WENHAM S BINOCULAR. 



117 



subjective to two reflections within the prism, passing 

 through only two surfaces of glass. The prism is held 

 by the ends only on the sides of a small brass drawer, 

 so that all the four polished surfaces are accessible, and 

 should slide in so far that its edge may just reach the 



Fig. 67. 



Fig. 68. 



central line of the objective, and oe drawn back against 

 a stop, so as to clear the aperture of the same. In this 

 case the straight tube acts as a single microscope. 



" Both the transmitting and reflecting surfaces of the 

 prism should be accessible, for the purpose of wiping off 

 particles of dust or mildew — particles of any kind ad- 

 hering to the prism will prevent total reflection at the 

 point of contact. If the prism is well made and polished, 

 and of the smallest size possible for admitting the pencil, 

 the difference between the direct and reflected image is 

 scarcely appreciable. 



" The binocular constructed aswe have described performs 

 satisfactorily up to the £th inch; but for powers above 



