AND DISPERSION IN ICELAND SPAR. 
441. 
Before considering the results for the rays F and g we must return to the experiments 
with prism II. 
The large differences it gives, overlapping as they do values given by experiments 
on prism I., in which the differences are small, pointed clearly to errors of experiment. 
On referring to my note-book containing the direct results of experiment, I found that 
the observation of deviation and incidence for the face P had been made on March 29, 
while the observations for the face Q and the angle between the faces were made on 
April 1. 
It seemed possible that the temperature of the prism had been different on the two 
occasions, and that this was the cause of the error. I therefore proceeded to observe 
afresh the angle between the faces P Q of the prism. The result differed by 34" 
from that found on April 1. I therefore recalculated the experimental results for 
the prism II. so far as the face P was concerned. 
Table X. (a) gives the results of the calculations. In Table X. (b) these are com¬ 
pared with the theory. 
Table X. (a). —Prism II. Bay C. 
i= 43° 36' 53" 
D + i. 
<P- 
<p'. 
A 
84 39 59 
71 51 30 
35 46 16 
1-62569 
82 16 14 
67 51 30 
34 47 22 
1-62341 
80 9 4 
63 51 30 
33 38 22 
1-62053 
78 18 19 
59 51 30 
32 19 32 
1-61724 
76 42 24 
55 51 30 
30 51 53 
1-61332 
75 21 29 
51 51 30 
29 15 39 
1-60906 
74 14 19 
47 51 30 
27 31 46 
1-60425 
73 20 59 
43 51 30 
25 40 41 
1-59897 
72 41 14 
39 51 30 
23 43 3 
1-59336 
72 15 24 
35 51 30 
21 39 24 
1-58730 
72 4 29 
31 51 30 
19 30 14 
1-58091 
Table X. (b). —Theory for same. 
p. 
p 
Prom Theory. 
p 
From Experiment. 
Excess of 
Experiment. 
22 35 27 
1-62560 
1-62569 
+ 9 
23 34 21 
1-62326 
1-62341 
+ 15 
24 43 20 
1-62043 
1-62053 
+ 10 
26 2 10 
1-61711 
1-61724 
+ 13 
27 29 48 
1-61329 
1-61332 
+ 3 
29 6 1 
1-60897 
1-60906 
+ 9 
30 49 54 
1-60418 
1-60425 
+ 7 
32 40 59 
1-59894 
1-59897 
+ 3 
34 38 36 
1-59326 
1-59336 
+ 10 
36 42 14 
1-58721 
1-58730 
+ 9 
38 51 23 
1 -58083 
1-58091 
+ 8 
