24 PHILOSOPHICAL TRANSACTIONS. [anNO 1672. 



colours. So that the business is not only to show, how rays, which before the 

 concourse exhibit colours, do in the concourse exhibit white; but to show, 

 how in the same pllace, where the several sorts of rays apart exhibit several 

 colours, a confusion of all together make white. For instance, if red alone be 

 first transmitted to the paper at the place of concourse, and then the other 

 colours be let fall on that red, the question will be, whether they convert it 

 into white, by mixing with it only, as blue falling on yellow light is supposed 

 to compound green ; or, whether there be some further change wrought in the 

 colours by their mutual acting on one another, until like contrary peripatetic 

 qualities, they become assimilated. And he that shall explicate this last case 

 mechanically, must conquer a double impossibility. He must first show, that 

 many unlike motions in a fluid can by clashing so act on one another, and 

 change each other, as to become one uniform motion ; and then, that a uni- 

 form motion can of itself, without any new unequal impressions, depart into a 

 great variety of motions regularly unequal. And after this he must further tell 

 me, why all objects appear not of the same colour, that is, why their colours in 

 the air, where the rays that convey them every way are confusedly mixed, do 

 not assimilate one another and become uniform, before they arrive at the 

 spectator's eye ? 



But if there be yet any doubting, it is better to put the event on further cir- 

 cumstances of the experiment, than to acquiesce in the possibility of any hypo- 

 thetical explication. As, for instance, by trying, what will be the apparition of 

 these colours in a very quick consecution of one another. And this may be 

 easily perfcH-mfcd by the rapid gyration of a wheel with many spokes or cogs in 

 its perimeter, whose interstices and thicknesses may be equal, and of such a 

 size, that, if the wheel be interposed between the prism and the white con- 

 course of the colours, one half of the colours may be intercepted by a spoke or 

 cog, and the other half pass through an interstice. The wheel being in this 

 posture, you may first turn it slowly about, to see all the colours fall suc- 

 cessively on the same place of the paper, held at their aforesaid concourse ; and 

 if you then accelerate its gyration, until the consecution of those colours be so 

 quick, that you cannot distinguish them severally, the resulting colour will be a 

 whiteness perfectly like that, which an unrefracted beam of light exhibits, when 

 in like manner successively interrupted by the spokes or cogs of that circulating 

 wheel. And that this whiteness is produced by a successive intermixture of the 

 colours, without their being assimilated, or reduced to any uniformity, is cer- 

 tainly beyond all doubt, unless things that exist not at the same time may not- 

 withstanding act on one another. 



