224 



NA TURE 4{ \.^, [Jan. 9. 1 890 



Some verbal inaccuracies which had crept into the first 

 translation have been corrected, and in every respect the 

 editor may be congratulated on the work in its present 

 form. It will be of the greatest use to students — espe- 

 cially, perhaps, to those who have to work alone. 



D. H. S. 



Traiid d^Optique. Par M, E. Mascart. Tome I. (Paris: 

 Gauthier-Villars, 1889.) 



This is the first half of a very elaborate treatise on 

 optics, the full scope of which we cannot tell till the 

 second volume appears, as no hint is given of what is 

 yet to come. This first volume begins with the funda- 

 mental principles of the wave-theory of light, deduces 

 from them the elementary laws of geometrical optics, 

 discusses the properties of a co-axal system of refracting 

 surfaces, describes the structure of the eye, expounds the 

 facts of colour-mixture, points out the conditions which 

 determine the resolving power of a telescope, develops at 

 great length the theories of diffraction and interference, 

 with some of their principal applications, and devotes 

 about 80 pages to polarization and double refraction. 

 There is practically nothing about the microscope, and 

 nothing at all about the paths of rays in media of con- 

 tinuously varying density. 



The book is by no means easy reading, and the labour 

 of perusing it is increased by the smallness of the refer- 

 ence letters (with their numerous accents and suffixes) 

 which occur in the figures. The plan involves much 

 specialization. For instance, the proof of the formula for 

 retardation on which the theory of Newton's rings de- 

 pends is not given in the sections devoted to Newton's 

 rings and colours of thin plates, but some 370 pages 

 earlier. In many cases, when the student has found a 

 formula which appears to contain the information of 

 which he is in quest, he has to search carefully through 

 a long series of preceding pages before he can find the 

 meaning of some symbol which occurs in it. The volume 

 contains a vast store of information, but not generally in 

 a form to suit hasty seekers after truth. It requires to 

 be studied at leisure, and the time so spent will not be 

 wasted. Great pains have obviously been taken to em- 

 body the latest information and present it in the clearest 

 form. We may instance the spiral curves which illustrate 

 the values of Fresnel's integrals, and the curve (to which 

 a folding-plate is devoted) showing the relations of 

 the colours of diffraction fringes to the three primary 

 colours. There is an excellent discussion of the theory 

 of concave gratings, both for reflection and refraction. 

 The least attractive chapter is that entitled " Properties 

 of Vibrations." It is a discussion of the composition of 

 simple harmonic motions, and occupies 40 pages bristling 

 with elaborate formulas. We think a more moderate 

 display of mathematics under this head would have 

 sufficed. 



The order of arrangement adopted in the volume is 

 rather peculiar, and baffles all a priori conjecture. For 

 instance, the discussion on colour-mixtures occurs in a 

 chapter on " Interferences," and the investigation of the 

 conditions which determine the resolving power of a 

 telescope is given in the introductory chapter under the 

 head of " Preliminaries." 



The book is essentially a mathematical treatise, all ex- 

 perimental descriptions being reduced to the narrowest 

 possible limits. 



The preface states that the work is addressed mainly 

 to " pupils of the Faculties and Schools of higher in- 

 struction," but we think its principal use in this country 

 will be as a book of reference for teachers. Its value for 

 this purpose will' be greatly increased if a good alpha- 

 betical index is added at the end of the second volume. 



J. D. Everett. 



Bibliotheque photographique : Le Cylindrographe, Ap- 

 pareil panoramique. Par P. Moessard, Commandant 

 du Gdnie brevet^, attache au Service geographique de 

 I'Arme'e. (Paris : Gauthier-Villars, 1889.) 



This is a description of a photographic camera invented 

 by Colonel Moessard, in which the lens is pivoted on an 

 axis, and the sensitive film is arranged in a cylindrical 

 form about this axis, on a radius equal to the focal length 

 of the lens. By this means a panoramic view of angular 

 breadth up to 170° can be taken. The camera being fixed 

 in position, the lens is uncapped, and then rotated quickly 

 or slowly, according to the speed of the plate, and the in- 

 tensity of light in any direction. The author claims for 

 the instrument useful employment in surveying, either in 

 the carefully detailed plans of an ordnance survey, or in 

 the rapid views useful for warlike purposes, which the in- 

 strument can afford. Two photographs taken with the aid 

 of the instrument illustrate very favourably its powers, 

 especially for architectural purposes. 



A Hand-book of Modern Explosives. By M. Eissler. 

 (London : Crosby Lockwood and Son, 1889.) 



In this book the author of " Modern High Explosives" 

 has collected much useful information about the various 

 explosives now in use. The greater part of the work is 

 devoted to nitro-compounds, but short accounts of the 

 other types of explosives now being manufactured are 

 added. The manufactures of gun-cotton and nitro- 

 glycerine receive full treatment, together with the modifi- 

 cations introduced in the various large factories both of 

 America and Europe. The important subject of the use 

 of explosives in fiery mines has a chapter to itself The 

 description of the tests of flameless powders is of 

 especial interest ; in fact, the official reports of the tests 

 of many of the most important explosives are perhaps 

 the most instructive portions of the book. The chapter 

 dealing with the practical application of explosives should 

 be useful not only to the miner, but also to officers of 

 both services to whom blasting and the use of explosives 

 generally may at any time become a necessary auxiliary. 

 An interesting account of the history and trials of the 

 Lalinsky gun, together with the manufacture and use of 

 gun-cotton shells, is also well worthy of their perusal. 

 Little is said on the use of explosives below water, 

 especially on the subject of the removal of wrecks, which 

 would stand far fuller treatment. Four appendices are 

 added, two dealing with the analysis and determination 

 of stability of explosives, and one containing abstracts 

 from the principal provisions of the Explosive Act of 1875. 

 Although there is much that is necessarily old, still this is 

 a book that will be read with interest by most who are 

 accustomed to work with high explosives. The illustra- 

 tions are well executed, and the whole wonderfully free 

 from printer's errors. 



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, rejected 

 manuscripts intended for this or any other part of Nature, 

 No notice is taken of anonymous communications. '\ 



The Peltier Effect, and Contact E.M.F. 



Without any further reference to the heading of a letter on 

 p. 102, signed "The Reviewer," I wish to discuss an interesting 

 argument therein propounded as proving that a true electro- 

 motive force at contact between two metals cannot be the cause 

 or sole cause of the Peltier effect, unless the latter be simply 

 proportional to absolute temperature. The argument is very 

 like one that I indistinctly remember to have heard suggested 

 some time ago by Prof. Schuster, and it struck me at the time 

 as ingenious and not easily answerable. 



