618 



ON THE THEORY OF LIGHT AXD COLOURS. 



ment, it appears that any attempt, to produce 

 a musical eftcct from colours, must be un- 

 successful, or at least that nothing more 

 than a very simple melody could be imitated 

 by them ; for the common period, which in 

 fact Constitutes the harmony of any concord, 

 being a multiple of the periods of the single 

 undulations, would in this case be wholly 

 without the limits of sympathy of the retina, 

 and would lose its eflecl ; in the same man- 

 ner as the harmony of a third or a fourth is 

 destroyed, by depressing it to the lowest 

 notes of the audible scale. In hearing, 

 there seems to be no permanent vibration of 

 any part of the organ. [See the Account 

 of some cases of the production of colours.] 



Hypothesis iv. All mattrial Bodiesare to 

 be considered, with respect to the Phenomena of 

 ■ Light, as consisting oj Particles so remote from 

 tach other, as to allow the ethereal Medium 

 to pervade them with perfect freedom, and 

 either to retain it in a stale of greater den- 

 iity and of equal elasticity/, or to constitute, 

 together with the Medium, an Aggregate, which 

 may be considered as denser, but not more 

 elastic. 



It has been shown, that the three former 

 hypotheses, which may be called essential, 

 are literally parts of the more complicated 

 Newtonian system. This fourth hypothesis 

 differs in some degree from any that have 

 been proposed by former authors, and is, in 

 some respects, diametrically opposite to that 

 of Newton ; but, both being in themselves 

 equally admissible, the opposition is merely 

 accidental ; and it is onl}' to be inquired, 

 which is the most capable of explaining the 

 phenomena. Other suppositions might, per- 

 haps, be substituted for this, and therefore I 

 do not consider it as fundamental, yet it ap- 



pears to be the simplest and best of any that 

 have occurred to me. 



Proposition i. All Impulses are propa- 

 gated in a homogeneous elastic Medium with 

 an equable Velocity. 



Every experiment, relative to sound, coin- 

 cides with the observation already quoted 

 from Newton, that all undulations are pro- 

 p.igated through the air with equal velocity ; 

 and this is further confirmed by calculations. 

 (Lagrange. Misc. Taur. I. 91. Also, much 

 more concisely, in my Syllabus of a Course of 

 Lectures on Natural and Experimental Phi- 

 losophy, about to be published. Article 289.) 

 It is surprising that Euler, although aware of 

 the matter of fact, should still have main- 

 tained, that the more frequent undulations 

 are more rapidly propagated. (Theor. luus. 

 and Conject. phys.) It is probable, that the 

 actual velocity of the particles of the lumini- 

 ferous ether generally bears a much smaller 

 proportion to the velocity of the undulations, 

 than is usual in the case of sound ; for light 

 may be excited by the motion of a body 

 moving at the rate of only one mile, in the 

 time that light moves a hundred millions. 

 And if our sun's light reaches some of the re- 

 motest fixed stars, the utmost absolute ve- 

 locity of the particles of the ethereal medium 

 must be reduced to less than one thousandth 

 part of an inch in a second. 



Scholium 1. It has been demonstrated, 

 that in different mediums, the velocity vaiies 

 in the subduplicate ratio of the force di- 

 rectly, and of the density inversely. (Misc. 

 Taur. 1. 91. Young's Syllabus, Art. 294.) 



Scholium 2. It is obvious, from the phe- 

 nomena of elastic bodies and of sounds, that 

 tlie undulations may cross each other witli- 

 out interruption. But there is no necessity 



