220 
PROFESSOR POWELL ON A NEW CASE OF 
(27.) 
Arrangement. 
Plate. 
Medium. 
Number of bands. 
Glass. 
Oil of Cummin. 
B to D. 
D to E. 
E to F. 
F to G. 
G to H. 
Total. 
I. 
inch. 
r='04 
Broad. 
0 
0 
5 
11 
J Faint. 1 
1 15? / 
31 
11. 
No bands. 
(28.) 
Oil of Turpentine. 
I. 
No bands. 
11. 
•08 
•04 
Fine. 
r Clear in the red H 
< broader towards > 
(_ blue. J 
14 
20 
15 
10 
59 
(29.) 
Water. 
I. 
No bands. 
II. 
•04 
•015 
Too tine to eount. 
r Fine and faint; 1 
\ difficult to count./ 
19 
30 ? 
22? 
15 ? 
86 
(30.) 
Arrangement. 
Plate. 
Medium. 
Number of bands. 
Calc-spar (by 
natural cleavage). 
Oil of Cassia. 
B to D. 
D to F. 
F to G. 
G to H. 
1. 
inch. 
r=^04 
Position J 
vertical. ] 
Two sets of bands 
superposed. 
With TBroad set. 
j 
0 
Broad and fain 
14 
30? 
40 ? 
prism. i^Fine set. 
20 
36 ? 
Too fine to 
20 unt. 
Position 1. 
Position 2. 
Both sets broader. 
Both sets finer. 
N 
o 
CO 
Both sets too fine to count. 
(31.) To detertnine which set of bands belongs to the 
ordinary and which to the extraordinary pencil, we may 
proceed as follows -.— Placing a rhomb of calc-spar with 
a small aperture behind it, so as to give two images, in 
the same relative position as the plate (see fig. 6) when 
tne section ot the Nicol-prism has its short diagonal per- 
])endicular to that of the rhomb (1.), E disappears; when 
parallel (2.) O disappears. In the spectrum, in the former 
case the finer bands disappear, in the latter the broader. 
The fine bands therefore belong to E, the broad to O. 
Fig. 6. 
