496 PROCEEDINGS OF THE SOCIETY. 



or Podocyrtis, by the several circlets (as in fig. 82) of a lens of 40° 

 aperture, thougli actually different, will blend into a composite image 

 — a sort of visual average. But when the two lateral halves of such 

 a lens are made, by the stereoscopic binocular, to give to the right 

 and left eyes respectively separate and sensibly dissimilar pictures, 

 corresponding in their perspective projections to what the real object 

 would give to the right and left eyes, if enlarged to the same size, 

 and placed at 10 in. distance from them — then the visual con- 

 ception of solidity is vividly called up. As I have already shown 

 you, this conception may be excited also by other suggestions — a 

 one-eyed person being able to see objects in relief, as a microscopist 

 sees them in a monocular instrument. But there is nothing which 

 so strongly and uniformly excites this visual conception of solid 

 form, as the combination of the two dissimilar perspectives ; and this 

 seems to me to be effected by the instrumentality of the Stereoscopic 

 Binocular, exactly as (by Sir C. Wheatstone's admirable demon- 

 strations) we know it to be effected in ordinary binocular vision,* 



Mr. Crisp said that at that late hour he would compress into a 

 brief compass his remarks in support of Prof. Abbe's view. 



So far as Dr. Carpenter intended only to insist that stereoscopic 

 vision in the Microscope resulted from two dissimilar images, there 

 was no disagreement between himself and Prof. Abbe, and this being 

 so, nearly all of Dr. Carpenter's interesting discussion was not really 

 in controversy so far as Prof. Abbe was concerned. 



The ditference between Dr. Carpenter's view and that of Prof. Abbe 

 was as to the mode in which the two dissimilar images were formed. 

 Dr. Carpenter suggested that they are formed in the Microscope just 

 in the same way as in the case of the naked eye, i. e. perspectively ; 

 whilst Prof. Abbe insisted that oblique vision in the Microscope is 

 entirely different from that in ordinary vision, inasmuch as there is 

 no perspective ; so that we have no longer the dissimilarity which is 

 the basis of the ordinary stereoscopic effect, but an essentially different 

 mode of dissimilarity between the two pictures. 



If we look at a small cube with the naked eye from an oblique 

 direction it will be agreed that we shall see it as a perspective pro- 

 jection upon a plane at right angles to the direction in which we are 

 looking, with the well-known perspective shortening of all lines which 

 are not parallel to that plane. In the Microscope, however, according 

 to Prof. Abbe's view, there is no such perspective shortening, but 

 the cube is imaged in the manner described in his paper, "f 



That the latter is the correct view is proved by the fact that 

 there is no difference in the outline of an object viewed under the 

 Microscope by an axial or by an oblique pencil ; there is simply a 

 lateral displacement of the image — an entirely different phenomenon 

 to that which occurs in non-microscopic vision. Again, in ordinary 



* ^Addendum. — I wish it to be distinctly understood, that iu this discussion I 

 refer exclusively to microscopic images formed dioptrically by objectives of low 

 power and small angular aperture, and not to those formed (as Prof. Abbe has 

 shown) by the combination of diffraction-spectra. — \V. B. C] 



t Ante, p. 20. See also i. (1881) pp. 422-3. 



