Decembeb 23, 1910] 



SCIENCE 



921 



homogeneous vibration we have to perform the 

 summation for all wave-lengths before we can 

 arrive at a final result. It must be noted that 

 we are at present not concerned with the question 

 how the light originates; we take the disturbance 

 as it is and try to represent it analytically, and 

 just as there are many ways of resolving a system 

 of forces so there are many ways of resolving the 

 motion of light into elements with which we can 

 deal analytically. The i-esolution by homogeneous 

 waves is one, the resolution by impulses another. 

 Whenever the two methods seem to yield different 

 results a mistake has been made in their appli- 

 cation. 



The usefulness of this view-point is well 

 shown in art. 64, where the author explains 

 how a grating may impress its own periodicity 

 upon a luminous impulse just as a picket- 

 fence may turn an atmospheric impulse into a 

 musical note. 



3. The curiosity of every reader can hardly 

 fail to be stimulated by the last sentence in 

 the author's new preface, which runs as fol- 

 lows: 



My thanks are due to various friends and cor- 

 respondents who have kindly pointed out a num- 

 ber of errors, which were left standing in the 

 previous edition — but I feel a consoling tliough 

 unmerited sense of satisfaction at the one serious 

 blunder having remained unnoticed, and, I hope, 

 undetected. 



One change which the author introduces 

 into his treatment of interference is so radical 

 as to make one ask whether the remark just 

 quoted from the new preface does not refer to 

 the following sentence from p. 54 of the pre- 

 vious edition : 



There may, therefore, be interference of inten- 

 sity, though there is no interference of displace- 

 ment. 



The new and precise definition of inter- 

 ference which replaces this idea in the later 

 edition is as follows : 



If the oiserved iUumination of a surface bt/ 

 tico or more pcn<nls of light is not equal to the 

 sum of the illuminations of the separate pencils, 

 ice say that the pencils have interfered loith each 

 other and class the phenomenon as one of inter- 

 ference (p. 57). 



The advantage derived from the adoption 

 of this definition is obvious on reading art. 35. 



4. A new section on the interferometer of 

 Fabry and Perot is an addition of distinct 

 value. 



As to the mathematical treatment, it may in 

 general be described as clear and elegant. 

 There are, however, some distinct exceptions 

 to this statement, of which the following in- 

 stance wiU serve as a type. The neat ele- 

 mentary discussion of diffraction on p. 95 

 would be distinctly improved by deriving, in- 

 stead of assuming, the value of the intensity 

 factor 2/7r. 



Among the striking omissions in the two 

 chapters on diffraction is any mention of 

 Cornu's spiral. 



K the student is to be considered, the value 

 of the book might be greatly enhanced by 

 throwing into heavier type many of the more 

 important results. To illustrate, the funda- 

 mental theorem, that " the brightness of the 

 image [of a luminous surface] is equal to 

 ISoy/s" is buried in the contest of p. 152 

 without comment. Much time and energy are 

 saved the student when matters of first impor- 

 tance are emphasized in some way. 



The index is very meager and cross-refer- 

 ences contain numerous errors. It is to be 

 hoped both of these may be corrected before 

 the next printing and without waiting for a 

 third edition. 



The shifting of the article on Talbot's bands 

 from the chapter on the Nature of Light to 

 that on Diffraction is interesting in view of 

 the two explanations of these bands recently 

 given by the author and by Professor R. W. 

 Wood.' 



Even though this review is intended to deal 

 mainly with the differences between, the first 

 and second editions, there are many other fea- 

 tures in this admirable treatment which one 

 can scarce resist commending. As an example 

 may be mentioned the chapter on Eotary Ef- 

 fects, in which an excellent notation, descrip- 

 tion of phenomena and quantitative discussion 

 are condensed into a few brief, but lucid, 

 pages. 



Henry Ceew 



'Phil. Mag., 7 and IS. 



