18 PHILOSOPHICAL TRANSACTIONS. [anNO 1672. 



subject to the like difficulties, than suspect the animadversor should select the 

 worst for his own. 



What I have said of this, may be easily applied to all ether mechanical hypo- 

 theses, in which light is supposed to be caused by any pression or motion 

 whatsoever, excited in the aether by the agitated parts of luminous bodies. For 

 it seems impossible that any of those motions orpressions can be propagated in 

 straight lines without the like spreading every way into the shadowed medium on 

 which they border. But yet, if any man can think it possible, he must at least 

 allow, that those motions, or endeavours to motion, caused in the aether by the 

 several parts of any lucid body, that differ in size, figure, and agitation, must 

 necessarily be unequal: which is enough to denominate light an aggregate of 

 difform rays, according to any of these hypotheses. And if those original 

 inequalities may suffice to differ the rays in colour and refrangibility, I see 

 no reason why they that adhere to any of those hypotheses, should seek for 

 other causes of these effects, unless (to use the objector's argument) they will 

 multiply entities without necessity. 



The third thing to be considered, is the conditions of the animadversor's 

 concessions, which is, that I -would explicate my theories by his hypothesis: 

 and if I could comply with him in that point, there would be little or no differ- 

 ence between us. For he grants, that witiiout any respect to a different inci- 

 dence of rays there are different refractions ; but he would have it explicated, 

 not by the different refrangibility of several rays, but by the splitting and rarefy- 

 ing of aethereal pulses. He grants my third, fourth, and sixth propositions ; 

 the sense of which is, that uncompounded colours are unchangeable, and that 

 compounded ones are changeable only by resolving them into the colours of 

 which they are compounded; and that all the changes, which can be wrought in 

 colours, are effected only by variously mixing or parting them : but he grants them 

 on condition that I will explicate colours by the two sides of a split pulse, and 

 so make but two species of them, accounting all other colours in the world to 

 be but various degrees and dilutings of those two. And he further grants, that 

 whiteness is produced by the convention of colours ; but then I must allow it to 

 be not only by mixture of those colours, but by a farther uniting of the parts of 

 the ray supposed to be formerly split. 



If I would proceed to examine these his explications, I think it would be no 

 difficult matter to show that they are not only insufficient, but in some respects 

 to me at least, unintelligible. For though it be easy to conceive how motion 

 may be dilated and spread, or how parallel motions may become diverging; yet 

 I understand not by what artifice any linear motion can, by a refracting super- 

 ficies be infinitely dilated and rarefied, so as to become superficial ; or if that be 



