crystallized bodies on homogeneous light. 89 
detailing particular experiments. The following table ex- 
presses the final result of a great number of such measures. 
Colour. 
Values of l in 
inches. 
Number of 
Observations. 
Extreme Red. 
0-0056158 
64 
Mean Red. 
0-0050032 
*4 
Mean Orange. 
0-0041; 852 
24 
Mean Yellow. 
0-0040^83 
5 2 
Mean Green. 
0-0036549 
62 
Mean Blue. 
0-0032863 
22 
Mean Indigo. 
0-0029868 
5 2 
Extreme Violet. 
0-0025093 
49 
The observations from which this table was calculated, were 
made indiscriminately on the maxima and minima of all or- 
ders. Those of different orders were of course computed se- 
parately, and found to agree without exception in giving the 
same values of / within limits of error less than those to which 
the observations are liable ; thus affording another proof of 
the exactness of the law of periodicity above employed. 
Now, if we compare these, one with another, and with those 
of c as deduced by M. Biot from Newton’s observations, we 
shall have as follows : 
Colour 
Values of _il 
l 
Values of Jl. 
c 
Extreme Red. 
1-00000 
1 -OOOOO 
Mean Red. 
0-89093 
0-96215 
Mean Orange. 
0-81659 
0 90490 
Mean Yellow. 
0 72266 
085550 
Mean Green. 
0-65082 
079433 
Mean Blue. 
0-58520 
0-73725 
Mean Indigo. 
0-53156 
0-69641 
Extreme Violet. 
0-44684 
0 63000 
It appears from this comparative statement, that the forces 
of polarisation and double refraction in the body now ex- 
amined, act with much greater proportional energy on the 
more refrangible rays than in mica, sulphate of lime, and 
MDCCCXX. N 
