6o 
Mr. J. F. W. Herschel on the action of 
In order however to avoid the effect of the dispersive 
power, which at such considerable obliquities would render 
the observations liable to some uncertainty, I cut another 
plate, in such a manner that the perpendicular to its surface, 
instead of coinciding nearly with the optic axis, was directed 
very nearly to one of the virtual poles. Its thickness was 
then gradually reduced in the manner above described for 
sulphate of baryta, though, owing to the nature of the body, 
it was found impossible to avoid the necessity of re-polishing 
the posterior surface at each operation ; but as this was done 
with all possible care, only a very slight error can have arisen 
from this cause. 
in. 
Table V. Rochelle salt. Thickness — 0.11518. 
Remarks, &c. 
tions. 
Ordinary pencil. 
Extraordinary. 
O / 
277 8 
261 8 
Very pale pink 
Very pale bluish green . .j 
PoleP for mean 
red rays. 
Perpendicular 
• • . . • 
\ 
incidence. 
261 0 
.Vhite tinged with orange 
Very fine intense indigo 
260 25 
Yellowish or greenish white 
Purple rather sombre 
2 5 9 45 
Very pale greenish blue 
Indifferent purplish pink 
Dull blue .... 
Yellowish pink white 
258 50 
Fine deep indigo 
White inclining to orange 
258 30 
Violet purple 
Yellowish white 
2 57 35 
White, a little tinged with violet . 
White, not very brilliant . 
The virtual 
Yellowish white 
Pale violet blue 
pole p. 
256 30 
Pale yellow 
| Sombre indigo inclining to 
{ violet, narrow & well defined. 
Pale pink yellow 
Sombre violet white 
255 10 
Pinkish purple 
Very pale greenish yellow 
'-54 3 ° 
Rich sombre purple somewhat fiery . 
White tinged with greenish yel- 
' - ' 
Pale green 
Fine crimson [low 
252 30 
Extremely pale green 
The richest deep damask crim- 
White 
Livid imperfect purple [son 
250 27 
The richest damask crimson 
Fine pale green 
Livid imperfect purple 
White 
248 5 
Pale bluish green 
Fine rich crimson 
2 45 45 
Pink approaching to red 
Pale blue green 
2 43 3 ° 
Sky blue 
Light pink strongly inclining to 
240 25 
Pink orange 
Pale greenish blue [orange red 
Sky blue inclining to lilac 
Fine yellow 
2 37 3 C 
Pale bluish green 
Fine pink a little purple 
237 c 
Splendid yellow green 
Lilac 
234 2C 
Rich lilac 
Splendid green 
230 1C 
Splendid green 
Rich lilac 
White 
White 
225 45 
Lilac blue 
Splendid yellow green 
White 
White 
220 4c 
Splendid green yellow 
Pale lilac blue 
White 
White 
21 1 4c 
Pale lilac 
Pale greenish yellow 
197 2C 
Fine yellow green 
Fine lilac 
