REVIEWS 163 



or of overcrowding pages with facts. The wealth of material in the book is 

 arranged in an orderly fashion, and each portion of it serves in its own way to 

 illustrate the principles of the subject. 



The treatment of geometrical optics, which constitutes the first part of the 

 book, is unusually good. The theory of lens combinations is based on Helmholtz's 

 tangent law ; and there follows a chapter on the defects on the image formed by 

 a single lens. There is a very judicious selection from the many methods for 

 measuring the various constants of mirrors and lenses and the optical properties 

 of various transparent materials, together with a very satisfactory account of 

 various optical instruments. 



The chapters on Interference and Diffraction, in the section of the book which 

 deals with Physical Optics, are thorough in the sense that the physical details are 

 carefully expounded without any appeal to an elaborate mathematics, which 

 makes it all the more remarkable that Dr. Houstoun should, after pointing out the 

 insufficiency of Huygens' principle even after Fresnel's amplification of it, sud- 

 denly startle his readers with Kirchoffs formula in its baldest mathematical form, 

 but without the slightest indication as to how it is developed, or how it removes 

 the defects of Fresnel's more elementary treatment. It is not very difficult, after 

 all, to explain to a student, who has grasped the full meaning of propagation with 

 a finite velocity, the meaning of " retarded " amplitudes and potentials, nor to 

 demonstrate that the effect of a secondary source depends on the velocity of 

 vibration there as well as on the displacement. 



The treatment of Polarisation leaves little to be desired. Perhaps a little 

 fuller account of the Fresnel-Arago laws on interference with polarised light, 

 together with the bearing of the third of these laws on the nature of unpolarised 

 light, might have been given with advantage ; the treatment of interference 

 phenomena in biaxal crystals is also somewhat too brief. 



It is undoubtedly in the third part, on Spectroscopy and Photometry, that the 

 book presents a marked advance on the customary treatment of these subjects. 

 The account of spectroscopic work is roughly on historical lines, a chapter being 

 devoted to earlier and a chapter to later research. There are separate chapters 

 on the ultra-violet and on the infra-red regions, while the description of some of 

 the most recent forms of interferometers and spectrographs is excellent in its 

 lucidity and terseness. 



The mathematical treatment of the subject is contained in the fourth and last 

 part. The first chapter of it makes a bold innovation considering the type of 

 student to whom the book is addressed. It essays to give some account of the 

 " pulse " theory of light to the " second-year man," and to remove from his mind 

 the all too prevalent notion that, because we can by a mathematical device treat 

 any light disturbance as simultaneous streams of quasi-homogeneous light, we 

 must therefore imagine the atom to be a collection of equally numerous harmonic 

 vibrators. It must be admitted that the object of this chapter is to be warmly 

 commended. It is a hopeful sign for the literature of any science when the 

 results of the most recent thought and research are so rapidly incorporated in 

 standard text-books. Doubtless this chapter will make a larger demand on the 

 student's mathematical equipment than any other part of the book, but the struggle 

 to grasp its underlying meaning will amply repay him. In the remaining chapters 

 of this part, Dr. Houstoun wisely abandons the old elastic, solid theory, except 

 for a brief reference to some of Fresnel's work, and applies the electromagnetic 

 theory to the explanation of the laws of reflection, refraction, and dispersion, 

 magneto-optics and radiation. A chapter on t.he relative motion of ether and 



