FRESNEL 207 



Fresnel then built further on this foundation, which how- 

 ever led, in the case of interference phenomena, to very diffi- 

 cult calculations, which could never have been carried out 

 without the calculus of Newton and Leibniz. But he could 

 show that the explanation of diffraction by the wave hypo- 

 thesis is completely successful, and that it suffices without 

 exception even when the experimental conditions are greatly 

 changed. In this matter, and also later, Fresnel worked in 

 part together with Arago (Member of the Paris Academy, 

 lived 1786-1853), whose goodwill he had won through his 

 first communication on diffraction to the Paris Academy. 



But Fresnel was not satisfied with a complete explanation 

 by means of the wave theory of diffraction phenomena and 

 the colours of thin plates, but wished to test this assumption 

 on his own account in as clear an experiment as possible. 

 This would have to be an interference experiment of the 

 simplest description without any diffraction, in which two 

 rays would simply be exposed to measurable differences of 

 path, but not treated otherwise differently, so that alternate 

 extinction and intensification as the difference in path grows 

 greater, could be taken directly and exclusively as a sign of 

 the presence of periodically changing opposite states along 

 the ray, that is to say, a wave phenomenon. 



Newton's experiments with thin plates appeared almost as 

 simple as this; here however, a bundle of rays is split up into 

 a reflected and a refracted path, which is later reflected, and 

 the possibility cannot be dismissed, that the periodically 

 changing states in the ray already assumed by Newton, 

 might be states of easy reflection and easy refraction, and not 

 states of opposite nature and hence capable of mutual anni- 

 hilation when they meet, as would be the case with a wave. 

 Such an interpretation was excluded in the case of Fresnel's 

 mirror experiment, which has with justification become 

 famous, since here the two parts of a bundle of rays are 

 reflected at two perfectly equal mirrors which meet at an 



