February i, 1906] 



NA TURE 



317 



that the disturbance is zero over the opaque portion 

 of the diffracting screen, while over the transparent 

 portion it has the same value as though the screen 

 were absent. The results are applied to the problem 

 of diffraction by a straight edge leading to Fresnel's 

 integrals and the properties of Cornu's spiral. These 

 might have been obtained more simply, though the 

 rigorous method has its advantages in enabling one 

 to si.- the meaning of the various simplifications intro- 

 duced in the process. Later on in the discussion an 

 interesting account of Sommerfeld's theory is given. 



The latter part of the book is taken up with the 

 usual theory of reflection and refraction and of double 

 refraction. The surface conditions are deduced from 

 the electromagnetic equations, and the relations 

 between the incident reflected and refracted vectors 

 follow readily. Attention is directed to the fact that 

 the laws thus deduced do not hold for light, and the 

 effect of a transition layer is considered in a satis- 

 factory manner. 



In the last chapter we have the equations relating 

 to the propagation, reflection, and refraction of electro- 

 magnetic waves bv crystals. 



At present, part i. only of the whole treatise is 

 under consideration. This deals, as will have been 

 observed, with the analytical portions of the subject 

 for which Maxwell's theory g"ives a satisfactory ex- 

 planation. In part ii. the author hopes to consider 

 the really more interesting portions where the simple 

 Maxwell theory needs modifications before it will fit 

 the facts. Readers will await with interest Dr. 

 Curry's treatment of the phenomena of the rotation 

 of the plane of polarisation, absorption, metallic re- 

 flection, the Zeeman effect, and the relations generally 

 between magnetism and light. 



INDIA.X HERMIT CRABS. 

 Catalogue of the- Indian Decapod Crustacea in the 

 Collection of the Indian Museum. Part ii., 

 Anomura. Fasciculus i., Pagurides. Bv A. Alcock, 

 M.B., LL.D., F.R.S., CLE. (Calcutta: Indian 

 Museum, 1905.) Price 14 rupees. 



THE second instalment of Dr. Alcock 's fine " Cata- 

 logue of the Indian Decapod Crustacea " is now 

 before us. It deals with the hermit crabs (Paguridea 

 or Pagurides), and forms the first fascicule of the 

 second part, which is devoted to the Anomura. Dr. 

 Alcock is thus making use of the old classification 

 of the Decapoda into Brachvura, Anomura, and 

 Macrura, a course to which modern opinion seems n. 

 incline — and, as we think, rightly — in spite of the 

 many merits of Boas's arrangement of the group 

 under the suborders Reptantia and Natantia. In the 

 hands of different authors, the limits of the Anomura 

 have varied considerably, and Dr. Alcock takes the 

 term in the sense of Boas's Anomura, including under 

 it the Paguridea, Galatheidea, and Hippidea only. 

 Now there can be no question that Boas was righl 

 in excluding the sponge crabs (Dromiacea) and sand 

 crabs (Oxystomata) from the Anomura when he- 

 formed his tribe Anomala, but we believe that the 

 group thus constituted is still an imperfect one, in 



no. 1892, VOL. y$] 



that it is not true to genealogy, since it omits the 

 Thalassinidea, which are certainly more nearly akin to 

 the primitive hermit crabs than they are to the 

 lobsters, near which they are generally placed. This 

 is not denied by Dr. Alcock, but he gives as his 

 reason for taking the old course with the Thalass- 

 inidea that to include them with the Anomura " is 

 going too far, as being; likely to confuse the sys- 

 tematist " — a poor compliment to the systematist ! A 

 zoological classification must be one of two things 

 either purely empirical, or based on genealogical facts 

 so far as we can ascertain them, though no one is 

 likely to choose the former alternative at this time 

 of day — but in either case illogical concessions to 

 supposed infirmities of the human intellect do not 

 seem to us to be admissible. However, authorities 

 will never agree on questions of classification, and 

 ivt do not regard the author's decision as a serious 

 blemish on this otherwise wholly admirable work. 



In this volume, as in that on the Indian crabs, Dr. 

 Alcock starts with an introduction on the group as 

 a whole', in which he has condensed into a few pages 

 a great deal of very interesting and useful informa- 

 tion. In the tables of distribution which follow it 

 appears that the littoral forms are generally Indo- 

 Pacific in range, but that the more primitive sub- 

 littoral genera have a very distinct circumtropic.il 

 distribution. The bearing of this fact on geograph- 

 ical problems is, >*i course, an important one. The 

 bulk of the work is taken up with systematic de- 

 scriptions, which are as excellent as is all Dr. Alcock's 

 work in this line, and deal with some ninety species 

 of twenty-eight genera. At the end of the volume is 

 a " table of the genera and species of Pagurides," 

 with bibliographical references, which must have 

 been extremely laborious to compile, but will now lie 

 correspondingly helpful to systematists. The illustra- 

 tions are excellent. L. A. B. 



OCR BOOK SHI: I.C 

 Traditions of the Caddo. Collected under the auspices 

 of the Carnegie Institution of Washington by 

 George A. Dorsev, Curator of Anthropology. Field 

 Columbian Museum. Pp. 136. (Washington, 

 D.C. : Carnegie Institution, 1905.) 

 The make-up of this volume is somewhat curious. It 

 contains one hundred and one pages of texts, followed 

 by twenty-eight pages of abstracts of the same in 

 small type ; there is' no index, and the only notes are 

 almost monosyllabic, for they merely indicate b> 

 whom the story was told— a fact of little value, inas- 

 much as we learn absolutelv nothing of the narrator 

 bevond his (or her) name. 'This is the more regret- 

 table, as the Caddo, a tribe allied to the Pawnee- and 

 Arikara and associated more especially with tin- 

 Wichita, has retained none of its ancient culture, and 

 we must therefore know the history of the tribe and 

 of the individual narrators before we can judge of the 

 influences that have gone to shape their stock of folk- 

 tales. Equally regrettable is the absence of notes on 

 the stories themselves; it is true that native names 

 are translated, but there are many points on which the 

 editor could throw light with advantage; for example, 

 in tale 35 we find a dead man cannot get into Spirit 

 Land because he cannot fit his arrows to his bow- 

 string-, which has a knot in it; a living man puts in 



