674 KEPORT — 1893. 



century later was to remove the one difficulty which Newton felt. For there was 

 one fact which Hooke's theory could not then explain, and till that explanation 

 was given the theory must be rejected ; the test was crucial, the answer was 

 decisive. 



Newton tells us repeatedly what the difficulty was. In reply to a criticism of 

 Hooke's in 1672 he writes : — * For to me the fundamental supposition itself seems 

 impossible, namely, that the waves or vibrations of any fluid can, like the rays of 

 light, be propagated in straight lines without continual and very extravagant 

 spreading and bending into the quiescent medium where they are terminated by it. 

 I mistake if there be not both experiment and demonstration to the contrary. . . . 

 For it seems impossible that any of those motions or pressions can be propagated 

 in straight lines without the like spreading every way into the shadowed mediima.' 



Nor was there anything in the controversy with Hooke, which took place 

 about 1675, to shake this belief. Hooke had read his paper describing his dis- 

 co^ery of diSraction. He had announced it two years earlier, and there is no doubt 

 in my mind that this was an original discovery, and not, as Newton seemed to 

 imply soon after, taken from Grimaldi ; but his paper does not remove the diffi- 

 culty. Accordingly we find in the ' Principia' Newton's attempted proof (lib. ii. 

 prop. 42) that ' motus omnia per fluidum propagatus divergit a recto tramite in 

 spatia immota ' — a demonstration which has convinced but few and leaves the 

 question unsolved as before. 



Again, in 1690 Huygens published his great ' Traite de la Lumiere,' wi-itten in 

 1678. Huygens had clearer views than Hooke on all he wrote ; many of his 

 demonstrations may be given now as completely satisfactory, but on the one 

 crucial matter he was fatally weak. He, rather than Hooke, is the true founder 

 of the undulatory theory, for he showed what it would do if it could but explain 

 the rectilinear propagation. The reasoning of the latter part of Huygens' first 

 chapter becomes forcible enough when viewed in the fight of the principle of 

 interference enunciated by Young, November 12, 1801, and developed, independ- 

 ently of Young, by Fresnel in his great memoir on ' Diffraction ' in 1815 ; but with- 

 out this aid it was not possible for Huygens's arguments to convince Newton, and 

 hence in the ' Opticks ' (2nd edit., 1717) he wrote the celebrated Query 28 :— ' Are 

 not all hypotheses erroneous in which light is supposed to consist in pressure or 

 motion propagated through a fluid medium ? If it consisted in motion propagated 

 either in an instant or in time it would bend into the shadow. For pressure or 

 motion cannot be propagated in a fluid in right lines beyond an obstacle which 

 stops part of the motion, but will bend and spread every way into the quiescent 

 medium which lies outside the shadow.' These were his last words on the subject. 

 They prove that he could not accept the undulatory theory ; they do not prove 

 that he believed the emission theory to give the true explanation. Yet, in spite 

 of this, 1 think that Newton had a clearer view of the undulatory theory than his 

 contemporaries, and saw more fully than they did what that theory could achieve 

 if but the one difficulty were removed. 



This was Y'^oung's belief, who writes : ' — 'A more extensive examination of 

 Newton's various -writings has shown me that he was in reality the first who sug- 

 gested such a theory as I shall endeavour to maintain ; that his own opinions 

 varied less from this theory than is now almost universally believed ; and that a 

 variety of arguments have been advanced as if to meet him which may be found 

 in a nearly similar form in his own works.' I wish to call attention to this state- 

 ment, and to bring into more prominent view the grounds on which it rests, to 

 place Newton in his true position as one of the founders of the undulatory theory. 



The emission theory in Newton's hands was a dynamical theory ; he traced 

 the motion of material particles under certain forces, and found their path to 

 coincide with that of a ray of light; and in the ' Principia,' prop. 96, Scholium, 

 he calls attention to the similarity between these particles and light. The particles 

 obey the laws of reflexion and refraction ; but to explain why some of the incident 

 light was reflected and some refracted Newton had to invent his hypothesis of fite 



' Fhilt Trans,, November 12, 1801. 



