430 C. S. Hastings—Triple Objectives 
Pi=(n," ae 1)A +(n,"—1 )B+(n,/"”—1)C= I 
LBL ATL) (BYR (@) TF (a) BA (BMF) 
+1" F'(#))C=0; 
but since the latter must hold true for all values of the variable 
« the final conditions for perfect color correction are: 
(n,/—1)A+(n,"—1)B+ (2,1? —1)0=1 
B’A+ BB+ B’"C=0 
IV”A+I"C+1"”"C=0 
the only practical limitation being that neither A, B or C sur- 
um. 
(2) 
pass thirty as a maxim 
Cauchy’s well-known formula as an expression for 7; 
there are two objections to such a course, the first and most 
important being that three terms of this series will not express 
the values within the necessary limits of accuracy, and the 
that of any other light glass. The form of the function isa 
trinomial of the second degree, thus 
| N=A+ Bn+In (3) 
Doubtless by not restricting it to the first and second powels 
ot ma formula might be shaped which would make the differ- 
of glass the optical constants of which I have been able to _ 
Siven with the requisite accuracy. Unfortunately there 4 
" w. Besides the five which I have determined an 
cited above, viz: 
Feil’s Crown 12] 9, 
ee B 
i. Win 1260 oo, ree a 
