1893.] MICROSCOPICAL JOURITAL. 331 



and tlie uiululations of the Hellopelta and the curved shape of 

 the Coscinodiscus are beautifully brought out. Of course, dia- 

 toms are not the only objects which can be equall}' beautifully 

 pictured. Stereoscopic photomicrographs of injected animal 

 tissues, which of necessity are cut rather thick, show the blood 

 vet^sels in natural position instead of on a single plane as in 

 ordinary photomicrographs. Insects, the rather thick sections 

 of vegetable histology, crystals, and the many objects which re- 

 quire relict to give a proper conception of their form, are all 

 besL represented by this kind of photograph. 



Stereoscopic photomicrographs may be taken in several 

 ways — the end to be attained being to produce a result similar 

 to that produced by ordinar}' binocular vision. In normal vis- 

 ion, the eyes being placed at some distance from each other, 

 each gets a slightly different lateral impression of the object 

 viewed and a combination of these impressions by mental pro- 

 cesses produces the effect of relief. In stereoscopic photomicro- 

 graphy the necessary different lateral views of the object are 

 obtained either by tilting the object or by using different halves 

 of the objective for each view. In the first method, which is the 

 one preferably t > be adopted whenever practible, the slide car- 

 rying the object is tilted on the microscope stage and a negative 

 is taken, the slide is then tilted in the opposite direction and 

 an exposure made on a second plate. In this way pictures are 

 obtained of the object partially from opposite sides, as with the 

 eyes in usual vision ; and a combination of tiese pictures when 

 viewed through a stereoscope gives stereoscopic effect. This 

 effect is strictly analog ms to that of b nocular vision, for fr.)m 

 the back and forth tilting of the object, the objective acts first 

 as does one eye and then as the other. The point of view in 

 both cases being not directly from the front but slightly from 

 opposite sides. 



Where stereoscopic effect is obtained by the us^ of different 

 halves of the objective, the cause is different, though the re- 

 sulting effect is practically always similar. This is due to the 

 fact, which has been fully demonstrated by Professor Abbe, 

 that in an aplanatic system pencils of different obliquities yield 

 identical images of every plane object, and that there is a par- 

 allel projection of all the successive layers in one common plane 

 perpendicular to the axis of the microscope. There is, therefore, 



