GKEENOUGH'S BINOCULAK MICROSCOPE 



103 



IB 



and has been constructed by means of a combination of Porro prisms 

 with a compound microscope of the usual optical type ; it possesses 

 many of the advantages of the compound micro- 

 scope, but inevitably loses light by the passing of 

 the ray through so many prisms, yet by means of 

 the Porro prisms the inverted image is rendered 

 erect. This may be practically illustrated by fig. 

 80, which shows that the rays of light in passing 

 from the object to the eye undergo* four succes- 

 sive reflexions at the surfaces of the prisms and 

 emerge from the last prism with undiminished 

 intensity. The prisms, it will be seen, have the 

 effect of erecting the inverted image formed 

 by the object-glass. But in this microscope 

 binocular vision is obtained, not as in the usual 

 form of binocular microscope, by the subsequent 

 division of a pencil of light passing through 

 one object- (/lass ; but two complete microscopes, 

 each having its own objective and eye-pieces, 

 are simultaneously directed upon the object. 

 This secures perfect stereoscopic (orthomorphic) j 



vision, but of course no power higher than if 



H inch can be employed. The path of the rays U 



is more clearly seen in fig. 81, giving a diagram FIG. 80. Showing the 

 by Mr. Nelson with one of the prisms turned ?&* SST 

 round 90 to make clearer the action of the rays (1894). 

 prisms on the ray. It is well to note that, 



when two of these erectors with a double objective binocular are 

 used, the distance between the eyes can be compensated for by 

 merely turning the erector adaptors round in the microscope tube. 



This method of erection, which is both valuable and practical, was 

 first described in Zahn's * Oculus Artificialis ' (1702), only reflectors 

 were used instead of prisms, but the path of the rays is diverted in 

 precisely the same way as with the Porro prisms. 



The stereoscopic binocular is put to its most advantageous use 

 when applied either to opaque objects of whose solid forms we are 

 desirous of gaining an exact appreciation or to transparent objects 

 which have such a thickness as to make the accurate distinction 

 between their nearer and their more remote planes a matter of im- 

 portance. All stereoscopic vision with the microscope, so far as it 

 is anything more than mere seeing with two eyes, depends, as already 

 seen, exclusively upon the unequal inclination of the pencils which 

 form the two images to the plane of the preparation, or the axis of 

 the microscope. By uniform halving of the pencils whether by 

 prisms above the objective or by diaphragms over the eye-pieces 

 the difference in the directions of the illumination in regard to the 

 preparation reaches approximately the half of the angle of aperture 

 of the objective, provided that its whole aperture is filled with rays. 

 By the one-sided halving we have been considering, the direct image 

 is produced by a pencil the axis of which is perpendicular to the 



