NA TURE 



THURSDAY, JULY 30, if 



IND US TRIAL PHO TOME TR Y. 

 A Treatise on Industrial Photometry, with special ap- 

 plication to Electric Ligfiting. By A. Palaz, Sc.D. 

 Translated from the French by G. W. Patterson and 

 M. R. Patterson. Pp. x -|- 322. (London : Sampson 

 Low and Co., Ltd.) 



IT is remarkable, considering the importance of arti- 

 ficial methods of illumination, that there is so little 

 literature dealing with the subject of light measurement 

 treated from an industrial point of view. The introduction 

 of electric lighting, and still more recently the success of 

 the incandescent gas systein, have aroused in the public 

 a demand for more powerful methods of illumination 

 than formerly were found to satisfy. But the problems 

 involved in the economical arrangement of artificial 

 light sources are very little understood. The scientific 

 aspect of the subject has, indeed, hardly advanced at all 

 during the last few years. Electrical engineers have made 

 great efforts to increase by one or two per cent, the 

 efficiency of dynamos intended for electric lighting pur- 

 poses ; but the study of arrangements of lamps and 

 reflectors, to produce the best effect, has been almost 

 ignored, with the result in many cases of losing quite 

 half the usefulness of the light. There are in fact many 

 problems involved in the economical distribution of lights, 

 the importance of which is little recognised, and the 

 solution hardly yet discovered. They are not to be found 

 in text-books, and are only alluded to in isolated papers 

 scattered about in the technical journals. The present 

 work is a valuable compilation of facts and experiments 

 obtained from the best technical authorities ; and is the 

 only book we know of in which this information can be 

 obtained. The matter has not been hastily put together, 

 since the book is a development of a long series of 

 articles published by the author in La Lumiere 

 Electrique in 1887, and now enlarged and brought up 

 to date. 



The work is divided into six chapters, the first of 

 which is introductory' and deals with the principles of the 

 subject. It contains some useful information, but is dis- 

 figured by an unfortunate choice of terms. The camile- 

 pcnuer of a source of light, or the quantity of light emitted 

 by it per unit solid angle, is here denoted by the objection- 

 able term total intensity. Now the candle-power of a 

 candle, like the horse-power of a horse, may vary from 

 time to time; but whatever the vagaries of a so-called 

 standard candle, the term candle-power is quite scientific. 

 To replace this simple, accurate, and much-used ex- 

 pression by a vague, misleading, and unknown term, 

 is most unfortunate, especially since, with all ordinary 

 light sources, the candle-power varies with the direction 

 considered, and one has to try and realise what the mean- 

 spherical-" total "-intensity of a light source may be. The 

 translators have, in fact, reproduced too literally the 

 author's word intensity, which turns up over and over 

 again in quite a needless fashion, as for instance in the 

 phrase intensity of illumination, which is used to denote 

 the quantity which English writers generally denote by the 

 single word illumination. It is a great pity to make this 

 NO. 1 396, VOL. 54] 



latter word synonymous with quantity of light, as is done 

 in this book. The ordinary idea conveyed in the assertion 

 that an area is well illuminated, is not that there is a large 

 quantity of light falling upon it, but that the quantity fall- 

 ing on it per unit area is considerable. The word 

 illumination when used scientifically should correspond 

 with this popular, and also precise, meaning. It should 

 denote the density of the flux, not the flux itself, as the 

 translators have unfortunately made it do. 



The second chapter, on photometers, is a most valuable 

 one. It is the longest in the book, and, like all the other 

 chapters, is well illustrated. Photometers are divided 

 into six classes, and over forty are described. The in- 

 formation given is by no means confined to instrumental 

 details, for matters connected with the theory and 

 practical use of the apparatus receive a good deal of 

 attention. For instance, many useful details of various 

 modes of making Foucault and Bunsen screens are 

 given, and the theory of the Bunsen photorneter is very 

 fully discussed. It is remarkable how complicated this 

 theory becomes when examined, and it does not appear 

 that the theory given is too elaborate. As a matter of fact 

 the optical properties of diffusing screens have been very 

 little studied, and what has really been done in this 

 direction seems to show that they cannot be treated in 

 so simple a manner as is here assumed. This chapter is 

 a useful summary of the different varieties of photometers, 

 and leaves little to be desired. Naturally soine of the 

 instruments alluded to are of small value, while others 

 depend on principles of a debatable character. This is 

 especially the case in connection with heterochroroatic 

 photometry, a subject on which opinions seem to differ 

 widely. The author is inclined to dissent from Helmholtz' 

 assertion that "of all the comparisons effected by the aid 

 of the eye between the intensities of different sorts of 

 composite light, there is not one which possesses an 

 objective value independent of the nature of the eye." 

 Apparently many disagree with this view, since several 

 photometers are described in which the judgment of the 

 eye is only appealed to after the contending lights have 

 been subjected to elaborate processes, which make it 

 doubtful whether the decision obtained has any reference 

 to their illuminating powers. Helmholtz' view seems to 

 be the true one after all, and in fact it is diflicult to dis- 

 pute it in view of the property of the eye known as 

 Purkinje's phenomenon, according to which the ratio of 

 the illuminating powers of two lights of different com- 

 position varies if the intensity of each light is reduced in 

 the same proportion. There is, no doubt, something to 

 be said for adopting methods of comparison independent 

 of the eye, owing to the fact that not only do two observers 

 differ from one another, but a single observer obtains dif- 

 ferent results at difterent times. This, however, is largely 

 a matter of practice, and we believe that Captain Abney, 

 and others who have worked much at colour photometry, 

 make little of the difficulty of judging when two illumin- 

 ated surfaces of difterent colours are equally luminous. 

 Certainly some of the photometers described here seem 

 of doubtful value, in spite of the skill shown in their 

 design, owing to the number of constants which have to 

 be determined experimentally for each instrument ; these 

 constants apparently depending not only on the instru- 

 ment, and the person using it, but also to some extent 



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