16S On the Decomposition of Light. 



glasses on each other, indicated as possible. I pcrceivei 

 too, that the tints of the seven orders of colours might be 

 imitated by the three primitive colours alone which I have 

 mentioned*. This was sufficient to suggest to me the idea* 

 that perhaps these three kinds of rays were all that really 

 existed ; a proposition that required to be examined with 

 care proportioned to its importance. Accordingly I inquired 

 into the probabilities that might be brought to support it, 

 and compared it with all the phenomena of colour that oc- 

 curred to me ; and lastly I proved it by direct experiments. 

 The details of these I shall reserve for the last place, begiriT 

 iiing with an account of the others. 



I have already stated that the supposition of three colours 

 was not inconsistent with the formation of all the tints of 

 the spectrum. Nor does it contradict the unchangeableness 

 of each tint by a second refraction ; for if a red ray of a certain 

 degree exists on the spectrum at the same place with a green 

 ray of a particular degree, their combination will give a yellow 

 of a particular tint ; and as these two rays have the same re- 

 frangibility, a similar refractive power cannot again separatp 

 them. Accordingly, to have a spectrum in all points simi- 

 lar to that which really occurs, nothing more is necessary 

 than to conceive it composed of three spectrums partly over-- 

 laying each other ; one formed of red rays, differently re- 

 frangHjle, and of different tints ; another trenching a little 

 upon the first, and having only green rays, but a, similar 

 gradation of tints corresponding to their refrangibility ; and 

 a third, exhibiting an analogous scries of violet rays, and iri 

 like manner trenching upon the green. On this hypothesis, 

 there will be no disruption of the whole image, whatever 

 extent be given to it by refraction ; besides, it accounts for 



formably to the idzn^ of Newton. See Optics, bock i. prob. 2. The author 

 ijas explained the principles of the constructloa of this dial, and its leading 

 properties, in his •.voik. 



• This proposition of three primitive colours is very different from that 

 formerly adopted; for the red, yeilow and blue, have hitherto been so con- 

 sidered; while here they nre the red, green, and violet, the exclusive exist- 

 ence of which is proved by the analysis of white light in several wayj, 4s will 

 be seen further oil. 



seven 



