92 Mr. J. F. W. Herschel on the action of 
violet illumination. Moreover, it is evident from the preceding 
theory, that any difference which may exist in their position, 
if too small to be sensible to the eye, can produce only an 
imperceptible deviation of tints. In fact, if we suppose a = o 
for any colour, we get, for the position of the virtual pole, 
. W i—i' - * 
sin 9 = v — 7 — • smjfl 
9 being the angular distance of the point of coincidence from 
the single axis of that colour. It is, consequently, insensible 
when $a is so. Now, the polarising force of the apophyllite 
being very feeble, the diameters of the rings in any plate of 
moderate thickness must so far exceed this very minute quan- 
tity, that the virtual poles, did any exist, must fall within the 
limit of the central blackness ; the Newtonian scale would 
still appear to commence from the centre, nor could any sen- 
sible deviation from it arise from this cause. 
a. When the prismatic spectrum is passed over an appa- 
ratus containing a plate of this mineral, no perceptible change 
in the magnitudes of the rings for different colours takes place. 
Hence it appears that the value of the function l for all the 
coloured rays is nearly alike. By measures taken on a 
divided apparatus, a slight difference is observed. Taking 
the mean refractive index R at 1-5431 (by a very careful 
measure) and the dispersion at 0-017, ^ formula 
l — t- • 5in61 = — • sin 9 . tan 9 
n . cos <p n 
gave as follows : 
Extreme Red. 
1 — 000930 66 
Mean Red. 
0-0092810 
Mean Orange. 
0-0092337 
Mean Yellow. 
00091 503 
Mean Green. 
00090643 
Mean Blue. 
0-0092059 
Mean Indigo. 
0-0093964 
Extreme Violet. 
o-oi 00660 ? 
