WENHAM S BINOCULAR PRISM. 



121 



dicular to the surface it meets. Within the prism it is 

 subjected to two reflections at b and c, which send it 

 forth again obliquely on the line b towards the eye- 

 piece of the secondary body, to the left-hand side of 

 the figure ; and since at its emergence its axial ray is 

 again perpendicular to the surface of the glass, it- 

 suffers no more refraction on passing out of the prism 

 than on entering it. By this arrangement, the image 



FIG. 72. 



received by the right eye is formed 

 by the rays which have passed 

 through the left half of the objec- 

 tive; whilst the image received by 

 the left eye is formed by the rays 

 which have passed through the right 

 half, and which have been subjec- 

 tive to two reflections within the 

 prism, passing through only two FlG< 72a 



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 central line of the objective, and be 

 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. 



The binocular constructed as we have described 

 performs satisfactorily up to the Jth inch; but for 



