Febkuakv i 2, 1903] 



NATURE 



enthusiasm and foresight of its curator, and the cost of 

 its production, which must have been heavy, and which 

 its council have so liberally borne. Inseparable from the 

 great collections it elucidates, this book should attract 

 workers to them. It furnishes the basis from which all 

 future research on the morphology of the mammalian 

 cerebrum that shall be exact must take its start. 



LIGHT FOR STUDENTS. 



Light for Students. By Edwin Edser, A.R.C.Sc, 

 &c. Pp. viii + 579. (London : Macmillan and Co., 

 Ltd., 1902.) Price 6s. 



r*HIS book is intended to meet the wants of the 

 -*- same class of students as the author's " Heat 

 for Advanced Students," published three years ago. 

 It gives a comprehensive account of the phenomena 

 and laws of geometrical and physical optics, with a 

 number of simple, illustrative experiments and ex- 

 amination questions. Special pains have been taken 

 throughout, as in the author's " Heat," to make all 

 the explanations as simple as possible, so that the 

 private student, who has not the advantage of a 

 teacher's assistance in explaining his difficulties, 

 should find the book particularly helpful. Advanced 

 mathematical methods have been scrupulously avoided, 

 and the calculus is rigidly excluded. This necessarily 

 limits the scope of the work, but the author has found 

 it possible to give a very good general idea of the 

 more difficult parts of the subject and of comparatively 

 advanced theories, such as Sellmeier's theory of dis- 

 persion, without making any extravagant demands 

 on the mathematical knowledge of the student. 



The first ten chapters are devoted to geometrical 

 optics, the last ten to the development of the wave 

 theory of light. A brief summary is given of the 

 properties of thick lenses, as introducing an account 

 of the eye and of vision through lenses and spectacles. 

 In the chapter on optical instruments, the construction 

 of eye-pieces is dealt with at unusual length, but on the 

 other hand, the account of telescopes is somewhat 

 scanty. Little or nothing is said about the conditions 

 affecting the brightness of the image or the extent 

 of the field of view. The ray diagrams are drawn, 

 following the prevailing custom, without indicating 

 the correct position of the eye. The diagram of 

 Galileo's telescope shows a pencil of rays full and 

 centrical on the object-glass, and small and excen- 

 trical on the eye-lens. This is the common practice 

 in text-books, but it does not correctly represent the 

 conditions of vision through this instrument. 



The following experiment is given as a proof that 

 the spherical aberration of the eye is over-corrected : — 



" Expt. 35. — Close one eye, and place the other 

 at a distance of less than ten inches from a printed 

 page, so that the type cannot be clearly seen. Then 

 place a pinhole immediately in front of the pupil. The 

 printing will become clearly visible, although rendered 

 fainter owing to the loss of light." 



Simple experiments of this kind are very helpful to 

 the student, but in this particular instance the con- 

 NO. 1737, VOL. 67] 



elusion is hardly justifiable. The pinhole would also 

 make the print clearer if held near the margin of the 

 pupil or if the print were beyond the distance of dis- 

 tinct vision of a short-sighted eye. The experiment 

 would be more appropriate as an illustration of in- 

 creased depth of focus produced by stopping down a 

 lens. An adequate test of the spherical aberration of 

 the eye is not quite so simple. 



The wave theory of light is introduced by a chapter 

 on vibrations and waves in general, including an ele- 

 mentary account of the propagation of transverse 

 waves in an elastic solid. This is followed by a 

 general explanation of the rectilinear propagation of 

 light, and of the reflection and refraction of waves. 

 The chapter on the spectrum contains many illus- 

 trations from astronomy, such as the proof of the 

 nature of Saturn's rings derived from the Doppler 

 effect. But no account is given of theories of colour 

 vision or of experimental methods of investigation. 

 The chapters on interference, diffraction and polarisa- 

 tion contain photographic illustrations by Mr. W. B. 

 Croft and others of fundamental phenomena. Some 

 account is also given of recent instruments and ex- 

 periments, such as the echelon grating and Rubens 's 

 experiments on infra-red rays of great wave-length. 

 Limits of space have prevented the author from giving 

 an account of the electromagnetic theory of light. 

 The advisability of this would also have been question- 

 able on other grounds. The book, considering its 

 size, already contains an unusually large amount of 

 information, and more could not reasonably be ex- 

 pected by the class of student for whom it is 

 written. H. L. C. 



OUR BOOK SHELF. 



Mr. Balfour's Apologetics Critically Examined. Pp. 



vi + 232. (London : Watts and Co., 1902.) Price 



y. 6d. 

 THIS book, issued anonymously by the Rationalist Press 

 Association, is explicitly directed against Mr. Balfour's 

 defence of Christianity (p. 10). To those who read with 

 an animus against this "decaying creed," the author's 

 vigour and lavish use of epithets may appear conclusive 

 reasoning. To the impartial, it will scarcely appear to 

 be criticism at all. Mr. Balfour's method in the 

 " Foundations of Belief" was .to advance from the more 

 general philosophic position to the problem of " Pro- 

 visional Unification." However much his critic believed 

 that Mr. Balfour's theism was based on "emotion and 

 sentiment " (p. 222), or that it could be explained by a 

 review of his pedigree (p. 221), he had no right to 

 rely too much on this application of the historical 

 method. 



At least, one expects to find that the "frontal attack "■ 

 which the author prefers to Mr. Balfour's " sap and 

 mine" (p. 222) shall be directed against the real strong- 

 hold. Yet, so far as this book goes, the author leaves 

 untouched the questions, Has experience any elements 

 which cannot be treated as we treat knowledge of 

 " things " ? If so, do these elements constitute data from 

 which we may infer that "the whole circuit of belief" has 

 wider foundations than "science" as such requires? 

 And lastly, if the foundations are thus widened, do they 

 admit Theism or Christianity as a form of it ? It is easy 

 to call the Incarnation a manifest absurdity ; what is 



