jSS On the Nurnhr of the Primitive Colorijit Rays in Solar tight. 



mixture of blue and yetiow circles, equally refrangible. Now this is fimpk, and con- 

 formable to the other phaenomena of the fpeftrum ; for if rays of the fame denomination 

 'of colour be differently refrangible, it is not unreafonable to fuppofe, that rays of a dif- 

 ferent denomination of colour may be equally refrangible ; and therefore fince the red rays 

 are unequally refrangible, and likewife the yellow, there is nothing incongruous in fup- 

 poGng (hat feme 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 intermixed vvijth 

 them : and the fame may be faid of the green. This hypothcfis likewife receives confir 

 derable ftrength from this confideration, that the orange, green, indigo, and violet occupy 

 thofe places \yhich t4iey 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 

 fpe£lrum, where the mofl refrangible red and blue rays are united, and gradually becom- 

 ing 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 flioald 

 have had fuch degrees of refrangibility affigned them, as that they {hould occupy the 

 places they do, rather than any other. 



Moreover, if thefe three colours red, yellow, and blue be the primitive colours, they 

 cannot themfelves be generated ; and accordingly 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 gene- 

 rate 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, there- 

 fore 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 affign no 

 reafon why blue (hould not be generated by the mixture of the adjacent colours, as well as 

 green and orange. But it is a received principle, that an hypotheCs fliould folve all the 

 phsenomena ; of the two hypothefes therefore, namely, that there are feven piimitive co- 

 lours, differently refrangible ; or that there are but three, fome of which, of each fpecies* 

 are equally refrangible ; the latter alone folves all the phenomena of the folar fpedrum, 

 and therefore is to be preferred. 



If it be faid, that thofe rays which are equally refrangible mufl excite the fame fenfatloa 

 •n the retina) becaufe tbey muft have, the fame oiomentum i it is replied, ift, That it has 



