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 + -00000414/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 y^ atmosphere, hydrogen at -$ atmosphere, or helium at 

 -fo 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 page, on which 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 



*y 



of a radial displacement l"-75 -^ , where / is the distance from the sun's centre and r a 



the radius of the sun. 



TABLE I. 



* ' Monthly Notices, R.A.S.,' LXXVII, p. 445. 



2 8 2 



