April 5, 1900] 



NATURE 



539 



Unith ilectrtques absolues. By Prof. G. Lippmann. 



Pp. ii + 240. (Paris : Carrd and Naud, 1899.) 

 This treatise is the reproduction of professorial lectures 

 delivered at the Sorbonne in the session of 1884-85, and 

 consists mainly of three parts. The first part deals with 

 the electrostatic system of units, the second with the 

 electromagnetic system, and the third with the electro- 

 magnetic theory of light. These are preceded by an 

 introduction, which treats of units in general and the 

 c.g.s. system. At the close of the book are two supple- 

 ments, dealing respectively with the conservation of 

 electricity and Lippmann's electrodynamometer. 



The treatment is chiefly mathematical, the experimental 

 methods referred to being described in outline. The 

 analysis is, however, simple and the text illustrated by 

 a hundred excellent figures. Indeed, the book is on the 

 whole so good and clear that one regrets the more that 

 the dimensional formulcE have not been brought up to 

 date by embodiment of the progress made in the fourteen 

 years which have elapsed between the delivery of these 

 lectures and their publication. In our view, the value 

 of the book would have been much enhanced by the in- 

 troduction in it of Prof Riicker's work on the usually 

 suppressed di\mtxvsi\ov\% of /x and k (see paper read before 

 the Physical Society of London, November 24, 1888 ; 

 Xature, vol. xxxix. p. 165). 



This, impossible in the lectures themselves delivered 

 <n 1884-85, was both possible and highly desirable in the 

 book of 1899. E. H. B. 



Elementary Practical Physiography {Section II.). By 

 J. Thornton, M.A. Pp. viii-l-208. (London : Long- 

 mans, Green and Co., 1900.) 

 This is an effort to meet the requirements of candidates 

 for the Queen's Scholarship in Section II. of the syllabus 

 of elementary science. Its scope is best described by 

 the sub-title ".A. Course of Lessons and Experiments in 

 Elementary Science," but it is necessary to add that the 

 only branches of science touched upon are chemistry 

 and astronomy. In both these subjects some knowledge 

 gained by experiment and observation is now expected ; 

 but though the author claims to have kept this in view 

 throughout, there is little in the book to entitle it to be 

 called practical. It is true that reference is made to 

 seventy-four experiments in chemistry, but they are for 

 the most part better adapted as suggestions for the 

 teacher than for performance by the student. In the 

 astronomical section an excellent course of reading 

 lessons is provided, but the author has by no means 

 taken sufficient advantage of the opportunity of directing 

 the student's attention to the heavenly bodies themselves. 

 Instead of the descriptions of simple apparatus for 

 making observations which might have been expected, 

 such, for instance, as the measurement of altitude and 

 azimuth, half-a-dozen class-room demonstrations are 

 alone given. 



Objection may be taken to the author's statement that 

 "most of the diagrams are new and original" ; many of 

 them seem familiar, though they may have been re-drawn 

 for their present purpose. A. F. 



Atlas d,' Photomicrographie des Plantes Mddicinalcs. 



Par M.M. les Drs. Braemer et Suis. Pp. vi -f- 230 ; 



76 plates. (Paris : Vigot Frcres, 1900.) 

 This book consists of series of plates derived from micro- 

 photographs of the ordinary medicinal plants. To the 

 plates relevant to each plant a descriptive text is added, 

 dealing with the morphology of the respective plant. 

 The microscopic sections are very clear and well re- 

 produced. 



The book ought to be useful to those interested in 

 materia viedica ; but although we know of no similar 

 work, we are afraid it will only appeal to a relatively 

 small circle of readers. 



NO. T588, VOL. 61] 



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 writers of, rejecttd 

 manuscripts intended for this or any other part of Naturb. 

 No notice is taken of anonymous communications. "[ 



Effects of Lightning upon Electric Lamps. 



In a communication to Nature (p. 391), Prof. Wood pointed 

 out the similarity of the features exhibited in Mr. Webb's photo- 

 graphs to the trails of luminosities exhibited in a picture taken 

 with a moving camera. 



To this I replied (p. 413), saying I had understood that the 

 camera was fixed, and calling attention to two features which 

 seemed to show that the phenomenon was real. 



An independent suggestion similar to that of Prof Wood, 

 from another quarter, accompanied by photographs purposely 

 taken with a moving camera/ subsequently came before me. 

 This helped to arouse suspicion, and it occurred to me as con- 

 ceivable that though the camera was fixed, Mr. Webb might as 

 a matter of convenience have taken it up before he capped it ; and 

 if so, trails might have been left from the short exposure between 

 lifting and capping. I wrote to him accordingly, suggesting 

 that he should try the effect of lifting before capping. This led 

 him to try the effect of exposure with a moving camera, with the 

 result that there appears to be no doubt now that such was the 

 origin of the supposed effects. In his reply, enclosing photo- 

 graphs taken with a camera which was purposely moved, Mr. 

 Webb writes : " I had made so sure that there was no shake of 

 my camera, in spite of your frequent suggestions to the contrary, 

 that I cannot even now understand it, placed as it was on the 

 balcony rail, excepting in the No. 6 or five-flash exposure, when I 

 wilfully raised or depressed the camera a little to avoid getting 

 two of the horizontal flashes on the same plane of horizon." 



I must now refer briefly to the two arguments I used (p. 413) 

 in support of the reality of the effects. I pointed out that in 

 Fig. 4 there was a real decrease of scale in the luminosities 

 about the nine more distant lamps, in accordance with the 

 increasing distance from the camera. I confess it seemed to me 

 that the difference of scale, though real, was not as great as I 

 should have expected, but I had no measurements of the 

 distances of the several lamps from the camera whereby to 

 calculate what the difference of scale should have been if the 

 luminosities were real, and of the same size for the diff'erent 

 lamps. A difference of scale might be produced in a moving 

 camera if the exposed plate had a movement of rotation about 

 the line of sight. Ttie other argument was founded on the 

 visibility of the discharge. It seemed to me that such a dis- 

 charge as those shown for instance in Fig. i, taken as real, 

 might be expected to be seen directly if the eye were defended from 

 too much glare of the lightning, and I suggested to Mr. Webb 

 to be on the look-out if an occasion should occur. He states in 

 his letter to Nature (p. 343), that he actually saw such a 

 discharge. I think it is not difficult to reconcile this with the 

 supposition that there is no real discharge. The observer on 

 the look-out would have his eye directed to the lamp, and when 

 the flash came might unintentionally look in a somewhat 

 different direction. In the rapid rotation of the eyeballs the 

 image of the lamp would leave a trail on the retina, which might 

 easily be mistaken for an actual luminous discharge. - 



The beading of the discharge now presents no difficulty. 

 Indeed, the first idea which naturally occurs to one on seeing it 

 is that it might be connected with the rapid alternation of the 

 current ; but so long as the picture is supposed to represent a 

 real discharge, it seems difficult to imagine how the alternation 

 could possibly account for the beading. 



Cambridge, March 23. G. G. SrOKES. 



The Absorption of the Becquerel Rays by Solid and 



Gaseous Bodies. 



I WISH in this note to give some observations recently made 



with regard to the absorption of the Becquerel rays. Though 



the experiments are not complete, it is hoped that the results 



MThe photographs referred to were taken by Mr. J. Williamson, of Hove, 

 the electric lamps towards which his camera was directed being those along 

 King's-road, Brighton. The effects were produced by giving a stand ex- 

 posure of from five to ten seconds, and then moving the camera about for a 

 few seconds with the, cap still off.— Editor, Nature.J 



- Mr. Webb has suggested to me another e.\planation. 



