SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 381 



applies with equal force when we use both. It would seem, 

 indeed, that the mere fact of using both eyes serves to strengthen 

 any impression of plasticity which may be formed when one 

 eye only is used, even when the images are entirely similar. The 

 experiment can be readily made with certain forms of binocular 

 microscopes, or by using the Abbe binocular eye-piece under 

 certain conditions. But far and away the most powerful aid 

 to our perception of solidity in the binocular microscope is the 

 mental blending of two dissimilar images, with its accompanying 

 changes in the degree of convergence of the eyes, precisely the 

 same as with the unaided e} T es. It is, however, not enough that the 

 images should be dissimilar. To yield the most correct result 

 they ought to differ, as nearly as possible in the same way and 

 degree, as do the images of objects on the retinae of the two eyes 

 in ordinary vision : for if this is not the case we may have an 

 impression of plasticity, it is true, but a false or distorted one. 

 A very neat way of regarding the matter has been given by 

 Czapski * in a paper on the Greenough binocular microscope. 

 He says that Greenough formulated the desire that " the 

 pictures presented to the two eyes, by the corresponding micro- 

 scopes, must be in all respects similar to those which would be 

 formed in the eyes of a dwarf, when regarding the object with 

 unaided eyes — the dwarf being supposed to view the object at 

 the very short distance commensurate to his own tiny size.'"' 



The question for consideration, therefore, is whether such a 

 similarity of conditions is possible ; if not, what nearest approach to 

 it can be obtained. In regard to the two eye pictures in unaided 

 vision, it is clear that difference of perspective plays the chief 

 part ; for we get a very considerable difference of " inclination 

 perspective," for the reason that the optic axes of the two eyes 

 are not parallel, but converge, when we regard near objects. 

 Moreover, since we change the convergence to suit the distance 

 of the point of the object we are looking at, the " inclination 

 perspective " changes, and this doubtless assists in our conception 

 of the form of the object, altogether apart from any judgment 

 due to the differing degrees of tension on the muscles from the 

 act of convergence itself. Further, a decided difference in 

 the " position in field of view perspective " follows from 

 the fact that the object looked at is either situated between 

 * Zeitschrift filr Wiss. Milrro^opie, 1897, pp. 289—312. 



