62 
Mr. J. F. W. Herschel on the action of 
in. 
Table VII. Rochelle salt. Thickness = 0.05437- 
Inclina- 
tions. 
262 
277 20 
278 25 
280 50 
282 O 
284 50 
288 25 
293 33 
294 50 
Ordinary pencil. 
Fine pink 
• • • » • 
Indifferent purple 
Indifferent lilac pink 
Pale yellow inclining to orange 
Fine pale yellow 
Yellowish white or pale yellow 
White 
Very pale blue 
Sombre indigo . 
Very pale blue 
Y ellow green 
Pale yellow green 
Greenish white 
White 
Very pale pink 
Deep fiery crimson 
Very dull purple (greenish) 
Blue . . , 
Very pale blue, &c. . 
Extraordinary. 
Fine light blue green 
Yellowish white 
Very pale greenish yellow 
Blue rather pale 
Beautiful sombre indigo 
Violet 
White with an almost imper- 
ceptible tinge between yellow 
Yellow white [and violet 
Pale yellow white 
Extremely pale pink white 
Lilac pink 
Deep lilac pink 
Rich but sombre purplish crim- 
Dull purple [son 
Good blue green 
White 
White 
Pink yellow [&c. 
Rich orange, bordering on red, 
PoleP for mean 
red rays. 
Perpendicular 
incidence. 
I Virtual pole 
Remarks, &c. 
III. On the causes of these phenomena. 
The developement of colour along the axis of double re- 
fraction, is at first sight analogous to the production of the 
secondary tints along the axis of rock crystal, discovered by 
M. Arago, and recently explained by M. Biot, in a masterly 
memoir communicated to the Academy of Sciences, on the 
hypothesis of a force inherent in its molecules independent 
of their state of aggregation, by which they communicate a 
rotation in an invariable direction to the axes of polarisation 
of the luminous rays. And this analogy is partially supported 
by the fact, that the tint developed along the axis, descends 
in the scale of colour as the thickness increases. 
