18 



SCIENTIFIC THOUGHT. 



13. 

 His 



' ' general 

 law of the 



he " discovered a law which appeared to account for a 

 greater variety of interesting phenomena than any other 

 optical principle that had yet been made known." ^ This 

 principle he familiarly illustrated by the well - known 

 observation that two series of waves of water entering a 

 channel reinforce or destroy each other according as their 

 elevations coincide or alternate in time. He main- 

 tained that similar effects take place whenever two 

 portions of light are thus mixed, and this he called 

 " the general law of the interference of light." He 

 SYight.^^*^^ showed " " that this law agrees most accurately with 

 the measures recorded in Newton's ' Opticks,' relative to 

 the colours of transparent substances, and with a great 

 diversity of other experiments never before explained." ^ 

 In three papers Young entered " minutely into the con- 

 sequences of the law of the interference of light.' 

 Especially in the case of the remarkable phenomena 

 discovered by Grimaldi, where Ught seems to bend round 

 the edge of screening surfaces, he showed how under 

 certain conditions light added to light would create 

 darkness, and, if removed, would leave light ; and he 

 ^^ boldly generalised the undulatory theory by maintaining 

 ^t^7 that * " a luminiferous ether pervades the universe, rare 

 ether!' ^™"*' and clastic in a high degree," that the sensation of 



»/ 



1 Works, vol. i. p. 202. 



- Ibid., p. 203. 



' ' ' This, I assert, is a most 

 powerful argument in favour of 

 the theory which I had before 

 revived : there was nothing that 

 could have led to it in auj^ author 

 with whom I am acquainted, ex- 

 cept some imperfect hints in those 

 inexhaustible but neglected mines 



of nascent inventions, the works of 

 the great Dr Robert Hooke, which 

 had never occurred to me at the 

 time that I discovered the law " 

 (ibid., p. 203). 



■^ The sentences in quotation 

 marks are the headings of the 

 different paragraphs in the " Baker- 

 ian Lecture " of November 12, ISOl. 

 Works, vol. i. p. 140 sqq. 



