R E F 



In order to find the principal focal distance, which we call / (see Itc- 

 fexion II) we have of course only to make A infinite in the equations just 

 given ; we have then 



1 _ m 1 _ w/__ 



I -rn i -. m 



Case 2. , or/ zz - 



/ ni r m 1 



Case 3. -f ^ , or/ r ? 



It is important to observe that in all cases the distance of the principal 

 focus from the surface, is to its distance from the centre, as the sine of 

 incidence to the sine of refraction. 



If we introduce the distance /into the formulae, we shall have in 



Cases 1 & 2,1=}+^, 



3& ^ = 7 + Z- 



The following are corresponding values of A and A', in different posi- 

 tions of the conjugate foci Q and q. 



00 



Co*e 2. 



( A = X , ^-r, 0, r. 00 



i **f-*r^ 



| A m r m r 



A> = - srrr 00 - - -"==! 



Ta^e 3. - 



234 



