94 VISION WITH THE COMPOUND MICROSCOPE 



In contradistinction to this explanation of binocular vision Dr. 

 Abbe, as we have seen, has demonstrated that oblique vision in the 

 microscope is wholly unlike ordinary vision ; there is, in fact, no 

 perspective. The perspective shortening of lines and surfacfs 1>\ 

 oblique projection is entirely lost in the microscope, and, as a con- 

 sequence, it is contended that the special dissimilarity which is the 

 raison d'etre of ordinary stereoscopic effects does not exist, but that. 

 an essentially different mode of dissimilarity is found between the 

 two pictures. The outline or contour of a microscopic object is 

 unaltered, whether viewed by an axial or an oblique pencil ; there is 

 no foreshortening, there is simply lateral displacement of the images 

 of consecutive layers. But Abbe contends that, whilst the manner in 

 which dissimilar pictures are formed in the binocular microscope is 

 different from that by which they are brought about in ordinary 

 stereoscopic vision, yet the activities of the brain and mind by which 

 they are so blended as to give rise to sensations of solidity, depth. 

 and perspective are practically identical. 



The fact that lateral displacements of the image are seen in the 

 microscope depends on a peculiar property of microscopic amplifica- 

 tion, which is in strong contrast to the method of ordinary vision. 

 It depends entirely on the fact, enunciated above, that the amplifi- 

 cation of the depth is largely exaggerated. Hence solid vision in 

 the binocular microscope is confined to large and coarse objects, the 

 dimensions of which are large multiples of the wave-length. It 

 therefore follows that when moderate or large apertures have to be 

 employed that is to say, whenever delineation requires the employ- 

 ment of oblique rays the elements of the object are no longer 

 depicted as solid objects seen by the naked eye or through the telescope 

 would be depicted ; nevertheless the brain arranges them so that 

 the characteristics of solid vision are still presented. 



Professor Abbe demonstrates l that in an aplanatic system pencils 

 of different obliquities yield identical images of every plane object, 

 or of a single layer of a solid object. This is true however large the 

 aperture may be. 



This carries with it, as we have said, a total absence of perspec- 

 tive and an essential geometrical difference between vision with the 

 binocular microscope and vision with the unaided eye. 



An object, not quite flat, as a curved diatom, when observed with 

 an objective of wide aperture will present points of great indistinct- 

 ness. This has been by some supposed to arise from the assumption 

 that there was a dissimilarity between the images formed by the 

 axial and oblique pencils ; but this is not so. It is wholly expli- 

 cable by the fact that the depth of the object is too great for the 

 small depth of vision attendant upon a large aperture. 



It will be seen, then, that so long as the depth of the object is 

 within the limits of the depth of vision, corresponding to the aperture 

 and amplification in use, we obtain a distinct parallel projection of 

 all the successive layers in one common plane perpendicular to the 

 axis of the microscope a ground plan, as it were, of the object. 

 Manifestly, then, since depth of vision decreases with increasing 

 1 Joitrn. If. M.S. series ii. vol. iv. pp. 21-24. 



