58 
Mr. J. F. W. Herschel on the action of 
In this plate the virtual poles correspond to the second 
maximum of the extraordinary pencil. It is needless to 
detail the tints between the poles. The same plate once 
more reduced, with the same precaution to leave the poste- 
rior surface untouched, developed the following series of 
colours beyond the poles. 
in. 
Table III. Sulphate of baryta. Thickness = 0*05758. 
Corresponding 
inclinations. 
Ordinary pencil. 
Extraordinary. 
Remarks, &c. 
" / 
60 57 
° / 
122 50 
Fine yellow 
Tndigo 
Poles, for the 
60 20 
123 27 
Pale yellow 
Purple 
mean red rays. 
White inclining to yellow 
Dull crimson red 
Tints beyond 
Bluish white 
Dull orange 
the poles. 
Indigo 
Yellow 
57 55 
126 3 
Sombre purple 
White 
57 5 ° 
126 „ 7 
Sombre reddish violet 
White 
57 2° 
126 33 
Dirty violet yellow 
White 
12 7 3 ° 
Pale yellow 
Violet white 
55 4 ° 
128 2 
White 
Sombre violet 
55 33 
128 20 
Pure brilliant white 
Black 
Virtual poles. 
128 30 
White . . . 
Sombre dirty green 
Pale orange 
Pale dirty bluish green 
53 40 
130 10 
Sombre orange or brick red . 
White 
53 27 
13° 3 ° 
Sombre and narrow purple 
White 
Blue 
Ruddy white 
Pale blue 
Orange 
Bluish white 
Orange red 
132 20 
White . . . 
Narrow crimson 
5 i 3 
132 4c 
Pale yellow, &c. &c. . . | 
Purple, & c, &c. 
Here the virtual poles/),/)' correspond precisely to the first 
minimum of the extraordinary pencil. 
In a plate of Rochelle salt, cut nearly, but not quite perpen- 
dicular to the optic axis, and whose thickness was 0.194,4,25 
inches, the rings beyond the poles were almost entirely obli- 
terated, while those between them exhibited the following 
singular succession of colours, which will show to what an 
extent the deviation from Newton’s scale is carried in this 
substance. 
