THE TWIN-OBJECTIVE BINOCULAR 



41 



When used on transparent o))jc('ts, the ()l)jectives with an 

 aperture of 0.1, or less, sometimes get obviously a too 

 large cone of light from the usual concave mirror. The 

 aperture of the mirror should then be cut down, as already- 

 stated. The plane mirror may well be cut across from 

 front to back (Fig. 12), and the halves set together at the 

 proper obtuse angle to light each tube (22); or an appro- 

 priate thin double prism (Fig. 12) may be fitted beneath the 



Xj Mirror 



Fig. 12.^ — Diagram of the twin objectives and object stage of the Greenough 

 binocular, showing the position of the double prism (Nelson) or of the double 

 plane mirror (Akehurst) used for equalizing illumination in the two tubes. 



stage (Nelson), if enough work is done with transparent 

 objects to require such help. 



The source of light for transparent objects may be a 

 white cloud, or a large enough ground or frosted sheet of 

 glass (with a screen of daylight glass) before a tungsten lamp. 

 Or a matt or frosted lamp can be used at the right distance. 

 A disc of flashed opal glass can replace the plane mirror 

 for low powers. Yellow-green color screens could be used, 

 to rest the eyes and improve the definition, when observa- 

 tion of the natural colors is unnecessary. 



A screen should be kept in front of the eyes to keep off 

 light from lamp or window. 



