crystallized bodies on homogeneous light , 
93 
This table, though not given as exact, owing to imper- 
fections in the specimen examined, agrees with the succession 
of tints which, as far as the fourth order, were as follows : 
in# 
Apophyllite. Thickness = 0*0829. 
Incidence. 
Ordinary pencil. 
Extraordinary. 
O / 
O O 
Bright white 
Black 
The axis. 
*3 5 ° 
White with a trace of purple 
White slightly greenish * 
1st, maximum of 
21 50 
Exceeding sombre violet 
Pure bright white 
illumination. 
25 12 
Pale greenish yellow 
Purplish white 
29 45 
White 
Sombre violet blue 
30 50 
White 
Extremely sombre violet 
1st. minimum. 
33 3 
White with a strong tinge of violet 
Pale yellow green 
35 5 ° 
Blue strongly inclining to purple 
Greenish white 
37 20 
Sombre indigo inclining to violet 
White 
38 5 
Sombre violet 
White 
2d. maximum. 
40 10 
Tolerably good yellow green 
Purplish white 
43 55 
White with a trace of yellow 
Obscure indigo inclining to purple. 
44 2 7 
White 
Sombre violet 
2d. minimum. 
46 45 
Pale purple 
Tolerable yellow green 
49 57 
Sombre purple blue 
Yellowish white 
50 30 
Sombre violet 
White 
3d. maximum. 
53 40 
Green yellow 
Pale purple 
56 0 
Yellowish white 
Sombre indigo blue 
3d. minimum. 
56 40 
Yellowish white 
Sombre violet 
White 
Livid grey 
59 35 
Pale purple 
Yellow green 
61 45 
Sombre indigo 
Pale yellowish white 
4th. maximum. 
62 10 
Sombre violet 
Yellowish white 
63 0 
Faint violet white 
White 
Livid grey 
White 
66 0 
Tolerable green yellow 
Purple 
69 30 
Yellow white 
Very sombre indigo 
4th minimum. 
In the colours of thin plates and others of the like composi- 
tion, the difference in the lengths of the periods of the different 
rays is so considerable, that after seven or eight alternations 
of colour the rings confound one another, and are blended 
into a uniform whiteness. Were the periods more nearly > 
equal, a greater number of rings should be visible, and were 
they strictly so, the succession of alternate whiteness and 
blackness should be continued to infinity. As the values of 
