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. 



Fresnel's 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 

 wave 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. 



