DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN’S GRAVITATIONAL FIELD. 293 
observed effect by refraction, the sun must be surrounded by material of refractive index 
1 + -OOGOOdld/r, where r is the distance from the centre in terms of the sun’s radius. 
At a height of one radius above the surface the necessary refractive index 1*00000212 
corresponds to that of air at atmosphere, hydrogen at atmosphere, or helium at 
t 2 ^ atmospheric pressure. Clearly a density of this order is out of the question. 
II. Preparations for the Expeditions. 
5. In March, 1917,* it was pointed out as the result of an examination of the photo¬ 
graphs taken with the Greenwich astrographic telescope at the eclipse of 1905 that this 
instrument was suitable for the photography of the field of stars surrounding the sun 
in a total eclipse. Attention was also drawn to the importance of observing the eclipse 
of May 29, 1919, as this afforded a specially favourable opportunity owing to the unusual 
number of bright stars in the field, such as would not occur again for many years. 
With weather conditions as good as those at Sfax in the 1905 eclipse—and these 
were by no means perfect—it was anticipated that twelve stars would be shown. Their 
positions are indicated in the diagram on next j)age> on winch is also marked on the 
same scale the outline of a 16 X 16 cm. plate (used with the astrographic telescopes of 
3*43 metres focal length) and a 10 X 8-inch plate (used with a 4-inch lens of 19 feet 
focal length). 
The following table gives the photographic magnitudes and standard co-ordinates 
of the stars, and the gravitational displacements in x and y calculated on the assumption 
of a radial displacement 1"*75 j, where r is the distance from the sun’s centre and fy 
the radius of the sun. 
Table I. 
Co-ordinates. 
Gravitational displacement. 
No. 
Names. 
Photog. 
Mag. 
Unit 
= 50'. 
Sobral. 
Principe. 
X. 
y- 
X. 
y- 
X. 
y- 
1 
B.D., 21°, 641 ... . 
m. 
7-0 
-hO-026 
-0-200 
n 
-1-31 
// 
+0-20 
If 
-1-04 
n 
+0-09 
2 
Piazzi, IV, 82 ... . 
5-8 
+1-079 
-0-328 
+0-85 
-0-09 
+ 1-02 
-0-16 
3 
Tauri. 
5-5 
+0-348 
+0-360 
-0-12 
+0-87 
-0-28 
+0-81 
4 
k' Tauri. 
4-5 
+0-334 
+0-472 
-0-10 
+0-73 
—0-21 
+0-70 
5 
Piazzi, IV, 61 ... . 
6-0 
-0-160 
-1-107 
-0-31 
-0-43 
-0-31 
-0-38 
6 
V Tauri. 
4-5 
+.0-587 
+ 1-099 
+0-04 
+0-40 
+0-01 
+0-41 
7 
B.D., 20°, 741 ... . 
7-0 
-0-707 
-0-864 
-0-38 
-0-20 
-0-35 
-0-17 
8 
B.D., 20°, 740 ... . 
7-0 
-0-727 
-1-040 
-0-33 
-0-22 
-0 29 
—0-20 
9 
Piazzi, IV, 53 ... . 
7-0 
-0-483 
-1-303 
-0-26 
-0-30 
-0-26 
-0-27 
10 
72 Tauri. 
5-5 
+0-860 
+ 1-321 
+0-09 
+0-32 
+0-07 
+0-34 
11 
66 Tauri. 
5'5 
-1-261 
-0.160 
-0-32 
+0-02 
—0-30 
+0-01 
12 
53 Tauri. 
5-5 
-1-311 
-0-918 
-0-28 
-0-10 
-0-26 
-0-09 
13 
B.D., 22°, 688 ... . 
8-0 
+0-089 
+ 1-007 
-0-17 
+0-40 
-0-14 
+0-39 
* ‘ Monthly Notices, R.A.S.,’ LXXVII, p. 446. 
2 s 2 
