Apparatus and Material used in micro-photography have, from time to time, been greatly varied by different workers, some preferring to use the microscope in the vertical position with the camera superimposed or fitted on the eye-piece of the microscope tube; others, again, prefer that both the microscope and the camera should be arrayed horizontally. In another form the ordinary microscope is dispensed with and the objective stage and mirror are adapted to the front of the camera, together with a suitable arrangement for holding the object. Lastly, the camera is lain aside, and an operating-room rendered impervious to light, takes its place, and the image is projected and focussed upon a ground glass screen held in its place by a separate support. This method has been made practical since the introduction into microscopy by Zeiss of the projection eye-piece. It is well known that micro-photographs can be produced by employing these projection eye-pieces, as well as for screen illustrations in the lecture-room.
Fig. 172.—Swift’s Horizontal Apparatus.
With regard to the position of the microscope and camera, the horizontal affords greater stability than the vertical, and is on this account to be preferred. The simplest apparatus consists of a camera fixed upon a base board, four or five feet in length, upon which the microscope can be clamped, and which also carries the lamp and bull’s-eye lens ([Fig. 172]). This arrangement I have found economical and useful. No more elaborate arrangement is actually necessary. Sunlight is no doubt the best, but a good paraffin lamp is a handy and available illuminant.
With the former, and rapid plates, a short exposure of three or four seconds, even when high powers are used, is found sufficient; whereas, with the paraffin lamp it will vary from three to ten minutes.
Walmsley gives the following table for exposures with the lamp:—
| 1½-inch | objective | 3 to 45 | seconds. |
| 2⁄3-inch | " | 7 to 90 | " |
| 4⁄10-inch | " | ½ to 3 | minutes. |
| 1⁄5-inch | " | 2 to 7 | " |
| 1⁄10-inch | " | 4 to 10 | " |
For micro-photography the following practical rules must be observed. The sub-stage condenser may be dispensed with when low powers are used, as well as the mirror, and the lamp so placed that the image of the flat of the flame appears accurately adjusted in the centre of the field of the microscope. The bull’s-eye lens is so interposed, that the image of the flame disappears, and the whole field becomes equally illuminated with high powers; the sub-stage achromatic condenser must be used, and a greater intensity of illumination is obtained by placing the lamp-flame edgeways. It is advisable to begin the practice of micro-photography with low powers, and a trial experiment should be made with some well-known object as the blow-fly’s tongue.
Dr. Crookshank is of opinion that, in the case of micro-organisms when their biological characters are studied under low powers of the microscope, photographs are preferable, because they give a more faithful representation of the object. A micro-organism, even under the highest powers of the microscope, is so minute an object, that to represent it in a drawing requires a very delicate touch, and it is only too easy to make a picture which gives an erroneous impression to those who have not seen the original. Photography enables the scientific worker to record rapid changes, and it is quite possible as the art advances we may find the film more sensitive than the human retina, and that it will bring out details in bacteria which would be otherwise unrecognised. The result, therefore, of experience is that in research laboratories it will come into more general use as a faithful and graphic method. I cannot better bring these observations to a close than by giving a quotation from Dr. Piersoll’s practical method of obtaining micro-photographs.
The three essential conditions to ensure success in micro-photography are:—(1) Satisfactory apparatus; (2) good illumination; (3) suitable preparations. With high amplifications (1,000 diameters and over), the conditions are greatly changed by the approach to the limit both of the shortness of the focus of the objective and of the length of the camera which can be advantageously used; for the first experience leads to the adoption of the 1⁄12-inch, for the second four feet is the limit, since a given high amplification, say 2,000 diameters, can be more satisfactorily and more conveniently obtained with a superior 1⁄12-inch connection with suitable optical means to increase the initial magnifying power of the objective, than with an unaided 1⁄25-inch lens, and the plate removed to a greater distance. Until quite recently the various amplifiers offered the best means of increasing the power of an objective, but the introduction of the projection-oculars of Zeiss is an accessory piece of apparatus, far superior to any older device. These projection-oculars resemble ordinary microscopical oculars or eye-pieces only in general form and name, being optically a projection-objective in connection with a collecting lens. The new oil-immersion apochromatic lenses, in combination with these projection-oculars, form undoubtedly the more efficient equipment for high-power work; it is as true for high-power photography as for microscopical observation in general, that the best results are obtained with fine and necessarily expensive, optical appliances. If for the satisfactory study of the intimate structure of a cell, or of a micro-organism, the most improved immersion lenses are necessary, it is to be expected that, for the successful photography of the same, tools at least as good are needed. Sunlight certainly affords the most satisfactory illumination whereby good micro-photographs can be obtained, as well as for recording microscopical images. That by good lamp-light fair impressions of objects under extreme magnification can be secured is encouraging, but the negatives produced by such illumination seldom, if ever, possess the characteristics of a really good sunlight negative, where the sharpest details are combined with an exquisite softness and harmony of half-tones.