250 Scientific Intelligence. 



optical theory of the spectroscope are subject to this criticism ; 

 and fig. 15, intended to show the path of the light through a 

 powerful prism spectroscope, could hardly be modified so as to 

 give more hopelessly false ideas as to the optical principles 

 involved. A critical reading shows an analogous lack of uni- 

 formity in the unexpected relative importance attaching to the 

 different topics treated. Probably few spectroscopists would 

 approve the ratio of eight pages devoted to absorption spectra to 

 only two appropriated to the explanation and application of Dop- 

 pler's principle. A single page is given to the subject of 

 spectroscopic binaries, of which o Ceti and j3 Cygni (with rela- 

 tive magnitudes reversed) are cited as examples. It would be 

 difficult to imagine errors which would be more confusing to a 

 beginner. This is immediately followed by more than six pages 

 on the spectra of comets. Another peculiarity is an utter lack of 

 system in the orientation of cuts representing spectra. In many, 

 the wave-lengths decrease from left to right, while in the others 

 the reverse order is employed ; nor is it always easy to tell, 

 either from the figure or from the text, which arrangement is 

 selected. 



On the other hand, there are features which will make the book 

 a welcome addition to every scientific library. The copious 

 references to sources and the extended tables of spectra at the 

 end of the volume will certainly prove conveniences. Then, too, 

 the chapter on the Michelson Echelon spectroscope is so satisfac- 

 tory that one wonders why the important theory of the concave 

 grating is wholly neglected. c. s. h. 



11. The Reflecting Telescope. From Vol. XXXIV of Smithsonian 

 Contributions to Knowledge. — This publication consists of two 

 parts, the first of which is a reprint of the celebrated paper by Pro- 

 fessor Henry Draper, On the Construction of a Silvered Glass Tele- 

 scope, published in 1864 in Vol. XIV of the same series. This is 

 so well known to all amateur and working opticians that a review 

 of it here is quite unnecessary. The second part, of nearly the 

 same length, bears the title, On the Modern Reflecting Telescope 

 and the Making and Testing of Optical Mirrors, by George W. 

 Ritchie of the Yerkes Observatory. This is a highly interesting 

 description of the refined methods which have been gradually 

 introduced during the forty years which have lapsed since the 

 appearance of the earlier paper, illustrated especially by the 

 experience of its author in constructing 24- and 60-inch parabolic 

 mirrors in the optical shop of the Yerkes Observatory. The 

 frontispiece is an enlargement of the admirable photograph of 

 the central portion of the Great Nebula in Andromeda, taken by 

 Mr. Ritchie with the 24-inch mirror. A comparison of the illus- 

 trations of the two papers — both of the highest excellence attain- 

 able at their respective periods — is most instructive. c. s. h. 



18. An Introduction to the Theory of Optics / by Arthur 

 Schuster. Pp. xv, 340. London, 1804 (Edward Arnold). — 

 This is a notable addition to the literature on optics. The first 



