86 
Mr. J. F. W. Herschel on the action of 
and c' respectively for the extreme red and violet rays,* we 
shall find by substitution in our formula ( h ) 
8a = 51' 10" 
But a red ray penetrating the surface from within the crystal 
at an angle a =i 8°12 / 30", and a violet one at an angle a -{- 
(S' a =19° 3' 40", would emerge at the respective angles 30° 59' 
and 32°58' 20", and would include between them an angle of 
i°59 # 20", which should be the apparent separation of the red 
and violet axes in the plate employed. Now, previous to 
the computation of this result, I had carefully measured this 
angle, by observing the incidences at which the extreme red 
and violet rays of the prismatic spectrum, received on the 
reflector of a graduated apparatus, respectively disappeared 
from the extraordinary image at the poles P, P'. I thus found 
Interval of the poles P, P' for red rays - 62° 2' 
Do. for violet - - 66 ° 5' 
Semi-difference, or apparent separation of the axes 2 0 l' 30" 
which differs from its computed value only by 2' 10". We 
may therefore fairly conclude, that in the case of sulphate of 
baryta, the hypothesis y = ~ does not deviate sensibly from 
the truth. 
If we apply our formula (z) to the measures above given 
for Rochelle salt, the result will be widely different. The 
same supposition as to the values of /, /' being made, we get 
8 a = 4 0 2' 50" 
The incidence being nearly perpendicular, and the angle small, 
we need only increase it in the proportion 1^499 : i,to have 
the apparent angle, which thus comes out 6° 4'. We have 
already found 9 0 46' for the same angle, by a method which 
* Biot, Traite de Physique. Vol. IV. 
