3^8 



NA TURE 



[January 31, 1901 



in 'this country to these equations, which are much 

 more convenient than Cauchy's formula, and which much 

 facilitate the reduction of measurements made with prism 

 spectroscopes. The chapter concludes with a complete 

 description of the various devices for compound and 

 direct vision prisms. The combination of prisms to 

 obtain great dispersion and resolving power has lost a 

 great portion of its interest since the more general intro- 

 duction of diffraction-gratings for spectroscopic purposes. 

 We therefore turn with special interest to the fourth 

 chapter, which deals with diffraction-gratings. 



After a short history of the methods of ruling gratings; 

 a discussion of plane gratings is given, which chiefly 

 follows Rowland's and Cornu's investigations. About 

 thirty pages are devoted to concave gratings. A very 

 clear and elegant theory of these gratings, due to Prof. 

 Runge, is, for the first time, published in full, and 

 deserves to be widely read. It includes the very im- 

 portant practical question of the easiest method of 

 adjusting the relative position of the slit, grating and 

 camera, so that when the carriages roll alongf the beams, 

 the spectrum should remain in focus and be displaced 

 only in a direction parallel to the plane containing the 

 two rectangular rails. 



A disadvantage of concave gratings, which has been 

 pointed out by Rowland in his first discussion, is its 

 astigmatism, a point on the slit being drawn out into 

 a line. It seems to me a curious fact that no one 

 should have attempted to correct this astigmatism by 

 means of cylindrical lenses. I was only waiting until 

 the large concave grating of the Owens College was 

 available, to try some experiments in this direction. 

 Prof Fitzgerald tells me that he has had the same idea, 

 and has already determined by experiment the proper 

 position of the two focal lines of the correcting lens. 

 In looking over the pages of Prof. Kayser's book, I 

 find that I had overlooked a paper by Mr. J. L. Serks, 

 in the Journal of Astronomy and Astrophysics, in which 

 the question is, in fact, solved theoretically. It is 

 curious, however, that the author does not seem to have 

 realised this application of his investigation, which he 

 only applied to proving the possibility of finding a 

 position for a comparison prism such that the hori- 

 zontal edges of the prism should appear sharp on the 

 spectrum plate. If the light coming from a luminous 

 point is passed through a combination of a cylindrical 

 and convex lens, placed So as to give a horizontal focal 

 line in the position given by Serks, and a vertical focal 

 line coincident with the slit, the astigmatism of the 

 concave grating will be corrected. 



The fifth chapter discusses the construction of spectro- 

 scopes, a good deal of space being devoted to the 

 various devices for securing minimum deviation. The 

 author seems to me to attach a somewhat exaggerated 

 importance to the minimum deviation as regards its 

 necessity to give definition. If the collimator is pro- 

 perly adjusted, and the faces of the prisms are plane, 

 the spectra should be equally perfect whether the prisms 

 are in the position of minimum deviation or not. When 

 many prisms are used it becomes, of course, necessary 

 that each prism should wholly take in the beam of light 

 which has passed through the previous prism, and, in 

 that case, the position of minimum deviation is most con- 

 NO. 1 63 I, VOL. 63] 



venient. For the usual prism, cut so that its base is 

 equally inclined to the faces, the position of minimum 

 deviation is also that of maximum resolving power ; 

 but the prism may be turned considerably out of the 

 symmetrical position without sensibly affecting its power 

 of resolution. 



The theory of the spectroscope, including the question 

 of resolving power and purity, is fully discussed ; but I 

 venture to think that the treatment of the brightness of 

 spectroscopic images might be made much simpler and 

 clearer, and in some cases more correct, by starting from 

 the following two very simple principles. 



It is a well-known proposition, in the formation of 

 images by lenses, that the brightness of the image, as 

 deduced from the laws of geometrical optics, simply 

 depends on the emitting power of the source and on the 

 solid angle of the converging beam forming the final 

 image. When the observations are taken by the eye, 

 and the whole pupil is filled with light, the last solid 

 angle is fixed ; hence the brightness cannot be altered 

 by any optical arrangement. The same proposition 

 also holds when the light is refracted through prisms, 

 provided the light is homogeneous. The second pro- 

 position, to which I have alluded, states that if the object 

 is linear, the width of the central image, due to the 

 finiteness of the wave-length of light, also depends only 

 on the solid angle of the conical beam forming the last 

 image. 



These two propositions enable us to draw all the 

 necessary conclusions without restrictions, such as that 

 made by Kayser as to the position of minimum devia- 

 tion of the prisms ; and the results of § 508, derived from 

 a paper by Wadsworth, will be found to need correction 

 in some important particulars. The latter portions of 

 this chapter deal with Michelson's researches, the appli- 

 cations of fluorescence, phosphorescence, and finally 

 with photographic and bolometric methods. 



The last chapter is devoted to spectroscopic measure- 

 ments. 



The value of the book is increased by the fact that 

 the author has not been satisfied with a statement of 

 results, but in many cases has added his own criticisms. 

 I entirely agree with the statement made in the preface, 

 that a mere compilation without critical discussion is of 

 very little value. In the present volume there has not 

 been so much opportunity of touching on tender spots 

 as will arise in subsequent divisions of the subject ; but 

 Prof. Kayser's evident fairness and knowledge of his 

 subject render it certain that no one need be afraid of 

 placing himself under the judgment of so competent an 

 authority. While congratulating Prof. Kayser on the 

 successful accomplishment of the first portion of his task, 

 we conclude with the hope that we may soon be able to 

 welcome a second volume. Arthur Schuster. 



LIFE AND WORK OF C. GERHARD T. 

 Charles Gerhardt: sa Vie, son Oewure, sa Correspondance : 

 1 8 16- 1 856. Document d'Hisioire de la Chimie. Par 

 M. Edouard Grimaux et M. Charles Gerhardt. (Paris : 

 Masson et Cie.) 



A BIOGRAPHY which involves the history of the 

 turning-point of a science is always interesting ; 

 and this one in particular, which tells the tale of the 



