"SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 383 



The latter point may perhaps be better understood from a 

 ■diagram. A and B (Fig. 5) are the two lenses, a and 6' their optic 

 axes, a and b the planes of the object in true focus for each 

 -objective. If be the plane of the object on the stnge, then 

 C will be the only point in true focus for both objectives. Any 

 •other points in this plane will be imaged as diffusion discs, and 

 owing to the same causes which govern the disproportionate 

 magnification in the direction of the depth dimension, the size 

 of this diffusion disc will not be the same in both objectives, so 

 that the point will be seen with greater clearness through the 

 •one objective than the other. 



If we consider a plane slightly lower or higher than 0, this 

 ^difference becomes still more apparent. For here we have the point 

 D in true focus for objective A, and represented by a large 

 diffusion disc for objective B. We see, therefore, that in micro- 

 scopes with two inclined objectives our two pictures have not 

 -only a different perspective, bat the corresponding parts of the 

 two eye pictures vary in distinctness. This fact, which becomes 

 more prominent the higher the power, coupled with the small 

 distance between the objectives permissible as we raise its power, 

 sets the limit to the application of microscopes so constructed ; 

 but within the range in which they are made they yield excellent 

 images, and it is noteworthy that the results of super-magnification 

 in the direction of the depth dimension of different parts of the 

 •object are compensated to a material extent, because any dis- 

 tortions in the two images due to this cause are opposed to one 

 another. 



The plan for binocular microscopes upon which most work has 

 been done, however, is that in which a single objective is used, 

 the image yielded by one half being brought to one eye, that by 

 the other half to the other eye. Since 1851, when the first 

 practical form of such a microscope was devised by Professor J. L. 

 Biddell, of New Orleans,* many have been the forms and devices 

 of prisms over the objective by which this result has been 

 ■obtained, the most convenient ones which are most generally 

 used at present being the Wenham form of prism, which covers 

 up the right half of the objective and directs the image towards the 

 axis of the left eye-tube, and the Stephenson form, which is 

 practically the same as Riddell's, in which two similar prisms 



* See Carpenter's Handbook, p. '.»7. 



