28 SCIENTIFIC THOUGHT. 



England and to Young, who learnt from them that, 

 mainly owing to Fresnel's labours, his own researches 

 had " attracted much more notice in Paris than in 

 London, . . . leading to some very warm discussions 

 among the members of the Institute on some public 

 occasions." l It is likely that this visit, as well as the 

 discovery of Arago that rays of light when polarised 

 i.e., possessed of laterality lose under certain condi- 

 tions their power of interference, induced Young to 

 resume seriously the consideration of the subject. In 

 19. January 1817, long before Fresnel had made up his 



Young and 



introduce mmc ^ to adopt a similar conclusion (suggested to him 

 tu>n C of ncei> kv Ampere), Young announced in a letter to Arago 

 that in the assumption of transverse vibrations, after the 

 manner of the vibrations of a stretched string, lay the 

 possibility of explaining polarisation or " laterality," and 

 the non - interference of rays whose sides are perpen- 

 dicular to each other. By introducing this conception 

 of a lateral or transverse movement into physical optics 

 a conception shortly afterwards adopted by Fresnel 

 the data were provided for a complete mechanical or 

 kinetic explanation of all phenomena of homogeneous 

 rays of light i.e., of such rays as, on passing through 

 refracting substances, are not divided into several 

 colours. 



Two great problems now presented themselves, one of 

 which Fresnel attacked with great success. The other is 

 hardly yet solved. Inasmuch as these two problems 

 have largely occupied physicists and mathematicians all 

 through the century, and guided their reasonings in other 



1 Peacock, ' Life of Young,' p. 389. 



