SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 389 



demonstration of the diversified action of the Abbe binocular 

 eye-piece.* 



We now come to some considerations which I believe to be 

 new, and which lead to the suggested improvement in binoculars, 

 which I desire to bring before you. But first I should like to 

 mention that it is really Mr. H. Taverner with whom the 

 methods in question have originated. Mr. Taverner a few 

 months ago showed me some exceedingly fine examples of stereo- 

 photo-micrographs, which he had taken by means of special stops 

 placed behind the objective, inquiring whether I could explain 

 the effects. On thinking the matter over, I discovered the 

 causes at once, and saw that the same methods might be applied 

 to the binocular microscope. This led me to a study of the 

 literature of the subject of stereoscopic vision with binocular 

 microscopes, and there I found my reasoning amply confirmed, 

 although the following further deductions do not seem to have 

 vet been drawn. 



If parallactic displacements of out-of-true-focus layers of the 

 •object constitute the mechanism by which stereoscopic effect is 

 produced, this in itself furnishes the necessary proof that the whole 

 image, barring the one plane in true focus, consists of diffusion 

 discs. The size and shape of these diffusion discs is therefore an 

 important matter. We saw in the first part of this paper that 

 the size of the diffusion disc varies directly as the size of the 

 portion of the objective used ; further, we saw that the shape it 

 assumes is the same as that of the portion of the objective 

 utilised. It is evident, therefore, that to have pictures of 

 maximum clearness it is desirable to have these discs as small as 

 the circumstances permit, and also that they should be circular 

 in shape. At present in binocular microscopes no regard is paid 

 to either of these matters ; the size of the diffusion discs is not 

 adapted according to the depth of the object to be viewed, and 

 the image is formed of overlapping discs .-emicircular in shape. 

 An unsymmetrical shape like this results in the image of the 



* For the first moment it may appear puzzling that the outer halves of 

 the Eamsden circles must be used for correct stereoscopic effect, and the 

 inner halves for the inverted effect, since this means that the nearer planes 

 of the object are shifted to the right on the retina of the right eye, and 

 the planes further away to the left. To understand this it is necessary to 

 remember that when we see an object moving from left to right, its image 

 is niovinsr on the retina from right to left. 



