164 THEORY Or LIGHT AND COLOURS. 



rencc between the periods of the undulation and of the natu- 

 ral vibration is greater. 

 Doftrint of heat Corollary. It was long an eftablifhed opinion, that heat con- 

 vftratJoni'" flfts in vibrations of the particles of bodies, and is capable of 

 being tranfmitted by undulations through an apparent va* 

 cuum. (Newt. Opt. Qu. 18.) This opinion has been of 

 late very much abandoned. Count Rumford, Profeflbr Pi&et, 

 and Mr. Davy, are almoft the only authors who have appeared 

 to favour it ; but it feems to have been rejected without any 

 good grounds, and will probably very foon recover its popu- 

 larity. 

 Suppafe the Let us fuppofe that thefe vibrations are lefs frequent than 



tovibrate ■ '" * n °fe of light ; all bodies therefore are liable to permanent 

 vibrations flower than thofe of light ; and indeed almoft all are 

 liable to luminous vibrations, either when in a ftate of ignition, 

 or in the circum fiances of folar phofphori; but much lefs ea- 

 fily, and in a much lefs degree, than to the vibrations of heat, 

 then the more It will follow from thefe fuppofitions, that the more frequent 



frequent lumi- i um i nous un dulations will be more retarded than the lefs fre- 

 nous undula- 

 tions will be quent ; and confequently, that blue light will be more refran- 



rnuft retarded, orible than red, and radiant heat leaf! of all; a confequence 

 Blue light will « . ' , n- 



be mod refraft- which coincides exactly with the highly lntereftmg experi- 

 ed, and radiant ments Q c Dr . Herfchel. (Phil. Tranf. for 1800, p. 284.) It 

 ' may alfo be eafily conceived, that the actual exiflence of a 

 ftate of flower vibration may tend flill more to retard the more 

 ana refractive frequent undulations, and that the refractive power of folid 



power will in- bodies may be fenhbly increafed by an increafe of tempera- 

 creafe with the . J - .. J . ' , • t- i > 



temperature. t» re > as li 'actually appears to have been in Euler's experi- 

 ments. (Acad, de Berlin. 1762. p. 328.) 



Scholium. If, notwithfianding, this propofition fhould ap- 

 pear to be infufficiently demonftrated, it mufl be allowed to be 

 at leaf! equally explanatory of the phenomena with any thing 

 that can be advanced on the other fide, from the doctrine of 

 Thispofition projectiles; fince a fuppofed accelerating force mufl act in 

 explains the phe- f ome other proportion than that of the bulk of the particles ; 

 TeaftaswelTas and, if we call this an eleaive attraftion, it is only veiling 

 the projectile under a chemical term, our incapacity of afiigning a mecha- 

 hypothefis. nica] caufe> Mr Shortj when he found hy bf e rv'ation tlie 



equality of the velocity of light of all colours, felt the objec* 

 tion fo forcibly, that he immediately drew an inference from 

 it in favour of the undulatory fyftem. It is affumed in the 

 , propofitiop. 



