556 Mr potter, on A NEW CORRECTION 





(q,R,-tA 



= y. (i - 



and 



ti 





since the incident rays are parallel, and neglecting the aberration, &c. : 



••• y-^ = y'(l-''-7^)' 

 and.-. .7 = y,(i + i-/,.;i^j; 



1 1 1 



or, putting — h — = - , 

 • ^ ^ r, ro |0, 



and since the angle of incidence and the angle of the prism are always 

 small, if i) = the angular deviation of the ray, after emergence in tlie 

 direction R.q, we have 



Z) = (m - 1) X ^ 1 



-("-«■*' l^'-:-^}' 



Ao-ain, if we consider the last lens as the outer lens of a double 

 achromatic object-glass, and that the ray emerging immediately into a 

 double concave lens is again refracted by it in the direction R, R, q.,, 

 as in Fio-. 2, the contiguous surfaces of the lenses having the same 

 radius, and q„ R„ M„ O, being the same points as in Fig. 1, we shall 

 have R, ?, the direction of the ray, when it meets the concave lens at 

 R,. Also let 



R, R, q, be the direction after refraction at R,, (see Fig. 2.) 

 ^3 q^ emergence at R^, 



