7 oo SCIENCE PROGRESS 



vacuum spectrograph, in which the prism has been replaced by a concave grating 

 of radius one metre approximately, with lines ruled on speculum metal, about 

 15,000 to the inch. By this modification, absorption of the light is much reduced, 

 accurate measurement of wave-lengths becomes possible, and manipulation 

 rendered less difficult. In fact the author has reached the line 905 A.U., thus 

 entering well within the third " octave " of the ultra-violet spectrum. 



After a brief account of earlier work and some reference to the life and 

 researches of Schumann, Dr. Lyman devotes a chapter to the details of his own 

 spectrograph, its construction and manipulation, and also to the preparation of 

 suitable photographic plates with an emulsion which must be almost gelatine-free, 

 as gelatine has a considerable absorbing power in the region considered. There 

 follow chapters on the absorption and emission by several gases and solids of 

 light of this extremely high quality. 



The author is not hopeless concerning still further extension of the limit 

 reached toward higher frequences, and of meeting, as it were, the school of 

 investigators who are seeking to produce X-rays of longer and longer wave- 

 length. The main difficulty seems to be connected with the use of the " window- 

 less " discharge tube, from which the discharge tends to spread into the receiver 

 of the spectograph and so fog the plate. Reduced sensitivity of the Schumann 

 plates, and decrease of reflecting power in the speculum grating, may also be 

 responsible for the failure of very high quality radiation to impress itself on the 

 sensitive surface, while small traces of impurity in the low-pressure hydrogen in 

 which the discharge takes place may exercise considerable absorption. 



Dr. Lyman draws attention to the importance of research in this region, in 

 connection with the photo-electric effect and Einstein's explanation of it in terms 

 of the " quantum " hypothesis. He refers to the strong bacteriacidal action of 

 such light, and points to the conclusion that interplanetary space is germ free, 

 if sun-spots are accompanied by jets of burning hydrogen projecting through 

 the solar envelope. Hydrogen, in fact, seems to emit light of this type in great 

 abundance ; and among the useful tables of emission spectra at the end of the 

 book, the longest is that concerning this gas, containing, as it does, some 400 

 lines between 1,675 3Ln( ^ 1,228 A.U. 



This little work is not only a valuable record of research for the specialist in 

 this branch of physics, but is also to be recommended to the general reader 

 seeking to keep in touch with the progress of physical science at all points. 



J. R. 



The Theory of Heat Radiation. By Dr. Max Planck ; translated by M. 

 Masius, M.A., Ph.D., from the second German edition. [Pp. xiv + 225, 

 with 7 illustrations.] (Philadelphia : P. Blakiston's Son & Co.) 



The present volume is a much-needed translation of a work which has already 

 had a deep influence on the development of physical theories. Many English 

 physicists, not too intimate with the German language, will welcome a translation 

 of a really important book as an aid to the comprehension of a new and somewhat 

 difficult line of reasoning. 



But apart from its exposition of the now famous " Quantum " theory of the 

 author, the book is valuable for the careful introduction to radiation phenomena, 

 and to the well-established laws of Kirchoff, Stefan, and Wien, given in the earlier 

 chapters. Physics manuals are frequently compelled by exigencies of space to be 

 brief, even on points where the most careful elucidation is vital to a correct under- 



