On the Number of the Primitive Colorific Rays in Solar Ligkt, 389 



not yet been proved, that the fenfation of different colours depends on the different mo- 

 mentum of the rays. 2dly, The rays may have different momentums, and yet be equally 

 refrangible j for fince refraftion is fuppofed to depend on the attra£live force of the denfer 

 medium, we muft fuppofe it analogous to the attraftive force of gravity, which is propor- 

 tional to the quantity of matter; and therefore the greater or lefs quantity of matter in af 

 particle of light would produce no alteration in its refraction. Neither can the different 

 refrangibility depend on the different velocity of the rays ; becaufe the difference of refran- 

 gibility of the red and violet rays is much greater in flirit glafs than in crown glafs ; and 

 this would require a proportionably greater difierence In the original velocities, which can- 

 not be. And this fame argument holds equally againft the former hypothefis, that the 

 difference of refrangibility depends on the different magnitude or denfity of the particles of 

 light. S'lly, Rcfraflion feems to arife from a fpecies of eledlive attra6lion, fince different 

 mediums which a£l: on the mean rays equally, ail on the extreme rays unequally: hence 

 rays of the fame quantity of matter and velocity, and therefore of the fame momentum,, 

 may be diverfely refracted ; and rays of different, momentums equally refraifted. 



Nor is it to be wondered at that the rays of light fhould be differently refrangible, inde- 

 pendent of any regard to their momentum, when we conCder, that the different coloured 

 rays appear to be combined with combuftible bodies, with different degrees of attraftive 

 force. For in combuftion we find, that different bodies are difpofed to part with different 

 lays with greater facility; but when the combuftion is fufficiently rapid, they part wifh all 

 the different coloured rays together, and the ftame is therefore white ; and this is what is- 

 called a white heat^. Dr. Fordyce in the Phil. Tranf. for 1776, tells us, that when the 

 heated fubftances are colourlefs, they firft emit a red light ; then a red mixed with yellow,, 

 and laftly, with a great degree of heat, a pure white. All this Is wonderfully conformable 

 to the refraction of light by tranfparent fubftances, which refraft, and therefore attract the 

 red light lefs,. and confequently in combuftion part with it more eafily. On the other 

 hand I know It is generally believed, that the light in combuftion proceeds from the air^ 

 but this circumftance of the different colour of the light in different cafes, feems to over-, 

 turn this opinion ; for If vital air were oxygen diffolved in caloric and light, then the oxy- 

 gen being abforbed by the burning body, the light extricated would in all cafes be of the 

 fame nature; the greater or lefs rapidity of the combuftion would only produce an extrica- 

 tion of a greater or lefs quantity of light, but could not produce any variation in its nature,, 

 it being neeeffariiy the fame in all cafes, to wit, that in which vital air is diffolved. But 

 the truth or falfhood of this reafoning will not affe£t the validity of the pofition, that the 

 refrangibility of the rays of light cannot depend on the^different magnitude, denfity,, or 

 velocity of the particles. 



But though fpcculation feems thus to render it probable, that there are but three parent 



colours ; our theory muft ever remain unfatisfadtory, unlefs it receives the fan£tion of dire£t 



experiment. In this howver there is no fmall difficulty ; for fince the rays of light which 



•ompofe any given individual point of the colours of orange, green, violet, and indigo are 



3. equally 



