FRESNEL 211 



also himself, in a certain sense?), he stated his deduction of 

 the transverse nature of polarised light accompanied by a 

 hypothetical explanation as to how transverse waves might 

 still be possible even in ordinary liquids. This mechanical 

 explanation of transverse ether waves later became of no 

 importance: after the results obtained by Faraday, Maxwell, 

 and Hertz, it has become clear also in detail, that the ether is 

 a thing by itself, not only as regards its intangibility (its in- 

 capacity for being confined in vessels). 



It is not superfluous to point out these peculiar difficulties 

 of Fresnel's time. It was the time of greatest departure of 

 the study of nature (and altogether of humanity's world 

 picture) from reality, as a result of complete submission to 

 matter. Kepler, like Newton, was still free from this ten- 

 dency. Huygens was not; for he says at one point^ of 'true 

 philosophy,' that in it 'the cause of all natural effects is found 

 in mechanical reasons.' Perhaps that was also the cause of 

 his not discovering gravitation, although no one at that time 

 was nearer to it than he. Newton calmly announced his law 

 of gravitation, although he had no mechanical explanation of 

 it to offer. Newton also remarks concerning polarised light, 

 a long time before Malus, and solely from the study of 

 Iceland spar as with Huygens, quite calmly: 'Has not the ray 

 different sides with different properties?' ^ and he re- 

 marks that this would not be possible in the case of a longi- 

 tudinal or pressure wave. The conclusion as to transverse 

 waves had to be drawn, when Fresnel had proved the 

 existence of waves in the ray. 



Fresnel then sought to discover the wave nature of 

 ordinary unpolarised light. In view of the complete absence 

 of unsymmetry, it might be longitudinal; according to the 

 manner in which polarised light was derived from ordinary 

 light another supposition was suggested: that the ordinary 



1 At the beginning of his essay on light, 1690. 

 " Newton, Opticks, 17 17, third book, query 26. 



