Miscellaneous Intelligence. 193 



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3. Physical Optics; by Robert W. Wood, Professor of Ex- 

 perimental Physics in the Johns Hopkins University. Pp. vi, 546, 

 with 325 figures. New York and London, 1905 (The Macmillan 

 Co.). — Prof. Wood's book is a remarkably interesting compen- 

 dium of our present knowledge of Physical Optics. It is very 

 complete and up to date on the experimental side, and while some 

 of the mathematical portion is abbreviated, many results are de- 

 rived which are found in few* if any other books on light. For ex- 

 ample, it is proven that, for regular reflection, a surface must be 

 smooth to within an eighth wave length (p. 36), also that Fer- 

 mat's Law requires that the time be a minimum or a maximum (pp. 

 55, 61), that the phase is uniform within a quarter wave length 

 over a surface of diameter equal to the wave length divided by 

 the apparent diameter of the source =-05 mm for sun light (p. 123); 

 further, Rayleigh's proof is given that the magnification equals 

 the compression of the wave front (p. 65). The reader is sur- 

 prised to find no reference to Wood's successful diffraction color 

 photographs. Considerable attention is appropriately devoted to 

 the author's work on sodium vapor in the region of the D-lines ; a 

 vapor with a refractive index as high as 1*38 and a dispersion 

 which separates a double line with components twenty times as 

 close as the sodium lines by an amount as great as the distance 

 between the red and the blue of the spectrum formed by a 60° 

 glass prism (p. 346), which shows enormous rotation in a magnetic 

 field (p. 426), which gives a series of fluorescent spectra, corre- 

 sponding lines in which will appear upon stimulation with proper 

 monochromatic light (p. 408) and which exhibits optical resonance 

 (p. 486) and lateral radiation of the stimulating light (p. 45J). 

 There is little to criticise. Misprints are few and not such as to 

 confuse the reader. Does not the graphical explanation of the 

 absence of a back wave ( p. 8) assume what is to be proven ? 

 The reviewer was unable to find a clear derivation of the con- 

 dition for resolution. As an American book upon Mathematical 



