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But though fpeculation feems thus to render it probable, that 

 there are but three parent colours ; Ar theory muft ever remain 

 unfatisfadlory, unlefs it receives the fandion of dired experiment. 

 In this however there is no fmall difEculty ; for fmce the rays of 

 light which compofe any given individual point of the colours of 

 orange, green, violet, and indigo are equally refrangible, they 

 will be alfo equally reflexible ; and therefore cannot be feparated 

 either by refraction or refledtion, fo as to exhibit the different 

 coloured rays of which they are compofed. It feems therefore, 

 that the only way remaining, by which we can experimentally 

 afcertain the compofition of thefe colours, if they be indeed 

 compound, is tranfmiffion. For fince tranfparent coloured bodies 

 are fuch merely by their letting pafs through them either folely, 

 or more copioufly, rays of a certain colour, and intercepting all 

 others, fuch tranfparent bodies, applied to compound colours, 

 will afcertain that compofition, by extinguifhing, in a great mea- 

 fure, all rays except fuch as are fo adapted to its conformation, 

 as to pafs through it, and give it its peculiar denomination of 

 colour. 



In order to try the truth of the hypothefis of feven colours by 

 this teft, I looked through a blue glafs at the red end of the 

 fpcdrum : now we are to conlider, that if that part of the fpedrum 

 was compofed of red rays, and none other, the only efFe£t of the 

 blue glafs would either be a total or partial fufFocation of the red 

 rays ; and therefore that part of the fpedrum, when looked at 



Vol. VII. R through 



