NA TURE 



261 



THURSDAY, JULY 17, 1SS4 



PROFESSOR TAIT'S "LIGHT" 

 Light. By P. G. Tait, M.A., Sec.R.S.E., formerly Fellow 

 of St. Peter's College, Cambridge, Professor of Natural 

 Philosopy in the University of Edinburgh. (Edin- 

 burgh : Adam and Charles Black, 1SS4.) 

 1"*HE issue of Prof. Tait's book on " Heat," recently 

 reviewed in these pages, has been quickly followed 

 by that of his book on " Light.'' 



The book we are told is " not designed for those who 

 intend to make a special study either of theoretical or of 

 experimental optics, but for ordinary students who wish to 

 acquire familiarity with the elements of the subject ;" and 

 its author has, as was to be expected, made it most 

 interesting and suggestive. 



The first three chapters are introductory and explanatory ; 

 we wish more space could have been found for the subject 

 dealt within Chapter II. — the theory of vision and colour 

 perceptions ; students' notions in general are so hazy on 

 these points, and they are not well treated of in most of 

 the books we are acquainted with. They fall between 

 two stools ; the physiologist considers them to belong to 

 physics, the physicist to physiology. 



In Chapter IV. the usual division of the subject into 

 geometrical and physical optics is made. " For the ex- 

 planation of the ordinary phenomena of light," says Prof. 

 Tait, " even with accuracy sufficient for the construction 

 of the very finest telescopes and microscopes, it suffices 

 that geometrical optics based on laws nearly verified by 

 experiment be followed out to its consequences. The 

 residual phenomena then come in to be treated by the 

 undulatory theory." Geometrical optics then is in the 

 main the subject of the next 140 pages of the book, the 

 remaining 106 pages being chiefly devoted to physical 

 optics. 



The geometrical part is excellent, and it, along with the 

 whole book, has been rendered much more interesting by 

 the frequent quotation, when describing some of the most 

 important steps in the subject, of the actual words of the 

 authors, or at all events of a close paraphrase of them. 

 Thus Newton's celebrated discovery of the dispersion of 

 white light is given as a quotation from his letter to 

 Oldenburg. We in the present day gain much by reading 

 the very words of the heroes of science, and learning with 

 what feeble instrumental means they made their great 

 discoveries ; there is a tendency, which it is well to check, 

 to think that nothing can be done in the way of scientific 

 observation and discovery without the most elaborate 

 instruments. Fresnel's apparatus in his country-house at 

 Matthieu was the work of the village blacksmith, and 

 Newton needed only his prisms, a measuring tape, and 

 some screens to prove that " light is not similar or homo- 

 geneal, but consists of difform rays, some of which are 

 more refrangible than others." 



The whole chapter on refraction — Chapter IX. — and 

 specially that part of it which deals with refraction through 

 a prism (§ 129) and the relation between the deviation of 

 a ray and the angle of incidence (§§ 125, 132, &c.) is par- 

 ticularly clear and good. The proof of the law that the 

 Vol. xxx. — No. 768 



deviation of a ray passing through a prism in a principal 

 plane is a minimum when the angles of incidence and 

 emergence are equal (§ 134) is much more elegant than 

 any we have seen before. 



But for all this we confess to a strong feeling of regret 

 when we read the statement of the author, quoted already, 

 as to the sufficiency of geometrical optics, a feeling which 

 was greatly intensified when we found how little use was 

 to be made of the all-important law of least time stated 

 for reflection in § 85, or in the more general form in § 82, 

 viz. " If / be the length of the part of a ray which lies 

 within a medium whose refractive index is p, the sum 2 p I 

 is the same for each ray of the group between any two 

 wave surfaces ;" that is, according to the undulatory theory 

 at least, the time is the same along all rays from one wave 

 surface to another of the same system. 



For the fundamental formula; for mirrors and lenses 

 can be deduced by a simple geometrical method from this 

 law ; this Prof. Tait points out, while the method has the 

 great advantage that it can be extended readily to include 

 problems in which spherical aberration is considered. 

 Lord Rayleigh's investigations in optics, published re- 

 cently in the Philosophical Magazine, are a distinguished 

 example of its powers. The full treatment of spherical 

 aberration is of course outside the limits of Prof. Tait's 

 work, but still it would have added to its completeness had 

 the book contained some elementary examples of the 

 use of the method in question. Besides the law is the 

 real link between geometrical and physical optics. That 

 one ray of light can exist only when accompanied by other 

 contiguous rays is a fact on which the teacher can scarcely 

 too often insist, and the solution of problems in geometri- 

 cal optics by the method of least time forms the best 

 introduction to the important principle of interference 

 required in physical optics. Prof. Tait's method is 

 admirable for a book on " Geometrical Optics " ; in a 

 book on " Light," however, we look for something more 

 than he gives us on the connection between geometrical 

 and physical optics ; and the development of some of the 

 elementary consequences of this law of minimum time 

 seems to us to afford the most suitable opening for con- 

 sidering that connection. 



Chapter XII. on absorption and fluorescence, is made 

 specially interesting by the quotation of an account of 

 an experiment of Fox Talbot on anomalous dispersion, 

 made about 1S40, but not published till 1870. The 

 method he adopted to obtain prisms of a substance show- 

 ing anomalous dispersion is very beautiful and ingenious. 

 The account is too long for quotation ; we must refer the 

 reader to Prof. Tait's book, p. 1 56. 



The fundamental difficulty of the undulatory theory — 

 the rectilinear propagation of light — is assailed in Chapter 

 XIII., and in dealing with this subject we think that Prof. 

 Tait gives too much credit to that great physicist, 

 Huyghens. He was of course the first to give the undu- 

 latory theory definite form, and if we allow him to make 

 one great assumption, he explained perfectly correctly the 

 rectilinear propagation, the reflection, and the refraction 

 of light. But in Huyghens' work there is an assumption 

 the importance of which it is impossible to overlook. 



" What Huyghens did not see sufficiently clearly," says 

 Verdet (" Optique Physique," tome i. pp. 33, 34), "was 

 why each of the elementary waves is only effective at the 



