DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 331 



The result from declinations is about twice the weight of that from right ascensions, 



so that the mean result is 



l"-98 



with a probable error of about 0"-12. 



The Principe observations were generally interfered with by cloud. The unfavourable 

 circumstances were perhaps partly compensated by the advantage of the extremely 

 uniform temperature of the island. The deflection obtained was 



The probable error is about 0"-30, so that the result has much less weight than 

 the preceding. 



Both of these point to the full deflection l"-75 of EINSTEIN'S generalised relativity 

 theory, the Sobral results definitely, and the Principe results perhaps with some un- 

 certainty. There remain the Sobral astrographic plates which gave the deflection 



0"-93 



discordant by an amount much beyond the limits of its accidental error. For the 

 reasons already described at length not much weight is attached to this determination. 

 It has been assumed that the displacement is inversely proportional to the distance 

 from the sun's centre, since all theories agree on this, and indeed it seems clear from 

 considerations of dimensions that a displacement, if due to gravitation, must follow this 

 law. From the results with the 4-inch lens, some kind of test of the law is possible 

 though it is necessarily only rough. The evidence is summarised in the following table 

 and diagram, which show the radial displacement of the individual stars (mean from all 

 the plates) plotted against the reciprocal of the distance from the centre. The displace- 

 ment according to EINSTEIN'S theory is indicated by the heavy line, according to the 

 Newtonian law by the dotted line, and from these observations by the thin line. 



RADIAL Displacement of Individual Stars. 



VOL. CCXX. A. 



2 Z 



