July 24, 1890] 



NATURE 



295 



for the output of the whole or part of the watch on the 

 interchangeable system, the ktter of those who still kept 

 to the old mode of making them " under a system of sub- 

 letting to small makers who work at their own homes." 



Messrs. Rotheram and Sons, of Coventry, about the 

 oldest and the largest firm of watch manufactures in 

 England, headed the list, and seem to have had a fine 

 display, theirs being one of the most striking exhibits in 

 the FIxhibition. 



In the remaining pages the author gives an account of 

 the merits and exhibits in the manufacture of clocks, 

 turret clocks, tools, watch-cases, &c., concluding with a 

 short summary. 



On the whole, the British section seems to have fared 

 very well, and to have held its own against foreign 

 competition, and to those interested in the subject this 

 work will afford a good insight into the present condition 

 of watch and clock making. 



The Harpur Euclid. Books V., VI., and XI. By E. M. 



Langley, M.A., and W. Seys-Phillips, M.A. (London : 



Rivingtons, 1890.) 

 This is an edition of Euclid's Elements revised in ac- 

 cordance with the reports of the Cambridge Board of 

 Mathematical Studies and the Oxford Board of the 

 Faculty of Natural Science. 



The books dealt with are V., VI., and XI. 



In most of the works on this subject Book V. is 

 generally omitted, and only the definitions are learnt ; but 

 the authors have thought it advisable for the reader to 

 acquaint himself with the terms used and with some of 

 the theorems which are established in it. Although he 

 is allowed to use these theorems as axioms, proofs are 

 given depending on the definitions, the notation used 

 being that recommended by De Morgan and adopted by 

 the Association for the Improvement of Geometrical 

 Teaching in its Syllabus and Elements. 



Preceding Book XL is a good and well worked out 

 series of propositions on loci, harmonic division, similarity, 

 maxima and minima ; and a few miscellaneous problems, 

 such as the nine-point circle, &c. 



The proofs in Book XL differ slightly from those 

 ordinarily given in text-books, but are made shorter and 

 perhaps clearer by the adoption of symbols. 



The method throughout of dotting all construction 

 lines is a great help to the reader, and is to be heartily 

 recommended, the figures in Book XL showing this off 

 to advantage. 



A large number of exercises are given here and there 

 for the student to practise his ingenuity on. 



The International Annual of Anthony s Photographic 

 Bulletin, 1890-91. Edited by W. Jerome-Harrison 

 and A. H. Elliott. (London : lliffe and Son, 1890.) 

 This is the third volume that has been published of this 

 most interesting Bulletin, and, glancing through its pages, 

 we conclude that it is one of the best publications of its 

 kind that we have come across. The articles, written in 

 great part by men of acknowledged ability, contain a 

 large amount of useful knowledge, forming a store of 

 information from which workers in every rank of the 

 art may obtain something that will interest them. 



One of the chief features of the volume is the great 

 increase in number of illustrations, which are printed by 

 the various kinds of processes now available, and which 

 show the advancement made in the application of 

 photography for purposes of illustration. 



The usual collection of tables is presented at the end. 

 Among them may be mentioned Dr. Woodman's table of 

 view angles, tables for the simplification of emulsion cal- 

 culations, and tables of comparative exposures. The work 

 concludes with a revised list of the Photographic Societies 

 of the British Isles, British Colonies, America, and most 

 of those on the Continent. 



NO. 1082, VOL. 42] 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the ivriters of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



The Discharge of Electricity through Gases. 



In the Bakerian lecture on " The Discharge of Electricity 

 through Gases," in the last number of the Proceedings of the 

 Royal Society, Prof. Schuster says : — " I do not see how the 

 insulating power of air at the ordinary temperature is consistent 

 with the presence of ions, however few in nnmber, for ultimately 

 a diffusion to the electrodes and a discharge would necessarily 

 take place. This seems to me to be fatal to J. J. Thomson's 

 view of the disruptive discharge." 



This statement implies a misconception of the theory of the 

 electric discharge advanced by me in the Philosophical Maga- 

 zine, June 1883, for there is nothing in the theory of the dis- 

 charge there given which makes the presence of free ions in air 

 at ordinary temperatures and pressures essential. I will quote 

 two sentences from the paper to show what the theory is : — " In 

 order to make the spark pass through an elementary gas, we 

 have to decompose the molecules into atoms. Thus the stronger 

 the connection between the atoms in the molecules, the greater 

 the electric strength." "Chemical decomposition is not to be 

 considered merely as an accidental attendant on the electrical 

 discharge, but as an essential feature of the discharge, without 

 which it could not occur." 



The misconception has, no doubt, arisen from my using in 

 the same paper the Clausius- Williamson hypothesis of the inter- 

 change of atoms among the molecules to account for the differ- 

 ence of pressure in different directions in the electric field. But 

 this hypothesis is not essential to the theory of the discharge 

 given in the paper, for on that theory the discharge does not 

 take place until ordinary dissociation of the molecules is pro- 

 duced by the electric field. The existence or non-existence of 

 the quasi-dissociation of the Clausius- Williamson hypothesis 

 which does not produce any chemical effects, does not affect 

 the theory of the discharge, though it does that of the inequality 

 of pressure. J. J. THOMSON. 



Cambridge, July 19. 



Birds and Flowers. 



In your note on Mr. G. F. Scott-Elliot's paper on this subject 

 (Nature, July 17, p. 279) you remark : "In accordance with 

 the view of Darwin, but opposed to that of Wallace, Mr. Scott- 

 Elliot believes that the identity of colour (an unusual shade of 

 red) in the majority of ornithophilous flowers and on the breasts 

 of species of Cinnyris is an important element in pollination by 

 birds." There must be, I think, some misapprehension here. 

 I am not aware that Darwin has anywhere referred to the colours 

 of birds as being generally similar to those of the flowers they 

 frequent. Mr. Grant Allen has done so in his work on "The 

 Colour-Sense," and I have opposed his views in Nature (vol. 

 xix. p. 501), because he founds the resemblance on the theory of 

 sexual selection, and because the facts do not support any such 

 general relation. That such a relation does sometimes occur I 

 have shown, by quoting Mrs. Barber in my " Darwinism " (p. 

 201) as to the scarlet and purple colours of a sun-bird being 

 highly protective when feeding among the similarly coloured 

 blossoms of the Erythrina caffra, which, at the time, has no 

 foliage. I have also called attention (in the same work, p. 3 1 9) to 

 the numerous flowers now known to be fertilized by birds, and to 

 the numerous large tubular flowers of a red and orange colour in 

 Chile and the Andes, which are apparently adapted to be fertilized 

 by humming-birds. The general uniformity of colour would 

 be advantageous as an indication of bird-flowers as distinguished 

 from insect-flowers ; but there is no similarity to the colours of 

 the birds. Curiously enough, the common Chilian Eustephanus 

 is green-coloured in both sexes, while its close ally in Juan 

 Fernandez is red in the male. Yet the flowers it frequents in 

 the island are not red, but mostly white and yellow (see " Tro- 

 pical Nature," p. 272). It is evident, therefore, that the preva- 

 lent colours of the flowers do not determine the colours of the 

 birds which frequent them, unless those colours are so pre- 

 dominant that a similar colour becomes protective, as is more 

 generally the case in the scantily-wooded plains of South Africa 

 than anywhere else. Alfred R. Wallace. 



