404 



NATURE 



[July 19, 1917 



British Insects and How to Know Them. By 

 Harold Bastin. Pp. ix+129. (London: 

 Methuen and Co., Ltd., 1917.) Price 15.' 6d. 

 net. 



The inquiry often made by beginners for a small 

 book giving trustworthy, if elementary, informa- 

 tion about the common insects of our countryside 

 may be safely answered by a recommendation of 

 this handy little volume. After a short introduc- 

 tory chapter on the general characters of the 

 Insecta and some of the varieties in life-history 

 to be observed among them, the author takes a 

 survey of the orders in ascending series, describing 

 the leading structural features, the transforma- 

 tions, and the habits of the principal families as 

 illustrated by their commoner and more con- 

 spicuous genera and species. The book contains 

 a relatively large amount of Information on sys- 

 tematic entomology, but Mr. Bastin has so much 

 of interest to tell about the mode of life of many 

 of the creatures which he mentions that the effect 

 is far from that of the dry, catalogue-like summary 

 which might easily have been the result of an 

 attempt to survey the whole class of insects in little 

 more than a hundred pages. The book is illus- 

 trated with twelve photographic plates, on each 

 of which five or six figures are printed with admir- 

 able definition and softness. The frenulum and 

 retinaculum of a hawk-moth's wings on plate ix. 

 mav be mentioned as treated with special success. 



G. H. C. 



Fresh-water Wonders and How to Identify 

 Them. By J. H. Crabtree. Pp. 64. 

 (London : C. H. Kelly.) Price is. 3d. net. 

 The author of this little volume is an enthusiast 

 on pond-life, and he seeks to introduce others to 

 what has been to himself a world of wonder and 

 beauty. He deals with diatoms, desmids, 

 confervae, Volvox, water-weeds of many kinds, 

 amoebae, infusorians. Hydra, rotifers, Bryozoa, 

 Annelids and some other worms, bivalves, water- 

 snails, water-fieas, crayfish, insect-larvae, and 

 amphibians. There are thirty photographic 

 illustrations, many of which will be useful to 

 beginners in identification. 



It is a simple, unambitious book, but the 

 author's standard of accuracy should have been 

 higher. The amoeba does not " flit about " ; the 

 young "volvoces" do not occupy "the parent 

 cell " ; the bell-animalcule does not feed on smaller 

 "hydrozoa"; nematodes are not Annelids, nor 

 " segmented like the river-worm " ; a Cercaria is 

 neither an Annelid nor a Planarian, as is alleged ; 

 the fresh-water mussel does not feed ravenously 

 on water-spiders ; the antennae of Daphnia are 

 not fringed with cilia, nor are the swimmerets of 

 the crayfish. Whatever one may say at the fish- 

 monger's, it seems a pity in a book to call the 

 crayfish a fish, especially after calling it a crusta- 

 cean. And why should one compare a tadpole 

 with a "fish without wings "? We are amazed at 

 the easy-going way in which the author has 

 tolerated numerous inaccuracies. It is not the 

 way of science. 



NO. 2490, VOL. 99] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications. \ 



Radiation-Pressure, Astrophysical Retardation, and 

 Relativity. 



The conclusion was reached by the late Prof. Foynt- 

 ing (Phil. Trans., 1903) that the radiation from a 

 material body in space gives rise to a small retarding 

 force, which acts cumulatively as a brake on its move- 

 ment through the aether ; and the consequence was 

 deduced, the significance of which has not yet been 

 exhausted, that the sun's radiation, acting in concert 

 with its gravitation, operates to keep the solar system 

 swept clear of fine cosmical dust. The system may 

 travel through nebulous clouds, but' no such clouds 

 can permanently- belong to it. 



A view seems to be prevalent that this con- 

 clusion contradicts electromagnetic theory, because for 

 an isolated radiator like a star this force of retardation 

 is specified as proportional to its velocity through the 

 asther, and this is said to violate the principle of 

 relativity (see, for example, the Observatory, July, 1917^ 

 p. 275, on "Radiation-Pressure and the Solar Rota- 

 tion "). The evolution of mathematical theories is now 

 carrying the modes of formulation of that principle far 

 away from the simple considerations on which it 

 originally reposed ; but it can fairly be said that none 

 of the original enunciations seek to apply the prin- 

 ciple that all motions are relative to systems that are 

 not self-contained. If a body is losing its energy by 

 radiation, it must surely stand in relation to the bodies 

 or to the medium to which it transfers that energy, 

 even though it be a star remote from all other bodies. 

 Any kind of relativity that supersedes this considera- 

 tion would seem to stand in self-contradiction. 



As a matter of fact, however, Prof. Poynting's prin- 

 ciple has nothing to do with the refined second-order 

 negative results which were the source of the very 

 interesting modern development regarding relativity. 

 His effect is proportional to the first power of the 

 velocity of the system ; it is thus a direct consequence 

 of the original Maxwellian theory, now universally 

 accepted ; to traverse it would appear to knock over 

 the whole fabric of modern mathematical physics. 

 How to reconcile it with special views on relativity is 

 another matter. 



The argument on this point may be found set forth 

 in Proc. International Mathematical Congress, Cam- 

 bridge, 1912 (vol. i., p. 213, "On the Dynamics of 

 Radiation "), or in the forthcoming collected edition of 

 Prof. Poynting's papers. It appears from it that the 

 effect of the solar radiation incident on a particle of 

 dust, in orbital motion round the sun, is simply to 

 reduce the factor of its gravitation, while the effect 

 of its own radiation again of the radiant energyj 

 which has been absorbed by it from the sun is toj 

 retard in a frictional manner its motion through the I 

 aether. There can be no question in general of thisj 

 retardation being exactly annulled or compensated by| 

 diminution of the inertia of the particle due to loss! 

 of its energy ; in the present case the particle, in fact, | 

 absorbs just as much energy as it radiates. Thei 

 principle and its cosmical results seem to stand firm on'' 

 established laws, and a priori views as to relativity 

 must adapt themselves to it. Any attempt in that 

 direction will have to take account of the inertia of 

 free travelling radiation. Joseph La'rmor. 



Cambridge, July 14. 



