72 Mr. J. F. W. Herschel on the action of 
little too large. In four experiments made at distant intervals 
of time, and by different modes of observation, I have found 
1-49640 1-50220 
1-49670 i‘498,53 
for the index for the mean yellow-green rays, of which de- 
terminations the last is to be preferred, having been made 
with great care. The same experiment gave 1*49293 for 
the index for mean red rays. The apparent angular distance 
of the axis for red rays from the perpendicular was 16 0 , 
which leaves 2 ° 33 ' f° r ^ ie an gular distance of the virtual pole 
from the perpendicular. These angles of incidence corres- 
pond to the respective angles of refraction io° 38' 20" and 
2 0 22' 40", of which the sum 13 0 i' is the real angle between 
the virtual pole and mean red axis in Rochelle salt. The 
series in table IV, gives 13 0 2' 25" for the value of 9 , which 
agrees completely with the foregoing determination. 
I took seven plates of nitre of various thicknesses, and cut 
from different crystals, and by a mode of observation to be 
described hereafter, found as follows : 
Table VIII. 
Designation 
of the plate. 
Mean distance of the virtual 
poles from the axis of mean 
yellow rays (apparent.) 
Excess 
above the 
mean. 
Order of the coincidence. 
I 
1° Co' 
-4' 
Between the 4 th minimum and 5 th maximum. 
2 
i 5 1 
-3' 
A little beyond the 3 d maximum. 
3 
1 57 
+ 3' 
Different at the two extremities. 
4 
1 53 
o' 
2 d maximum. 
5 
1 55 
+ 2' 
2 d minimum. 
6 
- 1 59 
+ 5' 
9 th minimum. 
*7 
/ 
1 53 
Mean i 53 
o' 
A little beyond the 8 th minimum. 
