DEFLECTION OF LIGHT DYSON AND OTHERS. 135 
this disagreement is to be taken as final it necessitates considerable 
modifications of Einstein's theory, which it is outside our province to 
discuss. But, whether or not changes are needed in other parts of 
the theory, it appears now to be established that Einstein's law of 
gravitation gives the true deviations from the Newtonian law both 
for the relatively slow-moving planet Mercury and for the fast- 
moving waves of light. 
It seems clear that the effect here found must be attributed to the 
sun's gravitational field and not, for example, to refraction by 
coronal matter. In order to produce the observed effect by refraction, 
the sun must be surrounded by material of refractive index 
l-|-0.00000414/r, where r is the distance from the center 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 
yItj- atmosphere, hydrogen at -^ atmosphere, or helium at ^V atmos- 
pheric pressure. Clearly a density of this order is out of the ques- 
tion. 
II. PREPARATIONS FOR THE EXPEDITIONS. 
5. In March, 1917,^ it was pointed out as the result of an examina- 
tion of the photographs taken with the Greenwich astrographic tele- 
scope 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 favorable 
opix)rtunity 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 
12 stars would be shown. Their positions are indicated in the dia- 
gram on page 136, on which is also marked on the same scale the 
outline of a 16 by 16 centimeter plate (used with the astrographic 
telescopes of 3.43 meters focal length) and a 10 by 8 inch plate 
(used with a 4-inch lens of 19 feet focal length). 
The following table gives the photographic magnitudes and stand- 
ard coordinates of the stars, and the gravitational displacements 
in X and y calculated on the assumption of a radial displacement 
1.75''^-^ where r is the distance from the sun's center and r„ the 
radius of the sun. 
6 Monthly Notices, R. A. S., LXXVII, p. 445. 
