line of demarkation between light and darkness in the field, will be indistinct, and the experiment must be performed in a dark room, with a ray of sunlight entering through a narrow perpendicular slit, by which means the exact angular aperture will be more readily determined. Without due precaution, errors of several degrees will be made in estimating the angular aperture of object-glasses. Adjusting the object-glass for a covered ob- ject, will increase the angular aperture of some object-glasses ten or fifteen degrees. Extending the draw-tube has a similar effect. Hence, in comparing the angular aperture of two objectives, they are to be examined with the same eye- piece, similar corrections for glass cover, and the same length of tube. Some methods employed to determine angular aperture, reall-y determine noth- ing but the angular breadth of the field of view, which is often less than the angular aperture for an object in the focus of the microscope. The real question to be determined is the angular aperture of the object-glass as ordinarily used hi the microscope ; not what is its angular aperture in other conditions where it is never placed for practical use. 3. Flatness of Field. To judge correctly of this quality, object-glasses should be tested with an eye-piece which gives a tolerably large field of view. In micro- scopes of inferior quality, the defects of the objectives are often concealed by eye- pieces so constructed as to give but a very limited field of view. 4. Depth of Definition. The qualities already enumerated, defining power, re- solving power, and flatness of field, may all coexist in the same object-glass, but there is another quality so essential to the prosecution of microscopical researches of a certain class, and which is generally so little understood and appreciated, that we shall dwell upon it more particularly. We refer to the quality which some object-glasses possess, in addition to clear definition in the focus, of giving tolera- ble definition of parts a little above or below the exact focus. This quality may, per- haps, be called Depth of Definition., as it refers to the distance above or below the focus where definition ceases, and where objects, by their distance from the focus, become invisible. The value of object-glasses used for viewing tissues containing cells or vessels variously related to surrounding parts, and, in short, for the practical every-day work of the microscopist, depends very much on the quality which we have here called depth or extent of definition. CATALOGUE OF ACHROMATIC MICROSCOPES. 7 98 PRACTICAL DIRECTIONS. As a general rule, it will be noticed that object-glasses, of the largest angular aperture, do not show parts above or below the focal plane as well as glasses of moderate aperture, and yet, in this respect, a great difference is perceived in the object-glasses of different makers. (a.) If two objectives, having the same magnifying power and angular aperture, are both perfectly corrected for chromatic and spherical aberration, the glass which has the greater distance between the anterior lens and the object, will have the greater depth of definition. (6.) If two objectives have the same magnifying power and the same distance between the anterior lens and the object, the glass having the smaller angular aper- ture will have the greater depth of definition, provided the angular aperture is not too small to admit a tolerable amount of light. (c.) It is possible, however, that an object-glass of small angular aperture may be made with its focus so near the anterior lens, that another glass of larger aper- ture could be constructed with a focus so far from the anterior lens that its depth of definition might exceed that of the objective of smaller aperture. Such a glass of large aperture would be, in every respect and for all purposes, greatly superior to the other. (c?.) But if an object-glass of moderate angular aperture, perfectly corrected, has its focus at i\\Q greatest possible distance from the anterior lens, every addition to the angular aperture, in another similar object-glass, necessarily (in the present state of science) requires the distance between the anterior lens and the object to be diminished. Consequently, with the enlargement of the angular aperture, the depth of definition is generally diminished. If opticians could procure, for the manufacture of object-glasses, glass of such refractive and dispersive power as they would prefer, it would enable them to improve very much the depth of defi . nition in object-glasses of large angular aperture. Some microscopists have had objectives of large angular aperture provided with a diaphragm, to be introduced behind the posterior lens, when viewing objects re- quiring greater extent of definition of parts above and below the focus. Some advantage may be gained in this manner, but object-glasses so arranged, cannot have as great depth of definition as if originally constructed with the greater distance between the anterior lens and the object, which would be possible with the limited aperture to which these glasses are thus temporarily reduced. In enlarging the angular aperture of our object-glasses, we have always sought to retain a reasonable working focus, or distance, between the anterior lens and the object, and so to combine depth of definition with the other qualities already enumerated, as shall give to our object-glasses the greatest degree of efficiency for the various uses of the practical microscopist. PRICE LIST OF ACHROMATIC MICROSCOPES AAD MICROSCOPICAL APPARATUS. PRICES OF MICROSCOPE STANDS,* Exclusive of Objectives, Apparatus and Cases, except where mentioned. No. 1. £ducatioaial IWlicroscope, (Sec. 45,) including two eye-pieces, (Nos. 1 and 2,) and 1 inch and ^ inch ob- jectives of second class, which, by combination with the two eye-pieces give magnifying powers respectively of 40, YO, 180, and 325 diameters, packed in a mahogany case, . $45.00 The same instrument, with the following additional apparatus, (if ordered at the same time,) viz, Polariscope, Camera lucida. Stage-micrometer, Bull’s-eye condenser. Animalcule cage. Stage-forceps, Hand-forceps, and a set of dissecting instruments, 15.00 No. 2. Studeiit’§ Microscope, (Sec. 46,) two eye-pieces, movable diaphragm, and 1 inch and ^ inch objectives of second class, (magnifying as above,) with mahogany case, . 50.00 Three dissecting lenses, magnifying 5, 10 and 15 diameters re- spectively, mounted so as to be inserted in the arm of the microscope when the body is removed, .... 8.00 No. 3. Student’s Microscope, (Sec. 47,) with the same eye-pieces, objectives and diaphragm as the preceding, in a mahogany case. This is the basis of a complete instrument, 60.00 * All the Microscope Stands are so constructed that additional apparatus can be supplied without requiring the instrument to be sent for fitting. It is only neces- sary to state the No. of the instrument in the Catalogue, and the No. engraved upon it. 100 PKICE LIST. No. 4, A. §tiideait’s l^arger Microscope §taiac3, (Sec. 48,) with two eye-pieces, (Nos. 1 and 2,) ... 870.00 No. 4, B. The same stand and eye-pieces, with stage movable in two directions by rack and screw, instead of a lever, (Sec. 49,) 75.00 Either form of stage above mentioned, constructed so as to re- volve around a steady centre, extra, .... 10.00 No. 4, C. The same stand and eye-pieces as No. 4, A, witli plain stage, 55.00 No. 5, A. Studeiirs liargea* Micrcscopc SJaad, made more portable than No. 4, and adapted to be used for dis- secting; Lever stage revolving around a fixed centre, (See Sec. 50,) with two eye-pieces, (Nos. 1 and 2,) . . . 90.00 No. 5, B. The same instrument as above, with stage movable in two directions by rack and screw, with two eye-pieces, . 95.00 No. 5, C. The same stand and eye-pieces, with plain stage, . 65.00 No. 6. Portable Microscope Stand, (Sec. 53,) with twm eye-pieces carefully packed in a mahogany case of very convenient dimensions, with fittings for apparatus, . . 105.00 No. 7, A. r