116 Mr. Arry on the use of Silvered Glass 
a 
neta Se 
oS ree te 
a 
Wee a =a We Wr 
i 
a we get for the determination of p the 
Substituting these values in the fourth, observing that D + - 4 
following equation : 
= n+3.F*—3 a’) at 2 3.a* D*+1.a@ F* ——— 
ee eee Te FDP 
Hoe: 2a FB G- D) a a*\2 
; nee (2741.F-2) ee 
3 @ (ag) x a ! 
8 
P ans 
+ F? mn Gey re F2 
4} 
[Ga ri). C8) soy er? 
(18). This equation att a cubic has at least one possible 
root; and p’, r, R, are then determined by the expressions 
Baa Grr). 
Thus it is always possible to correct the spherical and chromatic 
aberrations, except F* =a’; a case which can never occur in 
practice. 
