PROCEEDINGS OF THE SOCIETY. 493 



Stereoscope, any conceivable reversion can be made, no such conversion 

 of relief will take place when two portraits taken stereoscopically 

 are thus crossed — the mind refusing to accept the suggestion. 



Having thus fully prepared my ground, I shall briefly deal with 

 my proper subject. Prof. Abbe, as I understand him, says that the 

 perception of relief in the case of the Binocular Microscope is some- 

 thing different from that of ordinary stereoscopic vision ; and that it 

 depends more upon the relative planes of portions of the object. I 

 maintain, however, that it depends upon the combination (as in 

 the Stereoscope) of two dissimilar perspective projections. We 

 all know that the conception of solid form or projection which 

 we get with the stereoscopic Binocular (in which the prism divides 

 the cone of rays into its right-hand and left-hand halves), is very 

 different from that which we get with the non-stereoscopic Binocular, 

 in which half the rays of the entire cone are " sent iuto each of the 

 two bodies respectively. Every one also knows that in viewing a 

 solid object he cannot get adequate focal depth with a very wide- 

 angled objective. When our makers were bringing out 1/2 in. ob- 

 jectives of very wide angles, up to 90°, I tried one of them on a slide 

 of Polycystina, but could make nothing of it ; for a portion of a 

 spherical form (which was all that could be brought into focus) looked 

 very much like the small end of an egg. When I reduced the angle 

 to 60°, the same portion of a sphere looked like the large end of an 

 egg ; but when I further reduced the angle to 40°, I saw every form in 

 its true projection. I got Mr. Powell to construct for me a 1/2 inch 

 objective of 40° ; and this has been the progenitor of a goodly off- 

 spring of low-angled objectives, which give, in the Binocular, the real 

 solid forms of opaque objects. I maintain that the pictures which 

 we receive from the two lateral halves of such an objective, are as 

 dissimilar as two portraits taken at an angle of 15°; and that it is 

 by the stereoscopic combination of these^ that the impression of 

 solidity is suggested. 



It is interesting to go back to Mr. Wenham's first paper on 

 this subject,* written just thirty years ago. He was then working 

 out the problem of the Binocular Microscope : rightly apprehending 

 the principle of the Stereoscope, he attempted to reproduce its 

 effects in the Microscope ; and you know how he ultimately succeeded, 

 although his first results were unsatisfactory. Prof. Eiddell also 

 at first failed, and for the same reason, — that they both lost sight of 

 the fact that as the Microscope itself reverses the pictures, it is neces- 

 sary that they should be made to cross before reaching the eyes of 

 the observer. Some among you will no doubt remember, that the first 

 binocular Microscopes which were made, gave such a view of the 

 objects, that though you sometimes saw them stereoscopically, the 

 general effect was pseudoscopic. Now, Mr. Wenham in the course of 

 his investigations did this ; — he took a very suitable object for the 

 purpose, the egg of a bug, and having put it under a 2/3 in. ob- 

 jective, he covered up half the lens and made a drawing of the object 



* Trans. Micr. Soc, ii. (1854) p. 4. 



