in the polarized Tints of certain Crystals with one axis. 335 
* 
dence, or when inclined at any angle in the plane of primitive 
polarization, or in one perpendicular to it, it had no action upon 
the incident ray, however turned round in its own plane. Of 
course it has but one axis of double refraction, and that at right 
angles to its laminae. 
The best and most transparent portion being selected and in- 
sulated as before, the plate was inclosed in the oil apparatus, 
when the tints developed on inclining it in a plane making an 
angle of 45° with that of primitive polarization, were as follows : 
Table 1 V . — Apophyll ite, third Variety . Thickness— 94499. 
Incidence. 
Ordinary Pencil. 
Extraordinary Pencil. 
0° O' 
17 0 
28 15 
39 n 
47 3G 
White. 
Y ellowish-white. 
Pale yellow. 
Pale greenish-yellow. 
White, slightly greenish. 
Very pale green. 
White. 
White, scarcely perceptibly tin- 
ged with pink ? 
Black. 
Sombre indigo. 
Indigo inclining to purple. 
Pale lilac purple. 
Very pale reddish purple. 
Pale rose-red. 
White. 
White, scarcely perceptible green- 
ish ? 
This Table of tints indicates a much more energetic action on 
the red and violet ends of the spectrum than on the interme- 
diate colours, especially the yellow', and this was fully corrobo- 
rated by observations in homogeneous light, which gave the va- 
lues of l for the simple colours as in the subjoined Table. 
Table V .— -Scale of the Minimum Lengths of the Periods 
of the different simple Rays in the third Variety of Apo- 
phyllite , and their Reciprocals. 
Name of Colour. 
Minimum length of 
period value of l. 
Polarizing power or 
„ 100000 0. 
VcllUC of ^ 
Number of 
Observa- 
tions. 
| Extreme red. 
43634 
22.918 
10 
I Mean orange. 
101238 
9.878 
10 
1 yellow. 
366620 + 
2.728 — 
10 
| green. 
89646 
11.155 
10 
I blue. 
32211 
31.040 
10 
1 indigo. 
21947 
45.565 
10 
| Extreme violet. 
13704 
72.970 
10 
The curve representing the values of - or tin 
l 
polarizing ener- 
gy of the variety now under consideration, constructed as in the 
f/ 0 
