558 Mr potter, ON A NEW CORRECTION 



u — 1 i« — 1 



M |0i ^ r^ 



&c. ; 



•■• ^3 = ^2 li - 4 . y- ^r-\ \ ; 



or, substituting for 1/2 its value before found, 



^ I /ir, J I V M />i n-r. I] 



and ,1'^^+^ 

 r. n 



= „ jL ^. 1 _,. (_L ^ J_) (^1) _,. (^ _ ^)l. 



T . 1 1 1 



Let - + - = -, 



''2 Ti jOa 



then z/'=yJi-A.?i^ ^...C^-J-^]); 



and, if Z)' be the angular deviation caused by this virtual prisui, we 

 have 



^ ^"^1/02 /ir,p2 Vm ?-3|0, /ui r,rj\ 



Now when the dispersion by one of these virtual prisms is equal 

 and opposite to that of the other, we shall have 



S(D-iy) = SD-SD = 0. 



Before however we find the actual value of this expression, we must 

 shew that we may take t, and /, as constant quantities, namely, the 

 thicknesses of the lenses at their edges. 



In Fig. 3, let i?,, Rt,, q,, Mi, M^, represent the same points as ^n 

 Fig. 1 ; 



