371 



ON STEREOSCOPIC EFFECT AND A SUGGESTED 

 IMPROVEMENT IN BINOCULAR MICROSCOPES. 



By Julius Rheinberg, F.R.M.S. 



{Read April 20th, 1900.) 



It may seeni a commonplace to remark that the image plane 

 of a microscope, or for the matter of that of any optical instru- 

 ment, is only in perfectly true focus for one single plane of the 

 object we may be looking at ; and yet, on an adequate perception 

 of that fact and what it entails depends the whole subject of 

 stereoscopic vision with the microscope, and there are proofs in 

 abundance, both in the literature of the past and present, that 

 a want of proper appreciation of this simple matter has given 

 rise to much confusion. The very term " stereoscopic vision " — 

 and it is of course for this purpose that binocular microscopes 

 are chiefly used — implies that we are dealing with objects of 

 three dimensions. We desire to obtain a better sense of the form 

 of such objects — to see them standing out in relief, just as we see 

 objects with the naked eye. But if only one plane of the object 

 can be in true focus at anv time, it is evident that all the other 

 planes must be more or less out of focus, and we must therefore 

 see wherein these out-of-fecus images differ before it is possible 

 to discuss the subject of this paper. 



Kow if the object point is situated on a plane B, Fig. 1, a little 

 further away than the plane A, which is in true focus at the 

 view plane A 1 , the rays reunite in a plane B 1 before reaching 

 the plane A 1 ; if the object point lies a little nearer than the 

 plane in true focus (as at 0, Fig. 1), the rays reunite at C 1 after 

 they have passed the view plane. In both cases, therefore, they 

 are represented on the view plane it.- elf by a disc, a so-called 

 diffusion disc. If that disc be very small, the point of the object 

 it represents is clearly seen ; if it be larger, the image gets hazy 

 and confused. Depth perception in an optical instrument depends 

 upon the size of these discs. If they do not exceed a certain limit, 

 conventionally fixed — of which more hereafter — the object is 

 said to be in focus, although it is not in theoretically perfect 

 focus. It will at once be seen that the size of the disc varies 



