322 
MR. J. C. Me CONNER ON AN EXPERIMENTAL INVESTIGATION 
and five ciphers. This line shows that the large discrepancies between different 
measurements mean only very slight variations in a — b. They are no doubt due 
mainly to changes of temperature. The temperature of course varied one or two 
degrees during each set of observations, and no doubt the temperature of the quartz 
was slightly different from that of the thermometer. The ninth line gives the values 
of (f), and the tenth the values of a — b corrected to the temperature 22-|°. This was 
the mean temperature of the measurements of the large rings in Plate 2. The 
correction to a — b per degree of temperature is —'000000415. The last line gives 
the values of a — b, taken on the supposition that n=310 for the second band and 
corrected for temperature. We notice that the greatest difference between the 
numbers in the tenth line is '0000006, being about one part in six thousand. This is 
a strong confirmation of the truth of the Huyghenian construction for the region far 
removed from the axis, extending as it does from <£=54° to </>= 85°. The mean is 
a—b— '0037945. 
Table IV. (Plate 3). 
Temp. 
No. of Band. 
1. 
2. 
5. 
15. 
30. 
50. 
60. 
20° 
Analyser / . 
13° 34' 
19° 35' 
31“ 28' 
56° 104' 
82° 254' 
113° 37' 
20° 
„ /. 
19° 504' 
31° 42' 
56° 14“ 
82° 27' 
113° 354' 
22° 
„ \. 
. , 
19° 13' 
31° 25' 
56° 54' 
82° 204 ' 
113° 29' 
130° 354' 
21° 
„ \. 
19° 28' 
31° 27' 
56° 6' 
82° 23' 
113° 33' 
130° 394' 
20f° 
Mean diameter .... 
13° 34' 
19° 314' 
31° 304' 
56° 9' 
82° 24' 
113° 33p 
130° 374' 
a — b omitting ‘00 . 
Change in a — b per 10' 
37954 
37953 
37950 
37951 
37952 
37954 
37953 
omitting '00000 . 
030 
037 
055 
105 
140 
140 
140 
Value of 0 . 
a — b reduced to 224 ° 
85 c 37' 
83° 40' 
79° 53' 
72° 15' 
64° 45' 
57° IP 
53° 58' 
omitting '00 .... 
37944 
37946 
37943 
37944 
37945 
37947 
37949 
u — 310 for 2nd band . 
37823 
37823 
37820 
37816 
37811 
37802 
37804 
Referring back to Table III. we see that the numbers given there deduced from 
SAPR au’s theory are uniformly lower. This may be partly due to a wrong value of T, 
but, as I have said above, I do not think the error in the ratio of the values of T for 
the two plates exceeded 1 in 10,000. Moreover, Sarrau’s a — b diminishes steadily 
with (f) throughout. This is very apparent in the following figures, which are obtained 
by grouping three or four rings together, allowing as a possible error from the mean 
the combined effect of 4° in temperature and V in diameter. In the case of the three 
small rings, the 2nd, 3rd, and 4th, f' is allowed. In the case of the 1st 1'. 
From 7 bands </> = 90° to 53° a—b lies between '0037941 and '0037949 
4 rings </>=39° to 26° „ „ '0037928 „ '0037940 
3 rings 6= 25° to 14° „ „ '0037910 „ '0037934 
3 rings (j>=l‘3° to 8° „ „ '0037874 „ '0037916 
