\0L. VII.] PHILOSOPHICAL TRANSACTIONS; 739 



the influence and continuation of the subtile matter. Indeed I admit such an 

 hypothesis in " the Dissertation on the Motion of Undulation," which is the 

 sixth part of my mechanics, as I suppose that those apparent colours are the 

 sole effect of that communication of motion which is diffused laterally by the di- 

 rect undulations. As if the rays entering by the hole «, (fig. Q, pi. 15) should 

 proceed towards h^ the undulations ought indeed to terminate directly, with re- 

 gard to their direct and natural motion, at the right line ah\ yet nevertheless, 

 because of the continuity of the matter, there is some communication of the 

 motion towards the sides cc, where it becomes tremulous and undulatory. And 

 if colours be supposed to consist in the lateral undulation, all their phaenomena 

 may be explained in this manner, as I have shown in the dissertation before 

 mentioned; by which also the reason will appear, why the breadth of the co- 

 lours must be expanded beyond the divergency of the rays themselves. 



As to what he says of the error, which might arise in the calculation, from 

 what I mentioned like a hole made in the posterior face of the prism, that that 

 error could not cause any sensible variation ; his remark is very proper ; neither 

 have I judged that hence the breadth of the colours would be much in- 

 creased, but I wished only to indicate an accurate mode of calculation : and 

 therefore I also think that this caution may be neglected in practice. 



As to the Experimentum Crucis, I make no doubt that the incident rays had 

 an equal inclination, since the author expressly affirms it. But that is what I 

 could not gather from what I read in the Transactions; where it is stated, that 

 there are two small and very distant holes, and one prism near the first hole in 

 the window; through which prism the coloured rays escaping, fall on the other 

 distant hole. And it is added, that the first prism was turned round its axis, to 

 cause all the rays to fall successively on the second hole. Now in this case the 

 inclination of the rays which fall on the second hole, must necessarily be changed: 

 and I hinted in my letter, that it would be the same thing, whether the second 

 hole were raised or depressed, for all the rays pointing to the sun's image, to 

 fall successively on it, while the first prism was invariable; or whether, the 

 second hole being immoveable, the first prism were turned round, so that the 

 same image might change its situation, and all its parts successively fall on the 

 second hole. But no doubt the sagacious Newton used other precautions. 



As to what I objected about colours, I am well satisfied with the solutions. 

 And as to my calling the author's theory an hypothesis, that was done without 

 any design, having only used that word as first occurring to me; and therefore 

 request it may not be thought as done out of any disrespect. I have always 

 esteemed ingenious discoveries, and the excellent Newton I verj' highly admire 

 and honour. 



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