18 
PROFESSOR POWELL’S RESEARCHES TOWARDS ESTABLISHING 
Thus the hypothesis of undulations assigns the law and cause of dispersion in ten 
new cases, in addition to the ten considered in my former paper. 
Oxford, November 1 , 1835. 
Postscript. 
It may be right here to mention, that since my former paper was printed, I have 
learned from M. Cauchy that he has also investigated the relation between the 
length of a wave and the refractive index. And in a memoir on his new method of 
interpolation he has applied that method to this case, and has also given an example 
of the comparison of numerical values. This, however, is only made for one single 
case, viz. the Flint Glass, No. 23. of Fraunhofer. 
Also, while this paper has been passing through the press, some other important 
observations closely connected with the subject have been made, for which the reader 
must refer to the London and Edinburgh Philosophical Magazine and Journal of 
Science, Nos. 44 and 45. 
Comparison of Refractive Indices from Cauchy's Theory and from observation. 
Calcareous Spar. Rudberg. 
The edge of the prism parallel to the axis of the rhombo- 
hedron. 
Ordinary Ray. 
Ray. 
Observed value 
of ft,. 
( 4 ) 
Ratio (^) . 
\sine / 
Calculated va- 
lue of ft. 
/ arc \ 
= const x I — — - ) . 
\sine / 
B 
1-6531 
o / // 
13 16 0 
1-009 
1-6531 
C 
1-6545 
13 55 2 
1-010 
1-6547 
D 
1-6585 
15 29 59 
1-0123 
1-6584 
E 
1-6636 
17 19 45 
1-0156 
1-6638 
F 
1-6680 
18 47 30 
1-0181 
1-6680 
G 
1-6762 
21 14 30 
1-0233 
1-6765 
H 
1-6833 
23 1 30 
1-0277 
1-6834 
const = 1-6384 
Extraordinary Ray. 
B 
1-4839 
O / // 
9 30 0 
1-0045 
1-4838 
C 
1-4845 
9 57 59 
1-0051 
1-4847 
D 
1-4863 
11 5 58 
1-0063 
1-4864 
E 
1-4887 
12 24 38 
1-0080 
1-4889 
F 
1-4907 
13 17 20 
1-0092 
1-4908 
G 
1-4945 
15 12 30 
1-0119 
1-4948 
H 
1-4978 
16 29 15 
1-0140 
1-4978 
const = 1 -4772 
