262 



NA TURE 



{July 1 7- i ; 



point in which it touches the envelope. He contented 

 himself with saying that the movement which exists 

 over each of the elementary waves must be infinitely 

 feeble compared with that which exists over the en- 

 velope," while he remarks that " this must not be 

 examined with too much care or nicety " — " recherche 

 avec trop de soin ni de subtilite " are his words. Nor 

 was Huyghens in a position to give the necessary demon- 

 stration, for, to quote again from Verdet, " He never sup- 

 posed that there was any general relation between the 

 movements of these successive waves, he never combined 

 their effects, and in particular the notion of the constant 

 interference of two sets of vibrations bringing to one and 

 the same point movements of opposite phase is absolutely 

 foreign to him." 



Now it is this notion of interference, which is entirely 

 due to Thomas Young, combined with the excessive 

 smallness of the wave-length of light that renders 

 Huyghens' assumption correct. 



" It is to Young." says Verdet, "that the honour belongs 

 of having first applied to optical phenomena the principle 

 of interference," and Prof. Tait recognises to the full 

 Young's claims to this distinction ; the point on which we 

 would insist is that this principle is needed for the ele- 

 mentary explanation of the rectilinear propagation, the 

 reflection, and the refraction of light, as well as of diffrac- 

 tion and the colours of thin plates ; the principle is due 

 to Young, and Huyghens' explanation rests on an assump- 

 tion which he did not prove. 



Prof. Tait regrets in his preface that his book was all in 

 type before Prof. Stokes's " Burnett Lectures" appeared. 

 We v, ill quote a few lines from them bearing on the point. 

 After referring to Huyghens' principle, he says (p. 19) : — 

 " This principle does not by itself suffice for the explana- 

 tion of rays. It proves, or at least appears to prove, too 

 much. It is as applicable to sound as, on the supposition 

 that light consists in undulations, it is to light ; and if 

 Huyghens' explanation of rays were complete, there ought 

 equally to be rays of sound, and sound ought to present 

 the same sharp shadows as light." 



A- Young received in his own day the most unjust 

 treatment at the hands of the leaders of scientific opinion, 

 it is but fair that the full importance of his work should 

 be made clear, and that he should be given all the credit 

 he so richly deserves. In Chapter XIV. Prof. Tait brings 

 out and illustrates with his usual force and vigour the 

 value of Young's principle in explaining the phenomena 

 of the colours of striated surfaces and of thin plates. 

 Young's own attempt to account for the diffraction effects 

 produced by a wire or straight edge was incorrect ; it was 

 left for Fresnel in his great memoir on diffraction to show 

 that they too followed as a direct consequence of inter- 

 ference. 



It is of course impossible to give without the aid of 

 analysis a full explanation of the phenomena of double 

 refraction, and so Prof. Tait contents himself with show- 

 ing how Huyghens' construction, combined with the fact 

 that light-vibrations lie in the wave front, enables us 

 to account for many of the observed facts. 



When considering the subject of polarisation by reflec- 

 tion (§ 268, &c.)i we miss any reference to the experiments 

 of Janvin and others, besides Brewster, on the reflection 

 of light from transparent media. Jamin has shown that 



Brewster's law requires some modification, for in general 

 there is no angle at which light is completely plane- 

 polarised by reflection from a transparent surface. For 

 substances in which the refractive index is about i'4, an 

 angle of complete polarisation exists, but only for these. 



A short chapter on Radiation and Spectrum Analysis 

 concludes the book. K. T. Glazebrook. 



OUR BOOK SHELF 



A Pocket-Book of Electrical Rules and Tables. By John 

 Munro, C.E., and Andrew Jamieson, A.M.I.C.E., 

 F.R.S.E. (London : Charles Griffin and Co., 1884.) 



COLLECTIONS of rules and tables adapted to the wants of 

 civil and mechanical engineers have existed for a con- 

 siderable time, and now that practical applications of 

 electricity are becoming so many and important, a want 

 has been felt of a useful hand-book for those engaged in 

 this comparatively new branch of engineering. The 

 " Pocket-Book " before us is intended to supply this want, 

 and in many respects it does so very well indeed. It is 

 neat in appearance, handy in shape, and contains much 

 information in the form of tables of practical data, useful 

 rules and recipes, and specifications and directions as to 

 the performance of many different kinds of work. 



But although doing good service to electrical engineers 

 by collecting together so much that is useful in the form 

 of practical results, the compilers have, in their endeavour 

 (a mistaken one we think) to render their manual a guide 

 also in points of theory, fallen into many errors which 

 render the book an unsafe one to put into the hands of 

 any one who is capable of being misled in such matters. 

 In the first place there are many — we cannot call them 

 typographical — mistakes in equations given in different 

 parts of the work. As the process by which these equa- 

 tions are obtained is not given, and the formulas are in- 

 tended for reference and to be used in computation, 

 there is nothing to warn an inexperienced user of the 

 work of possible danger. The errors might, however, be 

 excused in a first edition if it were not that the for- 

 mula; in question are simply inaccurate copies of results 

 given in other works. For example, at pp. 123 and 

 125, a single glance at " dimensions " is enough to show 

 that several of the formula; for the localisation of faults 

 in aerial telegraph lines are erroneous, and the same 

 remark applies to the formula given at p. 174 for the 

 calculation of the distance of a fault in a submarine 

 cable from the testing station. 



As to the more theoretical portions of the work, we 

 have first a chapter headed " Definitions of Units." This 

 is in great part taken verbatim from Prof. Blyth's new 

 edition of Ferguson's " Electricity." An alteration on 

 Prof. Blyth's statement is made on p. 10, and confounds 

 the well-known and perfectly definite velocity v with the 

 velocity which is the proper expression of any given 

 resistance in electro-magnetic measure. Again, at p. 13. 

 v is said to be the ratio of the electro-static to the electro- 

 magnetic unit of quantity. The " derivation " of the prac- 

 tical units — volt, ohm, ampere — given in the table at p. 13 

 is a perfect maze of vicious circles, and in the same table 

 the "joule" is given as alternatively " volt X coulomb" 

 and "ampere 3 X ohm," — the confusion, which would seem 

 inveterate, between work and activity or rate of working. 

 The velocity of light, we may remark, is given at p. 1 1 as 

 3 X io 10 cm. per second, and at p. 382 this is given as the 

 " French value" of 192,000 miles per second ! Here, as 

 elsewhere in a few cases (the values of g given at p. 42 

 for example, where, besides, g is expressed as a velocity), 

 numbers evidently culled from different sources and sup- 

 posed to express the same quantity in different units are 

 given without verification of their equivalence. In p. 43 

 the venerable pendulum formula is terribly misprinted 



