57 
crystallized bodies on homogeneous light. 
In this plate the virtual poles correspond very nearly to the 
second minimum of the extraordinary pencil beyond the 
poles PP'. The same plate was now reduced by grinding to 
the thickness 0.08816 inches. In this operation, care was 
taken to grind away the side of the plate most distant from the 
eye only, leaving the other perfectly untouched and unim- 
paired in its polish. The plate being reduced to exact paral- 
lelism by the sphasrometer, was again examined, the same 
side still remaining next the eye. By this arrangement the 
same angles of emergence from the posterior surface cor- 
respond rigorously to the same directions of the ray in the 
interior of the crystal, with respect to the axes of its mole- 
cules ; and thus we avoid completely any errors which might 
arise from using plates cut at different angles, it being almost 
impossible to cut two plates precisely alike in this respect. 
in. 
Table II. Sulphate of baryta. Thickness =0.08816. 
Corresponding 
inclinations. 
Ordinary pencil. 
Extraordinary. 
Remarks, Src. 
O / 
58 c 
0 
120 
✓ 
c 
Pink, verging to orange yellow . 
Blue somewhat greenish . 
Poles for the 
Yellow 
Dark blue 
mean red rays. 
57 30 
120 
47 
Pale yellow . 
Purple 
Greenish white 
Pink 
Tints beyond 
Light blue 
Yellow verging to orange 
the poles. 
Dark blue 
Bright yellow 
55 57 
122 
15 
Sombre purple 
Yellow white 
Very indifferent sombre pink . 
Bluish white 
Pale yellow 
Dark indigo blue 
5+ 5 
124 
0 
White 
Sombre violet 
Very pale violet white 
Dusky greenish yellow 
52 57 
,2 5 
33 
Very sombre violet, almost black 
Pure brilliant white . 
Virtual poles. 
Sombre and dirty olive green . 
White rather ruddy 
Very pale blue 
Orange white 
White 
Sombre orange or brick red 
5‘ 3° 
127 
0 
White 
Narrow purple 
Orange white . v • 
Blue 
Bright orange , 
Pale blue 
Bright scarlet 
Bluish white 
Narrow crimson 
White 
;o c 
128 
3° 
Purple 
White 
Blue 
Yellow 
Bluish white 
Rich crimson 
48 40 
‘3° 
0 
Yellowish or greenish white . 
Purple 
Orange yellow 
Bright blue 
47 3 5 
IC 
Rich crimson 
Green 
47 ic 
‘31 
28 
Purple 
Good yellow 
Bright blue 
Pink yellow 
46 7 
‘3 2 
33 
Good green 
Rich pink or crimson 
45 3 
1.33 
45 
Rich pink 
Splendid green 
44 33 
‘34 
3° 
Greenish purple . 
Pinkish wtiite 
43 35 
•35 
>5 
Good green, but pale 
Fine pink 
42 23 
■36 
3° 
Pink red 
Greenish blue 
4° 55 
I .; 8 
c 
Pale bluish green 
Pale pink 
39 3 c 
‘39 
5° 
Pale pink 
Very pale greenish blue 
MDCCCXX. I 
