TRANSACTIONS OF SECTION A. 673 



the time has come when we may with advantage review our position with regard 

 to them, and sum up our knowledge.' 



That light is propagated by an undulatory motion through a medium which 

 we call the ether is now an established fact, although we know but little of the 

 nature or constitution of the ether. The history of this undulatory theory is full 

 of interest, and has, it appears to me, in its earlier stages been not quite clearly 

 apprehended. Two theories have been proposed to account for optical phenomena. 

 Descartes was the author of the one, the emission theory. Hooke, though his 

 work was very incomplete, was the founder of the undulatory theory. In his 

 ' Micrographia,' 1664, page 56, he asserts that light is a quick and short vibratory 

 motion, ' propagated every way through an homogeneous medium by direct or 

 straight lines extended every way like rays from the centre of a sphere. . . . Every 

 pulse or vibration of the luminous body will generate a sphere which will continu- 

 ally increase and grow bigger, just after the same manner, though indefinitely 

 swifter, as the waves or rings on the surface do swell into bigger and bigger 

 circles about a point on it ; ' and he gives on this hypothesis an account of reflexion, 

 refraction, dispersion, and the colours of thin plates. In the same work, page 58, 

 he describes an experiment practically identical with Newton's famous prism experi- 

 ment, pubhshed in 1672. Hooke used for a prism a glass vessel about two feet 

 long, filled with water, and inclined so that the sun's rays might enter obliquely 

 at the upper surface and traverse the water. * The top surface is covered by an 

 opacous body, all but a hole through which the sun's beams are suffered to pas? 

 into the water, and are thereby refracted' to the bottom of the glass, ' against which 

 part if a paper be expanded on the outside there will appear all the colours of the 

 rainbow — that is, there will be generated the two principal colours, scarlet and 

 blue, with all the intermediate ones which arise from the composition and 

 diluting of these two.' But Hooke could make no use of his own observation; 

 he attempted to substantiate from it his own theory of colours, and wrote pure 

 nonsense in the attempt ; and though his writings contain the germ of the theory, 

 and in the light of our present knowledge it seems possible that he understood it 

 more thoroughly than his contemporaries believed, yet his reasoning is so utterly 

 vague and unsatisfactory that there is little ground for surprise that he convinced 

 but few of its truth. 



And then came Newton. It is claimed for him, and that with justice, that be 

 was the true founder of the emission theory. In Descartes' hands it was a vague 

 hypothesis, Newton deduced from it by rigid reasoning the laws of reflexion and 

 refraction ; he applied it with wondrous ingenuity to explain the colours of thin 

 and of thick plates and the phenomena of diifraction, though in doing this he had 

 to suppose a mechanism which he must have felt to be almost impossible; a 

 mechanism which in time, as it was applied to explain other and more complex 

 phenomena, became so elaborate that, in the words of Verdet, referring to a period 

 one hundred years later, * all that is necessary to overturn this laborious scaflfold- 

 ing is to look at it and try to understand it.' 



But though Newton may with justice be called the founder of the emission 

 theory, it is unjust to his memory to state that he accepted i+ as giving a full and 

 satisfactory account of optics as they were known to him. "When he first began 

 his optical work he realised that facts and measurements were needed, and his 

 object was to furnish the facts. He may have known of Hooke's theories. The 

 copy of the * Micrographia ' now at Trinity College was in the Library while 

 Newton was working with his prism in rooms in college, and may have been con- 

 sulted by him. An early note-book of his contains quotations from it. Still 

 there was nothing in the theories but hypotheses unsupported by facts, and these 

 would have no charm for Newton. The hypotheses in the main are right. Light 

 is due to wave motion in an all-pervading ether ; the principle of interference, 

 vaguely foreshadowed by Hooke ('Micrographia,' p. 66), was one which a 



• This address was in the printer's hands when I saw Sir G. Stokes's paper on 

 ' The Luminiferous Ether,' Nature, July 27; Had I known that so great a master of 

 my subject had dealt with it so lately, my choice might have been different ; under 

 the circumstances it was too late to change. • 



1893. X X 



