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cordingly we find, that yellow cannot be generated by the mixture 

 of the adjacent prifmatic colours, orange and green ; and the reafon 

 of this is evident, becaufe orange is compounded of red and yellow ; 

 and green is compounded of yellow and blue ; but red and blue 

 compofe purple ; which added to the yellow will generate a new 

 compound colour, viz. a fickly green, differing manifeftly from 

 yellow, the colour which ought to refult according to the analogy 

 of the other primitive colours, in which the extremes, by their 

 mixture, generate that which is intermediate. In the fame manner, 

 blue cannot be generated by the mixture of green and indigo, 

 becaufe green is compofed of yellow and blue, and indigo of 

 blue and violet ; therefore the refulting colour is compofed of 

 blue, yellow and violet ; but yellow and violet do not compofe blue, 

 therefore neither will blue, yellow and violet compofe a blue 

 colour. Now if orange and green be primitive colours, in the 

 fame manner as red, yellow and blue, we can aflign no reafon 

 why blue fhould not be generated by the mixture of the adjacent 

 colours, as well as green and orange. But it is a received prin- 

 ciple, that an hypothefis fhould folve all the phsenomena ; of the 

 two hypothe'es therefore, namely, that there are feven primitive 

 colours, differently refrangible ; or that there are but three, fome 

 of which, of each fpecies, are equally refrangible ; the latter alone 

 folves all the pha:nomena of the folar fpedrum, and therefore is 

 to be preferred. 



1 1 it be faid, that thofe rays which are equally refrangible 

 mufl excite the fame fenfation on the retina, becaufe they muft 



have 



