February 13, 1902] 



NA TURE 



343 



A-iew of the student. It may be said finally that the book 

 is not merely useful as a work of reference, but it is 

 thoroughly readable throughout. M. S. 



Pleasures of the Telescope. By Garrett P. Serviss. 



Pp. vi + 2CX). (London: Hirschfeld Brothers, igoi.) 



Price bs. net. 

 This book is the result of the collection under one cover 

 of a series of articles originally published in serial form, 

 after considerable revision and insertion of matter neces- 

 sary to bring the information up to date. 



Chapter i. deals in a very interesting manner with 

 advice on the choice of telescopes, special characteristics 

 of refractors and reflectors, principles underlying the 

 achromatic corrections of refractors, and methods of 

 testing the performance of astronomical instruments. 



Following this, six chapters are devoted to a series of 

 descriptions of the constellations, numerical particulars 

 being furnished for all the more interesting objects. \ 

 very liberal supply of star-maps — twenty-si.x — serves for 

 the identification of all the objects mentioned in the te.xt. 



The main features of the planets are also described, 

 small cuts indicating the details to be seen with powers 

 usually at an amateur's command. P'our charts of the 

 moon are given, showing the more important formations 

 only, so as to avoid the confusion inseparable from the 

 complete maps. This section is made exceedingly in- 

 teresting by the various formations being compared with 

 each other, the reader passing from one to another much 

 more readily than by merely going over a list of objects. 

 Users of the book will recognise the treatment of the 

 subject as similar to that in '" .Astronomy with an Opera- 

 glass,'' by the same author, and it will doubtless be 

 welcome to many workers who only require information 

 concerning objects within reach of the instruments usually 

 possessed by amateurs ; but the size of telescope catered 

 for, of 5 inches aperture, is sufficiently large to render 

 the information of service to the more advanced 

 astronomer as well. There is only one slight criticism 

 ■which may be suggested regarding the preparation of the 

 star maps. On these there is no indication of either the 

 coordinates of right ascension or declination. In actual 

 practice, either in learning the constellations or in pass- 

 ing from one map to another, it is impossible to over- 

 estimate the help which is furnished by the graduated 

 position lines. The legibility and general arrangement 

 of the maps, however, are excellent and add greatly to 

 the value of the book, which there can be no hesitation 

 in recommending to the notice of all interested in 

 observational astronomy. C. P. B. 



Introductory Physics for Irish Intermediate Schools. By 

 R. \. Gregory and A. T. Simmons, B.Sc. Pp. ix + 2i8. 

 (London : Macmillan and Co., Ltd., igor.) Price 2^-. 

 This little book, as its emerald-green covers and title 

 suggest, is for the use of Irish boys and girls preparing 

 for the examination on the new syllabus in introductory 

 physics issued by the Department of .-Vgriculture and 

 Technical Instruction. A glance at the book shows that 

 Irish physics is the same as English, and those familiar 

 with the other books prepared by the same authors will 

 find here practically the same exercises. S. S. 



Algebraical Exaviples. By H. S. Hall, M.A. Pp. viii 

 -I- 172. (London: Macmillan and Co., Ltd.) Price 2j. 

 It will be a convenience to many teachers to possess this 

 collection of algebraical exercises to supplement those 

 given in Hall and Knight's " .-Mgebra for Beginners" 

 and "Elementary Algebra," up to quadratic equations. 

 The exercises are carefully graduated, and are classified 

 so that the teacher can easily select those referring to 

 the subject with which he is dealing. In addition, there 

 are a number of test-papers containing miscellaneous 

 examples to test the pupil's grasp of the principles of 

 algebra in which he has been e.xercised. 



NO. 1685, VOL. 65] 



LETTERS TO THE EDITOR. 



[ The Edilor 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 rejectei 

 7naniiscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.'\ 



The FitzGerald-Lorentz Effect. 



In the January number of the Philosophical Magazine I 

 published a discussion of the general theory underlying the 

 experiment of Messrs. Michelson and Morley on the drift of 

 the jether. As one result, it appeared that the effect to lie 

 expected in their special case was just the opposite of that usually 

 supposed, and that consequently the FitzGerald-Lorentz ex- 

 planation of the observed null effect would not hold. Mr. 

 H. M. Macdonald has pointed out the source of this discrepancy 

 in an algebraic slip in my paper ; when this is corrected, the 

 result comes into agreement with the special case treated by 

 Michelson and Morley. The exact effect on the displacement 

 of the interference-bands arising from a vertical component in 

 the ;iether-drift has not been hitherto directly considered. It 

 is probably null ; but this requires verification, which I hope to 

 be able to take up shortly on the basis of my analysis. As the 

 question stands at present, the corrected result shows that the 

 FitzGerald-Lorentz shrinkage would completely annul the shift 

 when the drift is tangential. Although Dr. Larmor has not 

 directly discussed the effect of an oblique drift in his " Aether 

 and Matter," I understand from him that he has come to the 

 conclusion (cf. loc. cit. §34) that complete annulment results 

 in all cases on the FitzGerald-Lorentz hypothesis. I think 

 further discussion on the lines of my own method of analysis 

 will verify that this is the case. 



Meantime I send this intimation in order that others may not 

 spend time in tracking out a discrepancy which has already 

 been cleared up. W. M. HiCKS. 



University College, Sheffield, February 10. 



Birds Attacking Butterflies. 



O-X July 22, 1901, a dull, sunless day, I pointed out to Prof. 

 Gotch a fine fresli male specimen of the " Holly Blue " (Lycaena 

 argiolus) at rest on the leaf of a shrub behind the Oxford 

 University Museum. Touching it with my finger, the butterfly 

 rose and fluttered feebly along the curved walk in the Parks. 

 At that moment a swallow (or a martin) came down the walk 

 from the opposite direction at full speed. It must have seen 

 the butterfly fluttering towards it from a considerable distance ; 

 for with the most perfect ease and control it diverted its course 

 and took the inseci: in its sweep. I felt, as I saw it, that only 

 by good fortune was it possible thus to obtain the most direct 

 evidence of events which are probably continually occurring. 



There are, however, other means by which evidence can be 

 obtained. One is the examination of the crops of dead birds. 

 Although we should be sorry for British birds to be killed with 

 this object (except in special circumstances), it is much to 

 be hoped that the observations will be made when birds are 

 killed, whether accidentally or otherwise. Mr. R. Newstead, 

 of the Chester Museum, has done excellent work in this way ; 

 but there can be no doubt that, taking the country as a whole, 

 only an insignificant proportion of the obtainable evidence is 

 utilised. 



Another line of evidence is afforded by specimens of butter- 

 flies which have their wings injured in a manner which is incon- 

 sistent with any interpretation except the snip of a bird's beak. 

 Thus it is common to find fresh and unworn specimens with a 

 notch or tear on the right side which exactly fits a correspond- 

 ing injury on the left side, indicating that the wings had been 

 torn when they were in contact. In one extreme instance, 

 presented to the Hope Department by Dr. F. A. Dixey, a deep 

 little notch had been cut out of all four wings of a " Red 

 Admiral " ( Vanessa atalanta), the four injuries exactly coin- 

 ciding in the true position of rest adopted by this insect. 



Oxford, February 2. Ed\v.\rd B. Poulton. 



P.S. — Mr. W. Holland, of the Hope Department, tells me 

 that about the middle of June 1901 he saw a swallow swoop 

 down from a great distance and catch a white butterfly (almost 

 certainly Pieris rapae) flying in front of the Museum. The bird 

 took the insect in a single sweep and then dextrously avoided a 



