REVIEWS 501 



The first chapter is devoted to simple experiments with pins. We then meet 

 the standard series of exercises on simple lenses, mirrors and optical instruments 

 and more advanced work on the optical bench, compound lenses, gratings and 

 polarised light follows. Two matters dealt with in unusual detail are inter- 

 ferometers and the compound microscope. There are also chapters devoted 

 respectively to photometry, colour measurement and vision. 



The chapter on photometry might well be extended somewhat in a subsequent 

 edition. A novice would probably experience some difficulty in understanding 

 the process from the brief directions given and some of the apparatus shown 

 seems somewhat primitive. One would like to see some reference to the in- 

 dustrial side of this subject, including the measurement of illumination. The 

 chapters on colour would gain in value if an account of some important physio- 

 logical phenomena (such as the Purkinje effect and the influence of adaptation 

 to strong and weak light) were included. (On p. 436 the author surely means 

 to say that the fovea centralis contains cones only and no rods, not " rods only and 

 no cones.") 



However, in a work covering such an exceptionally wide field it is naturally 

 difficult to enter deeply into details, important though some of them may be. 

 The work will doubtless prove very serviceable to teachers of optics and ad- 

 vanced students, who will find in it many suggestions for new experiments. 



J. S. D. 



Illumination, its Distribution and Measurement. By A. P. Trotter. [Pp. 

 xvii + 292.] (Macmillan & Co. Price 8s. 6d. net.) 



In his introduction to this work Mr. Trotter recalls the words of Count Rumford, 

 who, in 1789, expressed his wonder that so important an art as illumination should 

 suffer neglect. At that time methods of testing and even the necessary technical 

 terms were wanting. Bouguer (whom Mr. Trotter styles the " Father of Photo- 

 metry "), followed by other physiologists and physicists, had studied the measure- 

 ment of light ; but for many years the subject was regarded more as a perplexing 

 and fascinating field for study than as a process of any industrial value. 



Mr. Trotter's well-known paper before the Institution of Civil Engineers was 

 read just 103 years after Rumford's remarks. In the meantime the status of 

 photometry had improved materially ; the subsequent rapid progress in artificial 

 illuminants and the increased attention paid to their practical utilisation have 

 given it an added importance. In 1908 Mr. Trotter's paper began to appear in 

 a rewritten and expanded form in The Illuminating Engineer. It has since been 

 once more revised and brought up to date and now appears as a treatise on 

 modern photometry. 



The author's long identification with photometry and the study of scientific 

 lighting fits him for this task. The early part of the book is devoted mainly to 

 units, standards and terms used in connection with illumination. The chapters 

 on photometric apparatus following are perhaps as valuable as any in the book, 

 much of the advice given being clearly the result of personal experience. Useful 

 hints are given regarding the arrangement of photometric benches and scales, the 

 avoidance of stray light, etc. ; and a summary is presented of the important work 

 done during the last few years in connection with the distribution of light from 

 modern lamps and the determination of mean spherical candle-power. 



The author makes some illuminating comments on the nature of errors in 

 photometry and he discharges satisfactorily the difficult task of dealing with the 



