CHAPTER IV. 



THE LUMINIFEROUS MEDIUM, FROM BRADLEY TO FRESNEL. 



ALTHOUGH Newton, as we have seen, refrained from committing 

 himself to any doctrine regarding the ultimate nature of light, 

 the writers of the next generation interpreted his criticism of 

 the wave-theory as equivalent to an acceptance of the 

 corpuscular hypothesis. As it happened, the chief optical 

 discovery of this period tended to support the latter theory, 

 by which it was first and most readily explained. In 1728 

 James Bradley (b. 1692, d. 1762), at that time Savilian 

 Professor of Astronomy at Oxford, sent to the Astronomer 

 Royal (Halley) an " Account of a new discovered motion of the 

 Fix'd Stars."* In observing the star y in the head of the 

 Dragon, he had found that during the winter of 1725-6 the 

 transit across the meridian was continually more southerly, 

 while during the following summer its original position was 

 restored by a motion northwards. Such an effect could not be 

 explained as a result of parallax ; and eventually Bradley 

 guessed it to be due to the gradual propagation of light.f 



Thus, let CA denote a ray of light, falling on the line BA ; 

 and suppose that the eye of the observer is travelling ^ 

 along BA, with a velocity which is to the velocity 

 of light as BA is to CA. Then the corpuscle of 

 light, by which the object is discernible to the eye 

 at A, would have been at C when the eye was at 

 B. The tube of a telescope must therefore be pointed 

 in the direction BC, in order to receive the rays 

 from an object whose light is really propagated in 

 the direction CA. The angle BCA measures the 

 difference between the real and apparent positions 



of the object ; and it is evident from the figure that the sine of 



Phil. Trans, xxxv (1728), p. 637. 



t Roemer, in a letter to Huygens of date 30th Dec., 1677, mentions a suspected 

 displacement of the apparent position of a star, due to the motion of the earth at 

 right angles to the line of sight. Cf . Correspondance de Huygens, viii, p. 53. 



H 2 



