YOUNG, FRAUNHOFER, AND FRESNEL 197 



that their existence or non-existence could be subjected to 

 decisive tests by means of a series of properly planned ex- 

 periments. Just as, centuries previously, the physics of 

 matter had gradually proceeded from observation of what 

 offered itself more or less without preparation to thorough 

 and well-planned experiment, so now was a similiar tran- 

 sition accomplished in respect of the physics of the ether 

 founded by Huygens. 



But for the fact that the domain of the ether, the basis of 

 all known and unknown phenomena of light, electricity, and 

 magnetism, is obviously so infinitely more extensive than 

 that of matter (which is certainly nothing more than a pecu- 

 liar and tangible formation of ether), we might call Fresnel 

 the Galileo of the ether; but while Galileo was already able 

 in his experiments to proceed very far towards a knowledge 

 of the general behaviour of matter, we feel to-day as regards 

 the ether, even after the work of Faraday, Robert Mayer and 

 Hasenohrl, that we are still only quite in the initial stages of 

 knowledge. In any case, we owe to Fresnel the assurance 

 that the ether is able to form or produce or carry waves, 

 which are propagated with the velocity already measured 

 by Roemer. The length of these waves -measured from 

 peak to peak or valley to valley, without our yet being able 

 to say what peak and valley actually mean, other than states 

 of opposite nature^ - was determined by Fraunhofer for the 

 first time with very considerable accuracy by means of a new 

 optical apparatus designed for this purpose, the grating. We 

 are provided with a special organ, the eye, for the detection 

 of these ether waves. 



^ Fraunhofer, in a manner very characteristic of him as a true investi- 

 gator of nature, both in definite regions of known phenomena and in un- 

 known, says himself, concerning the wave nature of light: 'Whatever we 

 are to imagine under this term, it must in any case be of such a nature that 

 one half of the same is opposite as regards its effect to the other half, so 

 that if one half meets with a half of the opposite kind, the effect is elimin- 

 ated, whereas it is doubled when two like halves act together in the same 

 sense. This is the fundamental notion of interference. 



