OF THE PRODUCTION OF COLOULS. ] g£ 



The colours of mixed plates fuggeft to me an idea which Difperfion of 

 appears to lead to an explanation of the difperfion of colours by J?^",, J x ^j" 

 refraction, more fimple and fatisfaclory than that which I ad- ed from the afx 

 vanced in the laft Bakerian Mure. We may fuppofe that $*$**£** 

 every refractive medium tranfmits the undulations conftituting through the 

 light in two feparate portions, one paffing through its ultimate particles and 

 particles, and the other through its pores ; and that thefe por- pore sf 

 tions re-unite continually, after each fncceffive feparation, the 

 one having preceded the other by a very minute but conftant 

 interval, depending on the regular arrangement of the particles 

 of a homogenous medium. Now, if thefe two portions were 

 always equal, each point of the undulations refulting from their 

 re-union, would always be found half way between the places 

 of the correfponding point in the feparate portions; but, fup- 

 poting the preceding portion to be the fmaller, the newly 

 combined undulation will be lefs advanced than if both had 

 been equal, and the difference of its place will depend, not 

 only on the difference of the length of the two routes, which 

 will be conftant for all the undulations, but alfo on the law 

 and magnitude of thofe undulations ; fo that the larger un- 

 dulations will be fomewhat further advanced after each re- 

 union than the fmaller ones, and, the fame operation re- 

 curring at every particle of the medium, the whole progrefs 

 of the larger undulations will be more rapid than that of the 

 fmaller; hence the deviation, in confequence of the retarda- 

 tion of the motion of light in a denfer medium, will of courfe 

 be greater for the fmaller than for the larger undulations. 

 Affuming the law of the harmonic curve for the motions of 

 the particles, we might without much difficulty reduce this 

 conjecture to a comparifon with experiment ; but it would be 

 neceffary, in order to warrant our conclnfions, to be provided 

 with very accurate meafures of the refractive and difperfive 

 powers of various fubltances, for rays of all defcriptions. 



Dr. Wollafton's very interefting obfervations would furniih Comparifon of 

 great affiflance in this inquiry, when compared with the fepa- Dr « WollafWt 

 ration of colours by thin plates. I have repeated his experiments reparation o? 

 on the fpeclrum with perfect fuccefs, and have made fome colours by thiu 

 attempts to procure comparative meafures from thin plates ; p ates * 

 and I have found that, as Sir Ifaac Newton has already 

 pbferved, the blue and violet light is more difperfed by re- 

 fra&ion, than in proportion to the difference of the appropriate 



dimenfions 



