7 6 Mr. J. F. W. Herschel on the action of 
the centres of the last visible red and violet rings, as well as 
those of the intermediate colours, on a screen at a known 
distance seen with the other eye, I found the following values 
of the apparent and real separation of the several coloured 
axes : 
Between 
Apparent interval. 
Real Interval. 
Red and orange 
° f 
o 37 very uncertain 
0 1 
O 35 
Yellow 
l 50 rather uncertain 
1 13 
Green 
3 43 Do. 
2 29 
Blue 
6 5 
4 3 
Indigo 
8 19 
5 33 
Violet 
9 4 6 
631 
As a inode of measurement this method is very inaccurate, 
especially in the extreme red and violet rays, both of which 
would be copiously, and indeed almost entirely absorbed in 
their passage through two plates of tourmaline of a yellowish- 
green colour. Much more exact and unexceptionable mea- 
sures will be presently given, but these are quite sufficient to 
establish the reality of the phenomenon described. 
v. Of a secondary cause of the deviation of tints , subsisting in 
certain crystals, and of the anomalous tints of the apophyllite. 
To determine the dispersive power of any medium, and obtain 
some rough knowledge of its law, we make a prism of it act 
in opposition to one of a standard substance. To ascertain 
the total dispersion of the axes of a crystal, or the angle by 
which the extreme red and violet axes differ, we may make 
it act against itself. Since the violet rings are more elevated 
by refraction than the red, from the situation in which they 
