[ <25 ] 



the fame denomination of colour be differently refrangible, it is 

 not unreafonable to fuppofe, that rays of a different denomination 

 of colour may be equally refrangible ; and therefore fmce the red 

 rays are unequally refrangible, and likewife the yellow, there is 

 nothing incongruous in fuppofmg that Tome of the lefs refrangible 

 of the yellow may be equally refrangible with fome of the more 

 refrangible of the red ; and if fo, they will confequently be in- 

 termixed with them : and the fame may be faid of the green. 

 This hypothefis likewife receives confiderable flrength from 

 this confideration, that the orange, green, indigo and violet oc- 

 cupy thofe places which they ought to do, in cafe there were 

 but three primitive colours, red, yellow and blue: thus the 

 orange lies between the red and yellow, becaufe it is formed by 

 fome of "the extreme rays of red and yellow, which are equally 

 refrangible ; in like manner the green lies between the blue and 

 yellow, becaufe it is formed by the mixture of blue and yellow. 

 The indigo and violet muft alfo occupy the extreme part of the 

 fpedrum, where the mofl refrangible red and blue rays are united, 

 and gradually becoming more and more dilute, fade away, and 

 at length entirely vanifli. But if the orange, green, indigo and 

 violet be primitive colours, there is no apparent reafon why 

 they fhould have had fuch degrees of refrangibiiity affigned them, 

 as that they fhould occupy the places they do, rather than any 

 other. 



Moreover, if thefe three colours red, yellow and bluebe the 

 primitive colours, they cannot themfelves be generated ; and ac- 

 cordingly 



