March 31, 1898J 



NATURE 



507 



elementary theory of light and shadows, and closes with 

 some interesting details of Japanese magic mirrors, which 

 are, I believe, not generally known. We then pass on 

 in later lectures to the consideration of the visible spec- 

 trum and the eye, and here we are given an account of 

 some of the effects of persistence of impressions on 

 the retina, with its modern applications in the zoetrope 

 and animatograph. In a series of six lectures it is 

 manifestly impossible to treat of the whole subject of 

 light with any degree of fullness, so that no apology is 

 needed for the absence of any account of interference, 

 diffraction, spectrum analysis, and many other branches 

 of the subject, which are by no means the least im- 

 portant. The author has, however, a chapter devoted 

 to the polarisation of light, which he considers a subject 

 inherently simple rendered difficult by the nomenclature 

 applied to it. In his own words — 



" Scientific men often fall into the habit of using long 

 and difficult words to express very simple and easy ideas. 

 The natural consequence is that people are often led to 

 think that there is some difficulty about a really easy 

 subject, whereas the main difficulty is to understand the 

 meaning of the word selected to describe it. The word 

 ^polarisation' used in optics is one of these terms. It 

 sounds very learned and difficult, but the idea it is 

 intended to convey is really very simple." 



If this be true, the original inventor of the term must 

 turn in his grave at the thought of the way he has 

 unwittingly retarded the dissemination of scientific 

 knowledge. 



The elementary consideration of polarisation is ad- 

 mirably treated by the aid of simple mechanical analogies, 

 and is made so ridiculously simple that no one could 

 fail to grasp the fundamental ideas. The best and fullest 

 part of the lectures, however, is that which deals with 

 the ultra-violet portion of the spectrum, which is ex- 

 tremely well treated with a wealth of experimental illus- 

 tration, and will be read with great interest by all 

 students of physics. The discussion of the ultra-red 

 portion of the spectrum leads up to the consideration 

 of electromagnetic waves, of which a short account is 

 given. 



In the last chapter we reach the high-water mark of 

 popular literature in an account of Rontgen rays, in- 

 cluding, as it does, an interview of Prof. Rontgen by a 

 newspaper reporter, and photographs of the hands of 

 scientific celebrities, as well as of a cigar and spectacle 

 case, with whose contents we are all now so familiar. 



A short appendix is added to each of the lectures, 

 treating with more completeness of one or more of the 

 ideas which arose in the lecture. For example, we are 

 given an account of anomalous dispersion brought up 

 to date by the introduction of a brief account of Helm- 

 holtz's investigation of the change of refractive index 

 with wave-length. A brief summary of the elastic solid 

 and electromagnetic theories is appended, and also a 

 resume of recent work on invisible radiations. 



The author, in his first lecture, used the conception of 

 wave surfaces to explain the elementary theory of optics 

 rather than the time-honoured geometrical method. 

 Every one will agree with the author that the only true 

 method of treatment is to disregard the source, and to 

 consider only the march of the wave front ; and the 

 sooner it is incorporated in our elementary text-books on 



NO. 1483, VOL. 57] 



optics the better. In the appendix to the first lecture 

 a method of determining the ordinary optical formulae 

 from the consideration of wave surfaces is shortly ex- 

 plained, and in the author's words, "these [formulae] 

 are, in fact, established much more readily on this basis 

 than by the cumbrous methods that are consecrated by 

 their adoption in every text-book of geometrical optics." 

 This appendix is, however, rather out of place in a 

 popular treatise of this kind, for it will be passed over 

 by the ordinary reader, and it is not sufficiently complete 

 to be of much ser\'ice to the student of optics. 



In the beginning of this article attention was drawn to 

 the fact that the title of the volume hardly leads one to 

 expect the nature of its contents. The author is ap- 

 parently not quite sure whether he is justified in including 

 an account of electromagnetic waves and Rontgen rays 

 in a book on "Visible and Invisible Light" ; at any 

 rate, he considers it necessary to explain his nomen- 

 clature. On p. 272 (in a chapter entitled " Rontgen 

 Light ") we have the following : — 



"You will have noticed that I have spoken of these 

 rays as 'Rontgen Light.' But are we really justified in 

 calling it light ? It is invisible to our eyes ; but then 

 so also is ordinary ultra-violet light, and so is infra red 

 light and Hertzian light. And there are other kinds of 

 light, too, amongst them one discovered during last year 

 by M. Becquerel and myself, which are invisible. But if 

 the Rontgen light can be neither reflected nor refracted, 

 neither diffracted nor polarised, what reason have we for 

 calling it light at all.' In fact, direct proof that it con- 

 sists of transverse waves is wanting. Many conjectures 

 have been formed respecting its nature. Rontgen him- 

 self suggested that it might consist of longitudinal vibra- 

 tions. Others have suggested ether streams, ether 

 vortices, or even streams of minute corpuscles. At one 

 time the notion that it might be simply an extreme kind 

 of ultra-violet light of excessively minute wave-length was 

 favoured by physicists, who were disposed to explain the 

 absence of refraction and the high penetrative power of 

 the rays upon von Helmholtz's theory of anomalous dis- 

 persion, according to which the ultra-violet spectrum at 

 the extreme end ought to double bacjc on itself. The 

 most probable suggestion yet made, and the only one 

 that seems to account for the strange lateral emission of 

 the rays right up to the plane of the antikathode, is that 

 of Sir George Stokes." 



We cannot at all agree with the author in speaking of 

 Rontgen light, as it is a very misleading term, and pre- 

 supposes a knowledge which we do not at present possess. 

 The objection to the signification lies in the fact that the 

 underlying view of what we call light is that which gives 

 us a sense of vision. It is quite true that we call the 

 ultra-red and ultra-violet portions of the spectrum in- 

 visible light, but that is purely a matter of convenience, 

 as in the spectrum these waves are merely a continuation 

 of the visible spectrum, and are not bounded by any hard 

 and fast line ; but it is quite another matter to apply the 

 term to Rontgen rays, and when there is a perfectly 

 general term " radiation " ready to our hand, it is far 

 preferable to use it. It is far more accurate, as well as 

 more scientific, to speak of Rontgen and uranium 

 radiation rather than Rontgen and uranium light. It is 

 time that the term light was restricted to its original 

 signification, and it should not be carelessly extended to 

 forms of radiation the nature and properties of which 

 are probably very different to ordinary light vibrations. 



E. R, 



