JULIUS RIIEIXBERG OX STEREOSCOPIC EFFECT AND A 



the two eves, so that it is regarded from entirely different 

 directions, or, if not, because it lies much more sideways from 

 one eye than the other. 



A- regards binocular microscopes, which we may divide into 

 two classes— viz. those in which separate objectives are used, and 

 those in which separate parts of a single objective are utilised — 

 v» -rv different conditions prevail from those in unaided vision, 

 although in the former kind a clever attempt has been made to 



ilise the same conditions as nearly as can be done, the result 

 of which is seen in the Greenough binocular microscope, made by 

 Messrs. Zeiss.* In this instrument two separate objectives are 

 mounted with their optic axes converging towards the centre of 

 the object plane. Further, they are so mounted that the angle 

 of convergence approximates to that of the angle of convergence 

 of the eyes when regarding an object at 9 in. or 10 in. 

 distant — viz. about 14° — and this, of course, necessitates 

 a different separating distance between the two objectives, 

 according to the power used. It is easy to follow that if a 

 normal individual regards an object 10 in. away with eyes 

 separated 2| in., then the objectives which focus an object only 

 1 in. away may not be separated more than y4 = 4 m * 5 anQ, > ^ n 



it, the necessity for having the tw r o objectives so very close 

 together forms a restriction which prevents the principle being 

 applied to the higher powers. But our present point is that 

 here we have an instrument in which the object is imaged from 

 two different station points, so arranged that the " perspective of 

 inclination" and of "position in field of view " conforms to that 

 in naked eye vision as nearly as can be obtained. 



Where, however, the great difference comes in, is that : — 



1. In both of the single images the effects due to the dis- 

 proportionate magnification of the depth dimension are present, 

 which, as we saw before, is inherent in the microscope. 



2. \\ hilst in unaided vision we can direct both eyes to any 

 point of an object, and ste it with the same degree of clearness 

 with both eyes, the two microscope lenses remain directed 



one point of the object only, all other points being 

 imaged by them with a differing degree of distinctness. 



'Uld seem to be t lie only binocular microscope made upon this 

 long ago ;i> 1677 Cherubin d'Orleans suggested and 

 gram of such a microscope in his book, La Vision parfaite. 



