39« On (he Number of the Primitive Colorijic Rays in Solar Light. 



ftill remains fenfible, that is, liable to be excited into aftlon by any other colours at the 

 fame time ; and therefore the fpeflrum aflumes a green uppearance, becaufe if all the red 

 rays be taken out of the folar light, the remaining ray? will compafe green. See Phil, 

 T ranf. Vol. LXXVI. Converfely, a green obje£t produces a red ocular fpedlrum. Now 

 we may obferve, that if all the green rays be taken out of, the folar fpeftrum ot feven co- 

 lours, the remaining colours will not compound red. If indeed green be not a primitive 

 colour, but a Compofition of blue and yellow, then will the eye, ia looking on a green 

 obje£l, be at once affe£ted by blue and yellow rays ; and therefore become infenfible to 

 them both ; and confequently the fpetlrum will appear red. But if green be a primitive, 

 original colour, generated by its own peculiar green-making rays, the eye in contem- 

 plating a green objedl, will become infenfible only to the green rays ; and therefore the 

 Qtksr fix prifmatic colours, which are fpeclfically different from the green, ought to be 

 fenfible, and produce their proper compound effedt ; but this would not be the fenfation 

 of red. In like manner, if the obje£l be yellow, the eye will at length become infenfible 

 to the yellow-making rays, and the fp^dtrum will be violet. Now fince on the hypothefis 

 of feven original colours, the orange and green are primitive, though the eye be rendered 

 infenfible to tlie yellow rays, it will not be fo to the orange and green, which therefore, 

 together with the red, blue, violet and indigo, will produce their compound efFe£l; but 

 the colour, refulting from this joint a£tioii is not violet, which neverthelefs is the colour of 

 the ocular fpe£trum. On the other hand, if there be but three primitive colours, red, 

 yellow, and blue, when the eye is infenfible to the yellow-making rays, the fpe£trum 

 muft neceflarily be violet, which is the colour that refults from the mixture of red and 

 blue. If it be objected, that the eye is not only infenfible to the unmixed yellow rays, 

 but likewlfe to the yellow of the orange and the green, then it is admitted that orange and 

 green are compound colours. Befides, fince the colour which would refult from the mix- 

 ture of red, orange, green, blue, indigo and violet is not yellow, the eye ought not to be 

 infenfible to this colour j and confequently, fince by the exemption of the yellow rays from 

 the white folar light, that colour does not refult, but a diftinfl purple, it follows, that the 

 orange and green are not primitive colours inherent in folar light. 



It remains now only for us to fhew, that the tlirec colours of red, yellow, and blue, are 

 adequate to the folution of all the phenomena of chromatics. But in order to fhew this, 

 few words will be fulHcient, for having feen, that the feven prifmatic colours can be ge- 

 nerated by thefe three, it follows that all others can be generated from them, as Sir I. 

 Newton has proved at large. However, I think it will not be fuperfluous to obferve, that 

 white may be directly produced by thefe three colours, without the previous generation of 

 the other four prifmatic colours, in the fame manner as it is ufually generated with feven. 

 " I could never yet," fays Newton, " bymixing only two primary colours, produce a per- 

 *' fedt white. Whether it may be compofed of a mixture of three, taken at equal diflances 

 " in the'circumference, T do not know." Now to (hew that white may be thus generated, 

 let atiannujus.of about four inches diameter be divided into three parts by lines tending 



towards 



