194 PHOTOMICROGRAPHY 



half of its back lens and then the other, and making the 

 two exposures under these conditions. The best stop to 

 use for this purpose is one that will just fit over the back 

 lens, and provided with a small circular aperture excen- 

 tric in position (Fig. 61). After the first exposure this 



e 



Fig. 61 



OBJECTIVE STOP FOR STEREOSCOPIC PHOTOMICROGRAPHY 



stop is turned round through 180 and the second 

 made. 



Very beautiful stereoscopic results are obtained with 

 microscopes fitted with two exactly similar objectives, 

 whose axes converge on the object. These microscopes 

 have two tubes to which a camera for two plates is 

 attachable. 



Stereoscopic effect necessarily requires a considerable 

 depth of focus, and as generally only low-power objec- 

 tives are used, this is as a rule easily attained. The 

 circular stops behind the objective help in this way, and 

 a Davis diaphragm is also very useful. 



To obtain pleasing results it is well to give fairly full 

 exposures and make moderately soft prints : any very 

 harsh contrasts are unpleasantly obtrusive in the stereo- 

 scope, and mar the effect. When the two prints are made 

 on one piece of paper from the two negatives on one plate, 

 it is necessary to cut them and reverse their positions left 

 to right on mounting. 



Metallography. The photography of the minute struc- 

 ture of metals provides one of the most important 

 applications of photomicrography, and most of the 

 refinements devised for the study of opaque objects under 

 medium and high powers have been designed with the 



