no 



SCIENCE 



[N. S. Vol. XXXVII. No. 942 



but one can hardly avoid the conclusion that 

 the author himself had no very sharply cut 

 ideas associated with the various terms. It is 

 to be regretted that the table of " reduced 

 luminous efSciencies " taken from Ives is not 

 accompanied by a quotation to explain the 

 significance of this important conception and 

 its relation to the older ideas of luminous efii- 

 ciency and the mechanical equivalent of light. 



On page 64 the statement is made that 

 Houstoun's and Strache's specifications of a 

 standard of illumination in terms of radiation 

 having a spectral distribution corresponding 

 to the sensibility curve of the normal eye 

 " consist in specifying the light standard by 

 the least quantity of radiated energy which 

 can produce the standard intensity." This is 

 not true, and the error is especially striking 

 because immediately preceding it is a whole 

 page of discussion of " the most efiicient pos- 

 sible radiation " as the basis of the Ives 

 standard. 



It should be noted that the luminosity 

 curves given on pages 42 and 98 are not in the 

 form at present accepted, since they were 

 plotted from Koenig's data without correction 

 for the dispersion of his prism. Incidentally 

 the ingenious hypothesis of Dr. Bell that er- 

 rors in heterochromatic photometry with the 

 equality of brightness photometer are due to 

 " the shifting of colors by contrast along the 

 luminosity curve " can not be considered so 

 well established as the author seems to think. 



In view of the importance of the relation of 

 the old English candle to the international 

 candle now generally accepted as the unit of 

 intensity in England and France as well as 

 the United States, it is unfortunate that Pro- 

 fessor Barrows has felt it necessary to give a 

 new value for the old unit (p. 61) and has 

 thus helped to perpetuate the mistaken im- 

 pression that the value of the English candle 

 has been intentionally changed. The fact is 

 that the present unit is exactly the same as the 

 old English candle to the degree of accuracy 

 with which the British authorities could de- 

 termine the proper average value of the old 

 unit. Since there have been no British stand- 

 ard sperm candles made under authority of 



law since 1898, and the German candle was 

 superseded by the Hefner 25 years ago, it is 

 to be hoped that makers of text-books will 

 some time accept the simple ratios of units 

 now in use and will cease to give confusing 

 tables of uncertain historical ratios. 



The middle portion of the book gives de- 

 scriptions of a great variety of photometrical 

 apparatus and a very complete exposition of 

 various methods of calculating light flux and 

 illumination. A number of tables summarize 

 the published empirical data of the best-known 

 illuminating engineers and are valuable as an 

 approximate indication of the results to be 

 expected from various types of illuminants' 

 and methods of installation. In an appendix 

 are given several tables of constants which are 

 useful in the calculation of illumination. 



Two chapters devoted to principles of in- 

 terior and of street illumination are largely 

 quoted from papers of Mr. A. J. Sweet. Of 

 22 pages dealing with street lighting, 17 are 

 copied verbatim. Since Mr. Sweet's papers 

 were not intended to be text-books, it is no 

 disparagement of his work to say that the 

 chapter covers the subject very inadequately. 

 That the opinions so fully quoted are not 

 universally accepted is shown in a curious 

 way. Mr. Sweet takes twelve pages to estab- 

 lish the conclusion that an angle of about 65° 

 from the vertical " must be taken as a line of 

 absolute prohibition " for high intensity in 

 street lamps. " If it is to be exceeded at all, 

 it may as well be entirely ignored," he says, 

 and on the opposite page Professor Barrows 

 gives a curve for a lamp which has 75 per 

 cent, of its maximum candle-power as high as 

 80° from the vertical, with the statement that 

 it " closely approximates the ideal condi- 

 tions " ! 



The book as a whole would have been much 

 improved by a thorough editing, for in many 

 passages the language is crude, to say the 

 least. It is marred also by an unusual num- 

 ber of typographical errors. Nevertheless, in 

 spite of the many weak points, it must be 

 granted that Professor Barrows has collected 

 a large amount of valuable material, and it is 

 to be hoped that future editions will enable 



