866 



SCIENCK 



[N. S. Vol. IX. No. 234. 



' Light, including Invisible Radiation,' 

 is divided into six sections : Geometrical 

 Optics and Photometry ; Velocity, Wave- 

 length, Energy, etc., of Eadiation ; Inter- 

 ference and Diifraction ; Reflection and 

 Refraction ; Polarization ; The Emission of 

 Radiation, Phosphorescence, etc. 



' Electro-magnetism ' is divided into eight 

 sections : Electric and Magnetic Units ; 

 Electrostatics ; Magnetism ; The Electric 

 Current ; Electrolysis ; Electrodynamics ; 

 Electric Discharge; Terrestrial Magnetism; 

 The Compass, Earth Currents. 



These sections are divided further into 

 270 sub-divisions. The cards are to go to 

 the subscribers fully labelled, the marking 

 being done by expert assistants in London. 

 On being received they can be filed away 

 in suitable cases by a clerk, no expert 

 knowledge being required. With a suit- 

 able key as to symbols any desired reference 

 can be found quickly, and the work being 

 done in any subject can be ascertained 

 easily. Any system of classification, there- 

 fore, which is extensive, definite, and free 

 from ambiguity, will be satisfactory. 



In the main, the systems proposed by the 

 Committee of the Royal Society are most 

 satisfactory ; and the labor expended in per. 

 fecting them in the diiferent sciences, al- 

 though enormous, will be fully repaid. 



Unfortunately, the classification in 

 Physics does not entirely satisfy the require- 

 ments demanded. The primary divisions 

 are not altogether logical , nor is the classifi- 

 cation of certain subjects ; but this is com- 

 paratively immaterial. 



In some cases it would undoubtedy be 

 well still farther to sub-divide a subject. 

 For instance, the sub-divison devoted to the 

 ' discharge in rarefied gases, ' or the one de- 

 voted to the ' measurements of wave-lengths 

 by optical and photographic methods.' In 

 other cases this process has been carried 

 too far. For instance, there is no particular 

 reason why a special sub-division should be 



given to the ' vapor pressure near curved 

 surfaces.' 



Again, certain subjects seem to be en- 

 tirely omitted, such as ' spectrum analysis ;' 

 the 'effect of different external causes on 

 wave-lengths,' such as the Zeeman effect 

 and the pressure effect ; the ' numerical re- 

 lations between the lines of any one spec- 

 trum and between the spectra of different 

 elements ;' ' Doppler's principle ;' the ' laws 

 of radiation and absorption ;' ' forced vibra- 

 tions and resonance ;' the ' laws of steady 

 currents as distinct from alternating cur- 

 rents ; ' ' heat efiects of currents ; ' ' photog- 

 raphy ;' etc. 



There are sections which are almost 

 identical, such as the 'vibration of strings 

 and rods ' under ' Sound ' and the ' dynamics 

 of flexible strings ' under ' Elasticity.' It 

 is hardly an accepted fact that the Hall ef- 

 fect is due to changes in specific resistance, 

 and, therefore, one would not necessarily, 

 place it in that section. Again, in speak- 

 ing of dynamics, the word center of inertia 

 or center of mass is preferable to center of 

 gravity. The name ' Electro-magnetism ' 

 is not a particularly happy one for the last 

 primary division. 



The only points of importance in the 

 classification which need be criticised, 

 however, are, I think, the omissions, the 

 other matters being of very little importance, 

 owing to the fact that the classification has 

 a key, and the fact that anyone can, there- 

 fore, easily find the reference which he de- 

 sires. It would increase, however, the 

 value of the catalogue if the scheme of 

 classification could be somewhat remodeled, 

 and I venture to express the hope that some 

 action of this kind may be taken before the 

 recommendations of the committee are ac- 

 cepted by the countries concerned in the 

 proposal. 



There has been no plan proposed in re- 

 cent years which seems to be of so great 

 importance to the students of Physics 



