THEORY OF LIGHT AND COLOURS. §5 



tlons differing lefs or more from a perfect unifon ; for inflance, 



the undula ions of green light being nearly in the ratio of 6-§, 



will affect equally the particles in unifon with yellow and blue, 



and produce the fame effect as a light compofed of thofe two 



fpecies : and each fenfitive filament of the nerve may confift 



of three portions, one for each principal colour. Allowing this it is not to be 



flatement, it appears that any attempt to produce a mufical cx P efted that 



colours C3n nsvc 



effect from colours, muff be unfuccefsful, or at leaft that no- a mu f lca i effed. 

 thing more than a very fimple melody could be imitated by 

 them ; for the period, which in fact conftitutes the harmony of 

 any concord, being a multiple of the periods of the lingle undu- 

 lations, would in this cafe be wholly without the limits of fym- 

 pathy of the retina, and would lofe its effect ; in the fame 

 manner as the harmony of a third or a fourth is deflroyed, by 

 depreffing it to the loweft notes of the audible fcale. In hear- The ear not per- 

 ing, there feems to be no permanent vibration of any part™ nen ya 

 of the organ. 



HYPOTHESIS IV. 



All material Bodies have an attraction for the ethereal Medium, Hypoth. IV. 



by means of which it is accumulated within their fubjiance, and o^u^"] 1 ?- >* 



for a f mall Diflance around them, in a State of greater Denjity, of the ether, is 



but not of greater Elafticity. S reater witl ^ n 



° "T y and near other 



It has been fiiewn, that the three former hypothefes, which bodies, 

 may be called effential, are literally parts of the more compli- 

 cated Newtonian fyftem. This fourth hypothecs differs per- 

 haps in fome degree from any that have been propofed by for- 

 mer authors, and is diametrically oppofite to that of Newton ; Newton fup- 

 but, both being in themfelves equally probable, the oppofitionP ofed the con " 

 is merely accidental ; and it is only to be inquired which is the 

 beft capable of explaining the phenomena. Other fuppofitions 

 might perhaps be fubftituted for this, and therefore I do not 

 confider it as fundamental, yet it appears to be the fimpleft and 

 beft of any that have occurred to me. 



PROPOSITION I. 



All impulfes are propagated in a homogeneous elqfiic Medium with p r0 p f. I. im- 

 an equable velocity. pulfe is propa- 



tr • i • r i gated uniformly 



H.very experiment relative to found coincides with the ob- in an homoge- 

 fervation already quoted from Newton, that all undulations are neous elaftic 

 propagated through the air with equal velocity ; and this isfur- 

 G 3 ther 



i 



