104 The Luminiferous Medium , 



wave-length, he in this memoir supposed the frequency greatest 

 for red light, and least for violet ; but a few years later* he 

 adopted the opposite opinion. 



The chief novelty of Euler's writings on light is his 

 explanation of the manner in which material bodies appear 

 coloured when viewed by white light ; and, in particular, of the 

 way in which the colours of thin plates are produced. He 

 denied that such colours are due to a more copious reflexion of 

 light of certain particular periods, and supposed that they 

 represent vibrations generated within the body itself under the 

 stimulus of the incident light. A coloured surface, according 

 to this hypothesis, contains large numbers of elastic molecules, 

 which, when agitated, emit light of period depending only 

 on their own structure. The colours of thin plates Euler 

 explained in the same way ; the elastic response and free period 

 of the plate at any place would, he conceived, depend on its 

 thickness at that place ; and in this way the dependence of the 

 colour on the thickness was accounted for, the phenomena as 

 a whole being analogous to well-known effects observed in 

 experiments on sound. 



An attempt to improve the corpuscular theory in another 

 direction was made in 1752 by the Marquis de Courtivron,f and 

 independently in the following year by T. Melville These 

 writers suggested, as an explanation of the different refran- 

 gibility of different colours, that " the differently colour'd rays 

 are projected with different velocities from the luminous body : 

 the red with the greatest, violet with the least, and the inter- 

 mediate colours with intermediate degrees of velocity." On 

 this supposition, as its authors pointed out, the amount of 

 aberration would be different for every different colour ; and 

 the satellites of Jupiter would change colour, from white through 

 green to violet, through an interval of more than half a minute 

 before their immersion into the planet's shadow ; while at 

 emersion the contrary succession of colours should be observed, 



* Mem. del' Acad.de Berlin, 1752, p. 262. t Courtivron's Traite cfoptique, 1752. 

 JPhil. Trans, xlviii (1753), p. 262. 



