THE BINOCULAR MICHOSCOPE. 



123 



form a correct judgment of the solidity and distances of the various 

 parts of the object. 



If a rectangular plate of speculum-metal is ground and polished, 

 so as to form two reflecting facets inclined to each other at the required 

 angle, as represented at 4, fig. 89, and this is placed at an angle of 

 45° with the division of the facets intersecting the axis of the object- 

 glass, we shall divide the rays, and reflect them horizontally by one 

 single reflection. Any other direction than a right angle, with respect 

 to the axis of the object-glass, may, of course, be given to the rays by 

 inclining the reflector more or less. From the simplicity of this con- 

 trivance, and the facility with which it may be constructed, I shall take 

 an early opportunity of giving it a trial. The only question I have 

 is, whether a material may not be found that will reflect more light 

 than even speculum-metal. 1 have heard an alloy of cast-steel and 

 platinum well spoken of, but have never seen any of it. 



In considering the aberrations which the thickness of glass con- 

 tained in the reflecting prisms must inevitably produce, when placed 

 immediately behind the object-glass, it occurred to me that if the same 

 prisms were placed close to the top lens of the eye-piece, these errors, 

 not being magnified, would be less sensibly felt. 



fig. SO. 



Diagram 3, fig. 89, represents the methods that I have contrived for 

 obtaining the effect of bringing the two eyes sufficiently close to each 

 other to enable them both to see through the same eye-piece together. 

 aaa are rays converging from the field lens of the eye-piece; after pass- 



