372 SCIENCE PROGRESS 



temperature at Principe was in its favour. The final result 

 of the discussion of these plates was an outward deflection of 

 i"-6i, with a probable error of ±°"'3°- 



Summarising, the 4-inch lens at Sobral gave a value of the 

 deflection of i"'94 from declinations and of 2"-o6 from right 

 ascensions, these values being in excellent agreement and 

 giving a weighted mean value of i"-q8 = | = o"-o6. The Principe 

 observations have a much larger probable error (±°""3°) Dut 

 tend to confirm the result, and both these series point to the 

 full deflection i"75 of Einstein's theory and to the predicted 

 rate of falling off with increase in distance from the sun's 

 centre. There remain the Sobral astrographic plates which 

 gave a value discordant by an amount far exceeding the limits 

 of its accidental error, which discrepancy may possibly be due 

 to change of figure of the mirror. On the whole, therefore, 

 Einstein's predicted displacement is confirmed, but there is 

 sufficient uncertainty in the result to render it desirable that 

 further tests should be made at future eclipses. 



Finally it should be pointed out that it seems clear that 

 the deflection must be attributed to the sun's gravitational 

 field. Refraction by coronal matter naturally suggests itself 

 as a possible explanation, but a simple investigation at once 

 shows that an impossibly high value of the refractive index 

 would be necessary to account for the deflection. 



The announcement of the result attracted widespread 

 attention, and so many distorted accounts and unjustified 

 statements have appeared in the daily press that it is well to 

 consider briefly what interpretation is to be put upon it. 



First, then, the result may be considered independently of 

 any theory, as a purely observational fact. It cannot be 

 denied that it has been definitely established that the light from 

 a star is deflected in passing near the sun's edge, though there 

 may still remain some doubt as to the exact amount of the 

 deflection. Since it seems very improbable that the phenome- 

 non is due to refraction, it must be concluded that it is due to 

 the direct effect of gravitation on the field of light. The 

 establishment of this fact is a scientific achievement of the 

 greatest importance. Many attempts have been made to gain 

 some clue as to the nature of gravitation, but always with a 

 negative result. Gravitation has remained the most elusive of 

 the forces of nature. Theory after theory has been proposed 

 to account for it, but none could be accepted because none 

 could be tested. Now we know the nature of light and its 

 velocity can be measured ; the electro-magnetic theory of light 

 is to-day universally accepted. The present result therefore 

 shows a direct inter-relationship between gravitational and 

 electro-magnetic forces. 



