440 



NATURE 



[February i, 191 2 



of the directions of light-vibration in crystal plates, 

 nnd of refractive indices by means of prisms and by 

 total reflection, and the examination and measure- 

 ment of interference-figures. Plcochroism and the 

 rotation of the plane of polarisation are also dis- 

 cussed, nnd there are excellent chapters on the crys- 

 tallographic microscope and its accessories, with an 

 account of Fedorov's universal stage and the methods 

 due to him and to Becke for the location of the optic 

 axes in a crystal section. 



The descriptions are very full, and for the most 

 part clear, but some few points have been noted which 

 seem to call for criticism. Thus, for instance, on 

 p. 585, the mode of construction of the optical ellip- 

 soids on three rectangular axes the lengths of which 

 are made (either directly or inversely) proportional 

 to the values of the refractive indices of the crystal 

 for the vibrations taking place along them, is not 

 made clear at the start; and, moreover, the expression, 

 "the refractive index along" a piven direction, which 

 frequently occurs, is ambiguous, and does not seem 

 to be anywhere defined. A slip is noticeable on 

 p. 572, where the wave-front in a crystal is stated 

 in the general case to be an ellipsoid. 



The methods and apparatus described throughout 

 the book are principally those of which the author 

 has had personal experience in his well-known re- 

 searches, and the examples are mostly taken from 

 the same source. The treatment is very detailed and 

 almost unnecessarily circumstantial, and in many 

 places there is a good deal of repetition. Thus, for 

 example, in the seven chapters dealing with crystal 

 calculation, though it is no doubt desirable that the 

 advantage of systematic methods of computation 

 should be impressed upon the student, it was perhaps 

 scarcely necessary to insert the actual working (with 

 a diagram) of every spherical triangle and anharmonic 

 ratio computed; whilst in the physical portion of the 

 book a considerable amount of space might have 

 been saved by the omission of much of the large mass 

 of practical detail, particularly that concerning some 

 of the less readily determinable properties, such as 

 elasticity (of which the account occupies eight pages) 

 and thermal expansion (twenty pages), for which the 

 reader may fairly be expected to consult the original 

 memoirs. A reduction in the size and cost of the 

 book would have been advantageous, as rendering it 

 more accessible and at the same time more useful 

 to the student, who may be apt to be somewhat 

 bewildered by the amount of information provided 

 for him. 



The numerous illustrations, which have been mostly 

 drawn by the author or specially engraved for the 

 book, are, with a single exception (Fig. 440), all 

 excellent. 



(2) The smaller book is based on a lecture delivered 

 by the author at the meeting of the British Asso- 

 ciation in Winnipeg in 1909, and is intended for the 

 general reader, to whom it aims at presenting an 

 account of some of the more important properties of 

 crystals— geometrical, physical, and chemical— 

 especially in relation to their molecular structure. 



The task of conveying to a reader a clear idea of 

 this somewhat extensive subject within the compass 

 NO. 2205, VOL. 88J 



of tome 300 pages, without assuming some previoui 

 acquaintance with crystals, is no easy one. and the 

 difficulty has been increased by the inclusion ol 

 numerous practical experimental details, which, how- 

 ever appropriate in a lecture, are of little use in a 

 book of this kind. In the same way, a very largt 

 amount of space (twenty pages) has been devoted to the 

 behaviour of plates of quartz and amethyst in polarised 

 light, some of which might probably have been better 

 utilised for an expansion of other parts of the book 

 which have suffered from undue compression, such 

 as the discussions of interference and of molecular 

 "distance-ratios." 



Not a few incorrect or misleading statements have 

 been noticed. Thus, for example, the types of sym- 

 metry unrepresented among Sohncke's sixty-five point- 

 systems are not those having no plane of s>'mmetry, 

 as indicated on p. 117, but chiefly those showing 

 hemimorphic character. In chapter v., which deals 

 with crystal-axes and the law of rational indices, the 

 meaning of the axial lengths and the manner in which 

 they are determined by means of the intercepts made 

 by a parametral face are not explained early enough, 

 or with sufficient prominence, and the figures of 

 simple pinacoid forms, with inscribed axes, which 

 illustrate the axes of the various systems, are mis- 

 leading as to this point. Again, on p. 51, the letters 

 a, b, c, are used to denote the three crystal axes, 

 while on p. 57 (without further explanation) they are 

 used for the unit lengths along these axes. Similarly, 

 on p. 57, the " three numbers [tn, n, r] expressing 

 the intercepts " appear to denote the actual lengths 

 of the intercepts, while at the bottom of the follow- 

 ing page the intercepts are explained as meaning the 

 multiples of the unit lengths, a, b, c. The meaning 

 of the example on p. 58 is far from clear. 



There are numerous admirable illustrations of grow-, 

 ing crystals, and a good coloured plate of crystals 

 in polarised light, which have been reproduced from 

 the author's own photographs. ■ 



It is to be feared that the book will scarcely he] 

 intelligible to a general reader without some previous 

 knowledge of the subject, but the student who is 

 already familiar with the elements of crystallography 

 may find much in it that is interesting and sugges-. 

 tive. H. L. B. ] 



THE GEOGRAPHICAL BASIS OF HISTOKl. 



A Historical Geography of the British Colontes. 

 Vol. v., Canada. Part iii.. Geographical. By J. D. 

 Rogers. Pp. viii + 302. Vol. v., part iv., New- 

 foundland. By J. D. Rogers. Pp. xii + 274. ^ 

 (Oxford : The Clarendon Press, 1911) Price 45. 6d. 

 each vol. 



TWO more volumes of Sir Charles Lucas's valuable 

 " Historical Geography of the British Colonies " 

 have appeared, in which Canada and Newfoundland 

 are described. The Canadian volume deals with the 

 geography, but the historical aspect is still largely 

 to the fore. No general account of the whole region 

 occurs, in which the principal characteristics of this 

 portion of the North American continent is given, but 

 we are introduced at once to the far north land and 



