98 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(455.) 

 The prin- 

 ciple of 

 Huygens. 



(456.) 

 Newton's 

 opinions on 

 the nature 

 of light. 



principle that the impression of a wave of light 

 on a screen is to be found by considering the simul- 

 taneous effect of the wavelets propagated from all 

 the points disturbed at any previous moment (which 

 points form what is called the front of a wave). 

 It is only when these partial impressions concur 

 that they are strong enough to aft'ect the sense of 

 vision ; (2), the representation of the phenomenon of 

 double refraction by a wave of two sheets, or the 

 simultaneous propagation of spherical and spheroidal 

 waves in one and the same medium. Of the last of 

 these doctrines I shall speak in the next section in 

 connection with the discoveries of Malus. With re- 

 spect to the former I may here observe that it gives 

 the only satisfactory explanation of the primary diffi- 

 culty of the undulatory hypothesis, namely, that a 

 beam of light admitted by a hole in a screen pursues 

 a rectilinear course afterwards, instead of spreading 

 sideways, as do waves in water, and waves of sound. 

 Huygens shows, on elementary and convincing prin- 

 ciples, that the lateral impressions of the wave are 

 rapidly extinguished by the want of concurrence of 

 the impulses which they communicate to the ether. 

 This is necessarily true when the breadth of the aper- 

 ture is such as to exceed vastly the length of a wave ; 

 and such is always the case with light, but rarely in 

 any other instance. It is, in short, only imme- 

 diately in front of the aperture that the disturbances 

 originating in every part of the front of the original 

 wave embraced within the aperture, concur in pro- 

 ducing an accordant movement on the ether. 



This principle, more fully stated, by which every 

 luminiferous disturbance of the ether is considered 

 as the resultant of all the pre-existing disturbances 

 to which it is due, constitutes what is sometimes 

 called the principle of Huygens, of which I shall 

 have more to say hereafter. 



Neither at the time of its publication, nor for 

 more than a century afterwards, was the value of 

 these reasonings understood. It would be beside 

 our present object to discuss Newton's opinions ; but 

 it is too certain that he did not allow Huygens' argu- 

 ments on the undulatory nature of light to have any 

 weight with him. Not that he was averse (as is 

 often supposed) to the presence of Ether as modify- 

 ing the corpuscular theory of light ; on the con- 

 trary, in many of his minor writings he speaks of 

 its existence as all but certain, and as a requisite 

 adjunct to the corpuscular hypothesis to which he 

 had been led by the facts of reflection and refrac- 

 tion. 1 But he never adjusted the terms of a compro- 



mise, and must be held, we think, to have left behind 

 him no substantive theory of light worthy of the 

 name. The question never perhaps very seriously 

 engaged his attention after the publication of Huy- 

 gens' book ; 2 and we know that about that time his 

 intellectual energies received a shock which left him 

 indisposed for the fatigue of constructing new theories, 

 and still more disinclined to publish them. 



Euler, though he professed to defend an undula- (457.) 

 tory theory of light, treated the subject in a point of Euler. 

 view chiefly mathematical, and betrayed that uncon- 

 cern about physical theories which characterized a 

 mind steeped in geometric abstractions. He even 

 retrograded ; for he did not maintain the Huygenian 

 explanation of the cause of definite shadows. 



Young on the Undulatory Theory Diffraction. (458.) 

 One hundred and ten years after the publication of ^ oung j " 



.i m . ' j i T- . p TT TV mi. the un dul 



the 1 raite de La Lumiere ot Huygens, Dr Thomas tor y theo , 



Young re-opened the theory of light or "Physical Op Diffrac 

 tics," as he termed it. His experiments and reasonings tion - 

 will be found in a series of papers in the Philoso- 

 phical Transactions for 1800, 1801, 1802, and 1803. 3 

 These memoirs are of no great length, and deserve 

 the most careful study. They are perhaps among the 

 clearest and plainest of Young's writings, although 

 blamed at the time for defects precisely the reverse. 

 They are eminently marked by penetration, profound 

 induction, and candour of argument. Starting from 

 his studies in acoustics, the transition to optical 

 questions is extremely gradual. Young was, cha- 

 racteristically, a good musician in practice, as well as 

 a profound one in theory, and his paper of 1799 is 

 principally acoustical. In it he attaches conse- 

 quence to showing that the divergence of sound from 

 the direction of its emission is slower and less com- 

 plete than it is commonly believed to be, and he applies 

 the analogy to the existence of rays of light and definite 

 shadows. In one short section he sums up the chief 

 points of optical doctrine which lead him to prefer 

 the theory of Huygens to that of Newton. Amongst 

 the facts better explained by waves than corpus- 

 cles, we find reckoned Inflection and the Colours of 

 thin plates. But all this is stated in a very general 

 way, evidently rather as a conclusion towards which 

 his mind had for some time been tending, than as the 

 result of demonstrative proofs . In his paper of 1 8 1 

 the undulatory doctrine is methodically expounded in 

 a series of propositions, accompanied by proofs. The 

 accurate definition of shadows is shown to be possible 

 and natural on that theory, as well as the usual phe- 

 nomena of reflection, refraction, and total reflection. 



1 Thus Newton writes in 1675 : " Were I to assume an hypothesis, it should be this, if propounded more generally, so as not 

 to determine what light is farther than that it is something or other capable of exciting vibrations in the ether ; for thus it will 

 become so general and comprehensive of other hypotheses as to leave little room for new ones to be invented." And again, 

 " Do not the most refrangible rays excite the shortest vibrations [of the retina], the least refrangible the largest ? " Birch's Hist, 

 of the Royal Society, quoted in Young's Lectures, ii. 615, 617. Sir D. Brewster (Life of Newton, 1855, vol. i. p. 148) considers 

 that some passages in the later editions of Newton's Optics show that he had departed from any theory of undulations. 



2 The Optics, though published in 1704, had been written principally in 1675 and 1687 (see Preface). 



3 They may be more conveniently consulted as reprinted in the second volume of his Lectures on Natural Philosophy, and in 

 his Miscellaneous Works, vol. i. 



