16 THE STRUCTUKE OF 



nism of the instrument. Let G represent the body 

 of the ordinary Microscope and the secondary 

 tube attached to the side of the former, which it 

 will be seen has a portion of its surface cut away 

 at the point of junction, F, as a means of commu- 

 nication between them. The eye-pieces and draw- 

 tubes are seen at D and E. The object-glass G 

 is attached to the ordinary tube C in the usual 

 way. Just above it is the small prism, A, 

 mounted in a brass box, and so constructed as 

 to slide into an opening in the tube at the back 

 of the object-glass. By this arrangement it will 

 be found that while one half of the light passes up the 

 tube unobstructed the other half must first pass 

 through the prism, where, 

 after undergoing two re- 

 flections (fig. 7), it es- 

 capes in the direction of 

 the additional tube B. The 

 dotted lines in the diagram 

 show the direction the 

 light takes in its passage 

 to the eyes. At H the rays 

 are seen to cross each other. 

 Those from the left side of 

 the obj ect-glass traverse the 

 . _ right tube, while those from 



Double-reflecting Prism. ^ rigbt ' side of the lens 



are projected up the left tube. 



In using the Binocular it must be remembered 

 that the eyes of different individuals vary in their 

 distance from each other. It will thus be seen 

 that some contrivance is necessary to enable us to 

 increase or decrease the distance between the eye- 

 pieces to suit the requirements of all. This is 

 accomplished by the two draw-tubes, D and E, 

 which carry the eye-pieces. When drawn out, the 



