114 



THE MICROSCOPE 



The two different images just produced are sucL as are 

 required for obtaining stereoscopic vision. It is therefore 

 evident that if, instead of bringing them confusedly toge- 

 ther into one eye, we can separate them so as to bring fig. 

 96 a b into the left and right eye, in the combined effect 

 of the two projections, we shall obtain all that is necessary 

 to enable us to form a correct judgment of the solidity 

 and distances of the various parts of the object. 



" Diagram 3, fig. 66, represents the methods that I have 

 contrived for obtaining the effect of bringing the two eves 



Fig. 66. 



sufficiently close to each other to enable them both to see 

 through the same eye-piece together, a a a are rays con- 

 verging from the field lens of the eye-piece ; after passing 

 the eye-lens b, if not intercepted, they would come to a 

 focus at c ; but they are arrested by the inclined surfaces, 

 d d, of two solid glass prisms. From the refraction of the 

 under incident surface of the prisms, the focus of the eye- 

 piece becomes elongated, and falls within the substance of 

 the glass at e. The rays then diverge, and after being 

 reflected by the second inclined surface /, emerge from the 

 upper side of the prism, when their course is rendered 

 still more divergent, as shown by the figure. The reflecting 

 angle that I have given to the prisms is 47. I also find 

 it is requisite to grind away the contact edges of the 

 prisms, as represented, as it prevents the extreme margins 



