328 



NATURE 



\Aug. 17, 1876 



translated into English by Prof. Everett, there has been 

 an almost continuous stream of works upon Physics from 

 our neighbours across the Channel. French publishers 

 of technical works appear to be of opinion that the 

 production of a " Traitd de Physique" is an indispensable 

 part of their duty, and that their good name will suffer 

 unless they bring one out. France is fortunately rich in 

 physicists, so that there are always good men to be found 

 to do the work. The result is that each Paris season in- 

 troduces one or more new books upon Physics, which are 

 in most cases well written, and generally abound with 

 excellent illustrations. 



The " Cours de Physique " of Prof. Fernet is at once 

 both ably written and singularly incomplete. It is in- 

 tended as a text-book for the Classe de Maihe/naiiques 

 Speciales, and as such cannot altogether be classed with 

 the books of which we have been speaking. It is written 

 for a special purpose, and its value to the general student 

 is much impaired thereby. 



The chapters treating of the molecular construction 

 and of the various forms of matter is a concise digest of 

 the modern theories of this most speculative subject, and 

 the definitions of the solid liquid and gaseous states of 

 matter are particularly clear. On the other hand, no 

 reference whatever is made to the all-important subject 

 of gravitation, so that the laws of falling bodies and pro- 

 jectiles, centre of gravity, and even the pendulum itself, 

 are necessarily cut out. 



Again, hydrostatics is both fully and ably treated, the 

 law of Archimedes and the determination of specific 

 gravities being very clearly explained. 



The whole subject of heat is confined to the expansion 

 under its influence of solid, Hquid, and gaseous bodies, 

 which occupies one-fourth of the whole book. The 

 reader looks in vain for some reference to the laws of 

 freezing and evaporation or of conduction and specific 

 heat. More extraordinary still is that the entire subject 

 of radiant heat is conspicuous by its absence, no reference 

 being made to diathermancy or to the reflection and refrac- 

 tion of heat, and the dynamical theory is ignored altogether. 



In Optics the laws of reflection and refraction are more 

 fully treated than any other subject in the book, the pro- 

 perties of mirrors and lenses of various forms being 

 thoroughly and mathematically considered. No reference 

 is, however, made to dispersion, and the question of colour 

 is left out altogether, necessitating of course the omission 

 of the important subject of spectra and of the Fraunhofer 

 lines. Again, double refraction and all the phenomena 

 connected with polarisation are not even alluded to, nor 

 are the interesting subject of the velocity of light and 

 the beautifiil experiments of Foucault, of Fizeau, and of 

 Cornu for its determination. 



Acoustics is entirely left out, and statical electricity 

 occupies but a short chapter, in which induction is fairly 

 treated, and the various forms of electrical machines are 

 well described. 



What is perhaps the most remarkable omission of all 

 is that of the entire subject of Electro-dynamics. No 

 mention is made of the voltaic battery, of the great sub- 

 ject of electro-magnetism and the electric telegraph, of 

 electrolysis or of induced currents as exemplified in the 

 Ruhmkorff coil, neither are magneto-electricity or thermo- 

 electricity referred to. 



The only explanation offered for the omission of so 

 important a branch of physical science in a " Cours de 

 Physique " is the following foot-note to the chapter upon 

 Statical Electricity : — 



" By the rules for admission into the J^cole polytcclmique 

 — which are identical with those for the Classe de Mathd- 

 vialiques Spdciales — candidates are required to possess 

 an elementary knowledge of statical electricity and of 

 magnetism only. The importance, therefore, of these 

 two subjects in the present course does not admit of their 

 being treated as fully as those in the preceding chapters. 

 The further study of them must be reserved for the 

 course in the Ecole, where students are required to work 

 up the subjects of both dynamic electricity and electro- 

 magnetism." 



We may assume from this note that the other omissions 

 to which we have referred are due to the same regula- 

 tions. It is difficult to understand how such rules can 

 exist, or what considerations could have guided those who 

 framed them when they required candidates for admission 

 to read up statical electricity, leaving the more important 

 subject of electro-dynamics alone, as well as the science 

 of electro-magnetism, notwithstanding its important ap- 

 plications. 



From what has been said it will be seen that Prof. 

 Fernet's "Cours de Physique" is evidently a "cramming" 

 book for students seeking admission to a particular class 

 which has very exceptional requirements. For that pur- 

 pose it is no doubt of value, but it is practically unavail- 

 able to the general student of Physics by reason of the 

 number and the importance of its omissions. 



THE CHEMISTRY OF LIGHT AND PHOTO- 

 GRAPHY 



The Chemistry 0/ Light and Photography in their Appli- 

 cation to Art, Science, and Industry. By Dr. Hermann 

 Vogel, Professor in the Royal Industrial Academy, 

 Berlin. New and thoroughly revised edition, with 100 

 Illustrations. (London : H. S. King and Co., 1876.) 



LAST year, in reviewing in the columns of Nature 

 this volume of the International Scientific Series, 

 it became our duty to point out the very serious errors in 

 chemistry with which the translation abounded. 



We are happy to find that in this " new and thoroughly 

 revised edition," the whole of the objectionable passages 

 have been corrected, and that the same measure of cor- 

 rection has been extended to the English throughout, so 

 that the work is now a very creditable translation. With 

 regard to the author's share of the work, we can, on a re- 

 perusal, recommend it as a thoroughly good resume of 

 the principal photographic processes, and we note that 

 the very numerous variations of the photo-printing and 

 lithographic processes have been very fully noted and their 

 chief points described, for, although this description of 

 what are, as a rule, trivial variations of one or two processes 

 may seem useless, it cannot fail to call the reader's atten- 

 tion to some of the vagaries permitted by patent laws in 

 various countries. We also notice that some considerable 

 space has been given to astronomical photography (in 

 connection with which we would note that the name of 

 the eminent American astronomer who produced the 

 negative of the moon from which the frontispiece of the 

 book is taken is Rutherfurd, and not Rutherford, as 



