1893.] MICROSCOPICAL .TOURNAL. 331 



and tlie undulations oftlie lldiopclla jind the curved nliape of 

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

 toms are not the only objects which can be equally beautifully 

 pictured. Stereoscopic photomicrographs of injected animal 

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

 vessels in natural position instead of on a single [)lane as in 

 ordinary photomicn)graphs. Insects, the rather thick sections 

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

 quire relief to give a proj)er conception of their form, are all 

 best represt^nted by this kind of photograph. 



Ster oscopic ph()t()micrognii)hs may be taken in several 

 ways — thf end to be attained l)eing to [)roduce a result similar 

 to that jirodiiced by ordinary binocular vision. In normal vis- 

 ion, the e^'es being placed at some distance from each other, 

 each gets a slightly different lateral im[)res.sion of the olyect 

 viewed and a coml)ination of these impressions by nif-ntal pro- 

 cesses produces the effect of relief. In stereoscopic photomicro- 

 grnphy the nece-sary different Literal views of the object are 

 obtained either by tilting the ol)ject or by using different halves 

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

 one preferably t » be adopted wdienever practible, the slide car- 

 rying the object is tiltt-d on the microscope stage and a negative 

 is taken, the slide is then tilted in the opposite dir. ction 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 usial vision ; and a combination of t lese pictures when 

 viewed through a stereoscope gives stereoscopic effect. This 

 eff'eci is strictly analog ms to that of b nocular vision, for fr.>ni 

 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 b}' the use 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 demons'r.ited by Professor Abbe, 

 that in an aplanatic system pencils t)f dift'erent 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. 



