114 



THE MICROSCOPE 



The two different images just produced are such 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 eyes 



Fig. 66. 



sufficiently close to each other to enable them both to see 

 through the same eye-piece together, aaa 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 f, 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 47i°. I also find 

 it is requisite to grind away the contact edges of the 

 prisms, as represented, as it prevents the extreme margins 



