358 



SCIENCE. 



PHOTO-MICROGRAPHY. 

 By Dr. Carl Seiler. 



All workers in microscopy, doubtless, appreciate the 

 necessity of correctly recording, not only in writing, but 

 also by means of pictures or drawings, many of the ap- 

 pearances seen in the field of the microscope. We can 

 do this by drawing an outline of the objects observed, 

 by the aid of the camera htcicia ; but not only dees this 

 require some practice, but also a considerable amount of | 

 time, and even then the resulting picture will not be a 

 correct representation of the field of the microscope, 

 because it will always be tinged more or less by the I 

 imagination of the draughtsman, and will be more or 

 less diagramatical in consequence. 



With photograph} - , on the other hand, an exact repro- | 

 duction of the image thrown upon the screen can be 

 obtained, and in much less time than it takes to make 

 even a comparatively simple drawing with the camera 

 luctda. It is the object of this chapter to give an idea 

 of the means employed to obtain a photographic picture 

 of a microscopic object — means which are in the hands 

 of every microscopist, and which do not require a great 

 outlay of money. 



A room with a southern exposure, which can be 

 darkened ; a mirror, movable in all directions, outside 

 of the window ; an achromatic combination of lenses of 

 from eight to ten inches focal length ; a microscope 

 which can be tilted so as to be horizontal, and a stand 

 to hold the screen and sensitive plate, are all the appa- 

 ratus absolutely necessary besides the chemicals used in 

 ordinary photography. 



Fig. i. 



These different pieces are disposed of as follows (Fig. 

 i): The mirror (a), which should be eight or ten inches 

 long by about four inches wide, is attached to a board 

 which fist into an opening in the dark shutter of the 

 southern window, and is to be moved by rods from the 

 inside. Instead of this mirror, or in conjunction with 

 it a heliostat is of great advantage to throw the light of 

 the sun constantly in one direction, for, if once adjusted, 

 it need not be disturbed, and thus a great deal of time 

 is saved. Until recently such an instrument was too 

 costly for the use of students, but of late Mr. Kuebel, 

 of Washington, D. C, has put a heliostat in the market 

 which works very satisfactorily and which is sufficiently 

 low in price to be within the reach of many who desire 

 to work in photo-micrography. The board in the shut- 

 ter has in its centre a circular opening containing an 

 achromatic combination of lenses (b), such as the back 

 combination of a one-fourth portrait photographic lens. 



The microscope is secured on the window-sill in a 

 horizontal position, so that the axis of the tube is in a 

 line with the axis of the achromatic combination, and 

 at such a distance from it that the burning focus is 

 about half an inch from the back combination of the 

 sub-stage condenser (d). The eye piece is then removed 

 from the microscope, and the tube lined with black vel- 

 vet, to prevent internal reflection, as far as possible, and 

 the whole apparatus is covered with dark cloth, to pre- 

 vent stray rays of light entering the darkened room. 



This done, the sun's rays are reflected from the mitror 

 outside the window, through the achromatic combination, 

 which acts as a concentrator and throws a powerful 

 light through the condenser, through the object on the 

 stage (s), and thus a brightly illuminated image is formed 

 by the objective {o) on the screen, which latter, when 

 the negative is to be taken, is replaced by the sensitive 

 plate. 



This image, when thus formed, must be focused with 

 the greatest care and accuracy, in order to obtain a sharp 

 negative; and as the screen must be at some distance 

 from the microscope in order to obtain the necessary 

 magnification of the object, it is necessary to have some 

 conirivance for turning the fine adjustment at a distance. 

 For this purpose it will be found that a small pulley, 

 placed alongside of the microscope, having an endless 

 band running over it and the milled head of the fine 

 adjustment, answers the purpose very well, when the 

 axis of the pulley is connected by means of a universal 

 joint to a fishing-rod, which by its sections can be made 

 longer or shorter, thus bringing its end close to the 

 screen. 



The tube of the microscope, even when all internal 

 reflection has been obliterated, still remains a drawback, 

 inasmuch as it reduces the size of the image, or rather 

 the disk of light, the more, the longer it is. There are, 

 however, some stands made in which the tube can be 

 entirely removed, such as the old Ross stand, and they 

 are therefore very desirable for photc-micrographic pur- 

 poses. 



Any good objective of wide angular aperture and 

 good definition can be employed for photography, pro- 

 vided monochromatic light is used in making the nega- 

 tive. When such is the case the visual and chemical 

 foci fall in the same plane and a special correction of the 

 objective for photography becomes unnecessary. 



Such a light is obtained by passing the rays of the 

 sun through a cell containing a strong solution of am- 

 monio-sulphate of copper (c) before they enter the sub- 

 stage condenser. I have found some difficulty in making 

 the cell containing this solution, as the copper salt will 

 dissolve almost any cement, and if exposed to the action 

 of the air, very rapidly becomes decomposed, and the 

 solution is thereby rendered useless for the purpose. I 

 have used with satisfaction a cell made of a brass ring, 

 lined on its inner side with lead or tin, having a thread 

 cut on its outside, to which flanged rings are secured. 

 Upon the edges of the inner ring a ring ot rubber pack- 

 ing is applied, and upon it a disk of plate glass is laid, 

 which is tightly pressed upon the rubber by the flanged 

 ring. Thus a cell is obtained very similar to the round, 

 flat spirit levels, and which will hold the ammonia 

 sulphate of copper solution for months without change. 

 In filling the cell care should be taken to leave room 

 for a small air bubble, for if the cell is completely filled 

 the heat of the sun's rays will expand the solution 

 sufficiently to cause leakage. 



This solution, besides giving monochromatic light, at 

 the same time filters out almost all the heat rays from 

 the light, so much so that an immersion lens may be 

 used for any length of time without the drop of water 

 evaporating. 



At the present time, when dry plate photography has 

 been developed to such an extent that it has superseded, 

 in a great measure, the wet process, it has been thought 

 that it would be the most simple, economical and satis- 

 factory for photo-micrography ; but after repeated trials 

 by myself, as well as many others working in the same 

 direction, it has been found that it is not only more ex- 

 pensive, but also takes more time, in the long run. The 

 reason of this is that it is impossible to judge, with any 

 degree of certainty, as to the actinic power of the light 

 forming the image on the screen by merely looking at 

 it, and that a trial plate only will give an idea of the 

 length cf exposure necessary for a given day, time of day 



