FRESNEL 205 



activities which demanded a great deal of him, and he was 

 not able to obtain lighter work. In the year 1824 his health 

 was undermined; he began to suffer from haemorrhage of 

 the lungs. His life, like that of Fraunhofer's, only lasted 

 thirty-nine years. 



Fresnel's investigations started from simple cases of dif- 

 fraction, which were already known, though they are fairly 

 complicated even in the case of the shadow cast by a single 

 hair or aperture. These he investigated in all their details 

 with innumerable variations, and in the finest manner possible, 

 each experiment representing a definite question put to Nature. 

 He found that the phenomena entirely corresponded to a 

 wave motion according to the principle already put forward 

 by Huygens, if we take into account that waves can always 

 exhibit interference. Actually, all phenomena of diffraction 

 could be explained as due to interference of the diffracted 

 light. The diffraction itself, the possibility of propagation 

 in curved lines, is already contained in Huygens' principle. 



The further phenomena of interference were also in them- 

 selves not new, since they had already been taken into 

 account by Sauveur as regards sound, in the time of Newton; 

 they depend upon the fact that in every train of waves, no 

 matter of what kind, opposite states follow one another regu- 

 larly, as crest and trough in the case of water waves, conden- 

 sation and rarefaction in the case of sound waves, the distance 

 apart of these states being half a wave-length. If two 

 trains of waves meet at a point, having opposite states of 

 equal length, they must annihilate one another, and this 

 mutual destruction of two trains of waves is called inter- 

 ference. When like states meet, the trains of waves streng- 

 then one another. In the case of sound, beats, which result 

 from the meeting of two notes not exactly equal in pitch, had 

 already been thoroughly explained by Sauveur as an inter- 

 ference phenomenon. If light is a wave motion, as Huygens 

 already thought, it should also exhibit interference, and this 



