140 Mr Herschei on a Peculiarity in the extraordUnaty 
Extreme Red. Violet. 
15° 50' 29° 59' 
18 22 29 10 
17 38 30 58 
IT 17 32 2 
16 55 30 14 
29 53 . . Extreme Violet, 
Mean 17° 17' 31 41 
33 29 . . Violet bordering on Indigo.- 
30° 55' 
The inclination was therefore diminished for the red rays, 
and increased for the violet ; in other words, the interposition of 
the mica had contracted the red rings in a direction parallel or 
perpendicular to its principal section, but dilated the violet. 
I now (to confirm this indication) turned round the mica 
plate one quadrant in its own plane, so as to reverse its action 
on the polarised beam, and then, repeating the measures, found 
as follows. 
Extreme Red. 
Violet. 
20° 
53' 
22° 41' 
20 
10 
24 29 
21 
14 
21 36 
21 
15 
20 31 
21 
14 
21 58 
20° 
57' 
22° 15' 
The effect on 
the measures. 
as we see, corresponds to the 
change of circumstances, the red rings being now expanded and 
the violet contracted, by the action of the mica in the direction 
in which they were observed. To assure myself still more com- 
pletely of the identity of the ring examined, I detached the mi- 
ca, and observed the first evanescence of the extraordinary 
pencil in the extreme red rays, which took place at an inclina- 
tion of 27° 11'. The whole interval, then, from the maximum 
to the minimum of the first red ring, embraces an extent of 
27° 11'- 19° 28'= 7° 4S', while the displacement of the minimum 
by the mica did not exceed 2° 11' in one direction, and 1° 29' in 
the other. On the other hand, computing the first evanescence 
of the extraordinary pencil in violet light, from the formula * 
* n~ the number of periods, and parts of a period, performed by the polari- 
zed ray within the plate. 
I = the length of each period. t =; the thickness of the plate. 
0 z= the inclination of the ray to the axis. 
