816 SUMMARY "OF CURRENT RESEARCHES RELATING TO 



C4) Photomicrography. 



Stereoscopic Photomicrography.* — A special Microscope has been 

 devised tor the purpose of taking stereoscopic photomicrographs, the body 

 of which moves laterally in the arc of a circle, with the specimen as the 

 axis. After the first exposure the body-tube is slightly moved to one 

 side, the optical system keeping in a line with the object, and the second 

 plate exposed. This ingenious and successful instrument is, however, 

 says J. I. Pigg, rendered superfluous by a much simpler plan. 



Some years ago a comparatively easy and effective method was 

 introduced for taking stereoscopic photographs with the Microscope. 

 Instead of altering the position of either the specimen or the Microscope- 

 tube, the first exposure is made with one-half of the back-combination 

 of the objective, covered with some opaque material. The back lens- 

 coveriug is then moved to the other side of the objective, and the second 

 plate exposed. The two negatives are thus taken through different 

 halves of the back-lens, and the consequent separation of the two points 

 of view produces the stereoscopic effect. 



An improved variation of this mode of making a stationary lens 

 take two pictures from different standpoints, has since been adopted by 

 photomicrographers. Instead of using the two halves of the back- 

 combination of the objective, a diaphragm with a circular aperture cut 

 near the edge of the stop is fitted behind the lens. After the first ex- 

 posure the diaphragm is turned round, so that the aperture is at the 

 opposite side of the objective ; the second negative is then taken. The 

 use of a small aperture, in place of half the area of the lens, effects a con- 

 siderable improvement in the stereoscopic effect, owing to the increased 

 separation of the two points of view, but the exposure is proportionately 

 increased. The worker must exercise his own discretion as to the exact 

 size and position of the aperture. If the diameter of the lens will allow 

 of the aperture being placed at a fair distance from the centre, the 

 stereoscopic relief will be considerably improved. 



This method of stereoscopic photomicrography is obviously most 

 suitable for low-power work, and the beginner should confine his atten- 

 tion to low magnifications. The best results are obtained from objects 

 illuminated by reflected light, and really beautiful effects can be secured 

 from common microscopic objects. Unless an arc lamp is available, 

 specimens to be photographed as opaque objects should be of a light 

 colour, or the exposure will be abnormally prolonged, as a considerable 

 amount of light is cut off by the diaphragm. Diaphragms for stereo- 

 micro work may be purchased, but the amateur can easily make his own 

 by punching out a hole of the requii'ed size near the edge of a circular 

 piece of black paper. This diaphragm can then be fixed in position 

 at the back of the objective, by being pressed into the mount. When the 

 diaphragm is moved round for the second exposure, the aperture must 

 be exactly opposite the position it occupied for the first negative. 



Insects' eggs, foraminifera, small insects, and seeds of plants, make 

 excellent subjects as opaque objects for reflected light ; crystals, botanical 

 sections, filamentous alga3, and foraminifera mounted in liquid cells, are 



* Photographic Scraps, vi. (1911) pp. 279-80. 



