TBANSACTIONS OF SECTION A. 675 



of easy reaexion and transmission. These are explained in the 'Opticks,' book iii., 

 props. 11, 12, and 13 (1704), thus:— 



' Light is propagated from luminous bodies in time, and spends about seven or 

 eight minutes of an hour in passing from the sun to tbe earth. 



' Every ray of light in its passage through any refracting surface is put into a 

 certain transient constitution or state, which in the progress of the ray returns at 

 equal intervals, and disposes the ray at each return to be easily transmitted 

 through the next refracting surface, and between the returns to be easily reflected 

 by it. 



' Definition. — The return of the disposition of any ray to be reflected I will call 

 its fit of easy reflexion, and those of the disposition to be transmitted its fits of 

 easy transmission, and the space it passes between every return and the next return 

 the interval of its fits. 



' The reason why the surfaces of all thick transparent bodies reflect part of the 

 light incident on them and refract the rest is that some rays at their incidence are 

 in their fits of easy reflexion, some in their fits of easy transmission.' 



Such was Newton's theory. It accounts for some or all of the observed facts ; 

 but what causes the fits ? Newton, in the ' Opticks,' states that he does not inquire ; 

 he suggests, for those who wish to deal in hypotheses, that the rays of light 

 striking the bodies set up waves in the reflecting or reiracting substance which 

 move faster than the rays and overtake them. When a ray is in that part of a 

 vibration which conspires with its motion it easily breaks through the refractino- 

 surface — it is in a fit of easy transmission; and, conversely, when the motion of 

 the ray and the wave are opposed, it is in a fit of easy reflexion. 



But he was not always so cautious. At an earlier date (1675) he sent to 

 Oldenburg, for -the Royal Society, an ' Hypothesis explaining the Properties of 

 Light' ; and we find from the journal book that ' these observations so well pleased 

 the society that they ordered Mr. Oldenburg to desire Mr. Newton to permit them 

 to be published.' Newton agreed, but asked that publication should be deferred 

 till he had completed the account of some other experiments which ought to precede 

 those he had described. This he never did, and the hypothesis was tirst printed in 

 Birch's 'History of the Eoyal Society,' vol. iii., pp. 247, 262, 272, &c.; it is also 

 given in Brewster's ' Life of Newton,' vol. i., App. II., and in the ' Phil. Mao- ' 

 September 1846, pp. 187-213. 



' AVere I,' he writes in this paper, ' to assume an hypothesis, it should be this, 

 if propounded more generally, so as not to assume what light is further than that 

 it is something or other capable of exciting vibrations of the ether. First, it is to 

 be assumed that there is an ethereal medium, much of the same constitution with 

 air, but far rarer, subtiller, and more strongly elastic. ... In the second place, it is to 

 be supposed that the ether is a vibrating medium, like air, only the vibrations far 

 more swift and minute ; those of air made by a man's ordinaiy voice succeedino- 

 at more than half a foot or a foot distance, but those of ether at a less distance 

 than the hundred-thousandth part of an inch. And as in air the vibrations are 

 some larger than others but yet all equally swift, ... so I suppose the ethereal 

 vibrations diflPer in bigness but not in swiftness. ... In the fourth place, therefore, 

 I suppose that light is neither ether nor its vibrating motion, but something of a 

 difl'erent kind propagated from lucid bodies. They that wiU may suppose it an 

 aggregate of various peripatetic qualities. Others may suppose it multitudes of 

 unimaginable small and swift corpuscles of various sizes springing from shining 

 bodies at great distances one after the other, but yet ^^•ithout any sensible interval 

 of time. . . . To avoid dispute and make this hypothesis general, let every man here 

 take his fancy ; only, whatever light be, I would suppose it consists of success! ve rays 

 diflfering from one another in contingent circumstances, as bigness, force, or vigour, 

 like as the sands on the shore ; . . . and, further, I would suppose it diverse from 

 the vibrations of the ether. . . . Fifthly, it is to be supposed that light and ether 

 mutually act upon one another.' It is from this action that reflexion and refrac- 

 tion come about ; ' rethereal vibrations are therefore,' he continues, ' the best 

 means by which such a subtile agent as light can shake the gross particles of solid 

 bodies to heat them. And so, supposing that light impinging on a refracting or 



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