THE PROGRESS OF PHYSICS. 153 



to this singular result that light added to light could, 

 in certain cases, produce darkness, a paradoxical re- 

 sult contradicted by daily experience." * 



In spite of Young^s step, the emission-theory still 

 held the field, and new facts, such as the phenomenon 

 of polarisation discovered by Malus, lent support to 

 it rather than to its rival. 



FresneVs Experiments, — In 1816, however, a 

 young engineer, Augustin Fresnel (1788-1827), re- 

 discovered the principle of interference, applied 

 mathematical analysis to the vindication of the un- 

 dulatory theory, and devised the famous two-mirror 

 experiment, by which it was shown that " two rays, 

 issuing from the same source, free from any disturb- 

 ance, produced when they met, sometimes light, some- 

 times darkness.'^ Moreover Fresnel showed that 

 " light is propagated in straight lines because the 

 luminous waves are extremely small, while sound is 

 diffused because the lengths of the sonorous waves 

 are relatively very great,'' and that " the sound 

 vvave cannot be polarised because the vibrations are 

 longitudinal, while light can be polarised because the 

 vibrations are transverse, that is to say, perpendicular 

 to the luminous ray." " Henceforth the nature of 

 light is completely established, all the phenomena 

 presented as objections to the undulatory theory are 

 explained with marvellous facility, even down to the 

 smallest details." f 



To Fresnel and to Arago, Young " was first in- 

 debted for the restitution of his rights," and it is 

 pleasant to notice the entire absence of any discussion 

 as to priority. But the complete acceptance of the un- 

 dulatory theory was still distant. There followed a 



♦ Cornu, loc. cit., p. 295. 

 t Quotations from Cornu. 



