N.4 TURR 



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THURSDAY, APRIL to, 1SS4 



STOKES ON LIGHT 

 Burnt// I.ec/vrcs. On Light. First Course-, On the 

 Na/iire of Light. By G. G. Stokes. (London : Mac- 

 niillan, 1884.) 

 > YRON once wrote about a work of art : — 



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" What nature could, but would not, do. 

 And beauty and Canova can." 



A dozen, or more, years ago, the scientific world was 

 e.xcited by the announcement that a treatise on Light was 

 to be published by Prof. Stokes, as a companion volume 

 to Clerk-Maxwell's remarkable Theory of Heat. The 

 announcement was, however, ultimately withdrawn. 

 Nature could, but v.ould not, do it. 



But Beauty and Canova, in the form of a Conservative 

 Government and its En.lowcd Institutions {Scot/and) Act 

 of six years ago, did it : — and we now have, as a result^ 

 the first of a series of three volumes by Prof. Stokes ! 



The Burnett foundation, now about a century old, was 

 essentially of a Teleological character. It had been 

 applied, in accordrince with the Founder's will, at in- 

 tervals of 40 years, to the award of prizes for Essays : — 

 the competition being perfectly open. Those who know 

 what Teleology was, half-a-century ago or more : — even 

 at its very best, as in the once-celebrated Bridgewatcr 

 Treatises: — will probably be of opinion that the Com- 

 missioners, under the Act referred to, did real good by 

 their modification of the terms of the Burnett Endowment. 



True : — it is still possible that the Trustees may some 

 day appoint to the Burnett Lectureship a rabid Teleo- 

 logist of the old school. Even this might be defended 

 on the ground of even-handed justice ; though an ill- 

 informed, but zealous, champion of such a cause is 

 probably more dangerous to it than is a declared enemy. 

 But the appointment of Prof Stokes, as the first holder 

 of the new office, augurs well for the future. Beside his 

 scientific qualifications, he is possessed of that calm, 

 judicial, mind which is absolutely required in so delicate 

 a position : — where, indeed, to say too little might involve 

 the charge of luke-warmness, if not of positive unbelief; 

 while to say too much would display that presumption 

 which is usually characteristic of ignorance. 



As the title of the work implies, the subject is the 

 nature and extent of the present evidence in favour of the 

 Undulatory Theory of Light. The reader is supposed to 

 have an elementary acquaintance with the simpler facts 

 of geometrical optics, nothing further. Hence the work 

 is, in the true sense, a popular one ; suited to any reader 

 of average intelligence and information. The language is 

 simple and, as far as possible, devoid of technicalities : — 

 while it "goes without saying" that the author does not 

 condescendingly patronise the assumed weakness of his 

 reader, nor does he anywhere attempt to escape from 

 a difficulty by the use of mysterious or indefinite 

 expressions. 



As the book will, undoubtedly, find its way into the 



hands of every one who desires to see an important and 



difficult subject brought by a Master to the level of the 



understanding of any ordinary reader, we will content 



Vol. XXIX. — No. 754 



ourselves with a few selections. These have been made 

 on a very catholic principle, some to exhibit novelties of 

 matter or treatment, some for their historical bearing, and 

 one to show ho.v the special difficulty of the Lecturer's 

 position has, so far, been met. 



First, we have the two rival theories of Light compared, 

 with the chief arguments for and against each : — 



" Prima facie there is much to be said in favour of the 

 theory of emission. It lends itself at once to the explana- 

 tion of the rectilinear propagation of light, and the exist- 

 ence of rays and shadows. It falls in at once with the 

 law of aberration. The laws of reflection and refraction 

 admit of an easy explanation in accordance with it ; at 

 least if we accept the existence of both rellection and 

 refraction ; for according to this theory we should rather 

 have expected beforehand that light would have been 

 either reflected or refracted, according to circumstances, 

 not that incident light should have been divided into a 

 portion reflected and a portion refracted. 



" The theory of undulations on the other hand presents 

 at the outset considerable difficulties. In the first place 

 .it requires us to suppose that the interplanetary and inter- 

 stellar spaces are not, strictly speaking, a vacuum but a 

 plenum ; that though destitute of ponderable matter they 

 are filled with a substance of some kind, constituting 

 what we call a iiiediuiu, or vehicle of transmission of the 

 supposed undulations. When I speak of this medium as 

 a substance, or as material, I mean that it must possess 

 that distinctive property of matter, inertia; that is to say, 

 a finite time must be required to generate in a finite 

 portion of it a finite velocity." 



Then the special difficulty which msde Newton aban- 

 don the wave-theory : — 



" The necessity of assuming the existence of some kind 

 of substance in what we commonly speak of as a vacuum, 

 does not appear to have been a serious preliminary difti- 

 culty in the way of the reception of the theory of undula- 

 tions. A far more formidable difficulty appeared at first 

 to be presented by the existence of rays and shadows. It 

 was this that led Newton to adopt the theory of emission, 

 though even he was led in the course of his researches on 

 light to suppose that there was some sort of medium 

 through which the particles of light moved, and in which 

 they were capable in certain cases of exciting a sort of 

 undulation. But the supposition of particles darted forth 

 seemed to him necessary to account for shadows." 



How, mainly by the marvellous insight of Young, this 

 difficulty has been, not merely got over but, converted 

 into one of the strongest arguments in favour of the Un- 

 dulatory Theory : — 



" There is no difference of explanation as regards light 

 and as regards sound, save what depends on the differ- 

 ence of scale entailed by the dififei'ence of wave-length. 

 Take as regards light the case of a small circular hole, 

 say the tenth of an inch in diameter, and of distances 

 from the luminous point to the screen in which the hole 

 is pierced, and from that again to the screen on which 

 the light is received, of say 8 feet 4 inches, or 100 inches, 

 each. In this case, regarding the luminous patch on the 

 screen as a whole, there would be no great diffusion of 

 light, but the phenomena of diffraction would neverthe- 

 less be fairly pronounced. There ought to be a corre- 

 sponding case of diffraction for sound ; but on what 

 scale 1 Take 50 inches as the length of a wave of sound, 

 which would correspond to a musical note of moderate 

 pitch. Taking as before the 1/50000 part of an inch as 

 the wave-length for light, the length or the wave of sound 

 will be two-and-a-half million times as great as the wave- 

 length of light. Consequently to obtain the correspond- 

 ing cise of diffraction for so.ind, our 'small' circular hole 



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