April 8, 1909J 



NATURE 



153 



A short appendix is added by Mr. J. J. Lister on 

 Chlamydomyxa and Labyrinthula. He considers that 

 the former is not allied to the Mycetozoa, but rather 

 to the fresh-water forms with filose pseudopodia 

 classed under the order Gromiidea, while Labyrinthula 

 is regarded as a colonial organism the units of which 

 remain in connection by means of their pseudopodia. 

 Both organisms may be regarded as related in one 

 direction to the outlying members of the Gromiidea 

 and in the other to the Heliozoa and Proteomyxa. In 

 a final appendix there is a brief notice of the Xeno- 

 phyophoridae, which were formerly regarded as 

 sponges, but which, through the labours of F. E. 

 Schultze, are shown to be more nearly related to the 

 Foraminifera, to which subdivision they are now pro- 

 visionally referred. 



Throughout the volume the systematic characters of 

 each group and of its constituent orders and families 

 fire given, and each section concludes with a well- 

 selected bibliography. This volume worthily upholds 

 the high standard attained in its companion fascicle, 

 with which it forms a comprehensive treatise on the 

 Protozoa of outstanding excellence. 



THE TEACHING OF OPTICS. 

 Cours de Physique conforme aiix Programmes des 

 Certificats et de I'Agrt^gation de Physique. By 

 Prof. H. Bouasse. Quatri^me Partie : Optique. 

 Etude des Instruments. Pp. 420. (Paris : Ch. 

 Delagrave.) Price 13 francs. 



I3ROF. BOUASSE has pronounced opinions on 

 the subject of the teaching of optics, opinions 

 which he is vigorous in defending — a defence which 

 consists in a spirited attack on those who differ from 

 his views — and in the application of which to the 

 development of his subject he undoubtedly shows 

 ■originality and independence of thought. Into the 

 merits of his quarrel with the Sorbonne we have 

 no inclination, nor is this the place, to enter; but 

 his views on the presentment of optical theory and 

 on the relative value of the " ray " and " curvature '• 

 methods touch on a question which has been dis- 

 cussed a good deal of late in this country, and are 

 in themselves well worthy of attention. 



The advocates of the " curvature " method of teach- 

 ing optics proceed on the assumption that this method 

 is based on a close representation of the actual 

 physical phenomena, and that hence, if it can be 

 applied in a simple manner to the deduction of the 

 ordinary results of geometrical optics, it must neces- 

 sarily and from the nature of the case have an 

 advantage over any more artificial method. The 

 curvature method undoubtedly has advantages, but 

 they do not rest on the assumption thus made. The 

 ray and curvature methods are alike based on an 

 ideal representation of certain characteristics of the 

 phenomena, and the proposition that the latter mode 

 of representation has a greater physical significance 

 tlinn the former at least admits of argument. But 

 whatever opinion may be held on this point, it will 

 hardly be urged that the ray method, or an equiva- 

 lent procedure, can be dispensed with in the handling 

 NO. 2058, VOL. So] 



of many not particularly advanced optical problems, 

 and the view that it is desirable to limit the student 

 to the one method of attack seems to be founded 

 on a higher estimate than usual of his average 

 stupidity. We learn through Prof. Bouasse that there 

 exists at the Sorbonne a course of geometrical optics 

 in which " rays of light " are not so much as 

 mentioned. We can but trust that his information 

 is incorrect. 



The author himself urges strongly, whether from 

 the physical or the purely ideal aspect, the importance 

 of the ray in optical theory. He points out that the 

 wave surface is defined by Fermat's principle of 

 least distance independently of any theory as to 

 the propagation of light ; that the caustic, which is 

 a locus of concentration of energy, has the immense 

 advantage over the wave surface that it is directly 

 determinable bv experiment, and that it is this which 

 is first naturally met with ; but especially he insists 

 on the convenience and clearness of the ray theory 

 as a first approximation in the representation of the 

 phenomena. 



He is at some pains, too, to expose the inherent 

 absurdity of the practice of avoiding, in optical and 

 physical text-books, the use of such elementary 

 mathematics as is really essential to the development 

 of the subject. He points out that, so far as the 

 use of certain mathematical functions is concerned, 

 any person of average ability can readily make him- 

 self acquainted with their definitions and elementary 

 properties sufficiently, at the least, to be able to use 

 intelligently tables of their values. As to the general 

 knowledge of mathematics required, he insists — 

 with Prof. Perry — that it is the mathematicians who 

 must mend their teaching, and that it is not the 

 part of the physicist to attempt to make bricks with- 

 out straw. His final plaint on this subject, 

 " Je perds mon temps; les math6maticiens sont 

 pleins de bonne volont^ pour satisfaire nos d^sirs, 

 mais ils ne les comprennent pas," is not, perhaps, 

 altogether without application in Great Britain. 



The views of Prof. Bouasse above referred to are 

 laid out at some length in the preface, and, apait 

 from their intrinsic interest, are pertinent to the 

 present notice as indicating two main features of 

 the book — adherence to the ray theory as the founda- 

 tion of the earlier chapters on geometrical optics, and 

 the use of rather more mathematics than is perhaps 

 usual, where necessary for concise expression, and in a 

 form which, we think, will appeal to the intelligent 

 student. 



The volume is one of a series of advanced physics 

 text-books written to meet the requirements of the 

 French public examinations. It sufi'ers inevitably 

 from many of the disadvantages of the examination 

 text-book ; it has, on the other hand, more than an 

 average share of its merits. It is not a study of 

 optical instruments, nor is it a text-book on practical 

 optics ; it deals with the theory of geometrical and 

 physical optics so far as this is necessary as an aid 

 to the study of optical instruments. Electro-optics 

 is expressly reserved for another volume. On the 

 other hand, many matters usually treated under the 



