VISION THROUGH STEREOSCOPIC BINOCULAR MICROSCOPES. 225 



To what extent the depth of the field of view modifies the 

 stereoscopic effect we leave undecided. Without comparative 

 observations of objects with or without depth (e.g. photographs on 

 glass, &c.) nothing decisive can be said on this point. In 

 Harting's first edition it was stated that this factor is of special 

 importance in stereoscopic observation, which we considered quite 

 erroneous. In the second edition, however, he merely states that 

 "in forming a judgment on the performances of the binocular 

 Microscope " the depth of the field of view also comes into account 

 (loc. cit. i. p. 213). It is not however proved, but merely indicated, 

 that the field of view possesses a certain depth, which no one 

 has doubted. So much is at any rate certain, that the influence 

 in question is of slight importance, and in no case necessary 

 for supplementing the stereoscopic effect. For as in ordinary 

 stereoscopes two plane pictures are combined to form one stereo- 

 scopic image, the images of the binocular Microscope would also 

 produce the impression of solidity, even if the depth of the field 

 of view were nil. 1 



That the differences in the distribution of light and shadow 

 determine the stereoscopic effect is proved also by the above- 

 mentioned photo-micrographic process, but especially by the use ot 

 the so-called stereoscopic "see-saw." Instead of bringing into 

 action the right and left halves of the objective successively, in 

 order to observe the object equally from different points of view, 

 the object-carrier is tilted to the right and left, so that the 

 object appears to be seen under a different angle. The shifting is 

 effected between the two operations, and without alteration of the 

 focal adjustment. 2 The angle through which it is turned amounts 

 with low amplifications to 12, with medium to about 7 or 8, and 

 with high powers (e.g. with Nachet's No. 5 objective) to about 

 4 or 5. 



1 Helmholtz (" Handb. der Physiolog. Optik," p. 682) also explains the 

 origin of the stereoscopic effect by the position of the dispersion circles which 

 the points of the object situated in front of or behind the plane of adjustment 

 produce, according to which, therefore, a certain depth of the field of view 

 would be an important condition. But Helmholtz entirely disregards the fact 

 that the images formed by the two halves of the objective are actually 

 different even without the supposed dispersion circles, and would therefore of 

 their own accord produce a stereoscopic effect. 



2 Cf. on this point Benecke : " Die Photographic als Hiilfsmittel mikro- 

 skopischer Forschung." Braunschweig, 1868. 



Q 



