from Bradley to FresneL 115 



his experimental measures. Poisson, when reading the manu- 

 script, happened to notice that the analysis could be extended 

 to other cases, and in particular that it would indicate the 

 existence of a bright spot at the centre of the shadow of a 

 circular screen. He suggested to Fresnel that this and some 

 further consequences should be tested experimentally ; this was 

 done, and the results were found to confirm the new theory. 

 The concordance of observation and calculation was so admirable 

 in all cases where a comparison was possible that the prize was 

 awarded to Fresnel without further hesitation. 



In the same year in which the memoir on diffraction was 

 submitted, Fresnel published an investigation* of the influence 

 of the earth's motion on light. We have already seen that 

 aberration was explained by its discoverer in terms of the 

 corpuscular theory ; and it was Young who first showedf how 

 it may be explained on the wave-hypothesis. " Upon con- 

 sidering the phenomena of the aberration of the stars," he 

 wrote, " I am disposed to believe that the luminiferous aether 

 pervades the substance of all material bodies with little or no 

 resistance, as freely perhaps as the wind passes through a 

 grove of trees." In fact, if we suppose the aether surrounding 

 the earth to be at rest and unaffected by the earth's motion, 

 the light- waves will not partake of the motion of the telescope , 

 which we may suppose directed to the true place of the star, 

 and the image of the star will therefore be displaced from the 

 central spider-line at the focus by a distance equal to that 

 which the earth describes while the light is travelling through 

 the telescope. This agrees with what is actually observed. 



But a host of further questions now suggest themselves. 

 Suppose, for instance, that a slab of glass with a plane face is 

 carried along by the motion of the earth, and it is desired to 

 adjust it so that a ray of light coming from a certain star 

 shall not be bent when it enters the glass : must the 

 .surface be placed at right angles to the true direction of the 



* Annales de Chimie, ix, p. 57 (1818) ; CEnvres, ii, p. 627. 

 t Phil. Trans., 1804, p. 12; Young's Works, i, p. 188. 

 I 2 



