632 



PHYSICS, PROGRESS OP, IN 1898. 



or 4 centimetres apart, can be determined by throw- 

 ing simultaneously rays of two different known 

 wave lengths (say red and green), and noting the 

 positions of exact coincidence of a red and a green 

 ring. 



Magneto-optics. Further experiment on Zeeman's 

 phenomenon (" Annual Cyclopedia," 1897, page 673) 

 shows that the effect of a magnetic field is usually 

 to divide light of one refrangibility into three com- 

 ponents, two of which are displaced by diffraction 

 analysis on either side of the mean position and are 

 polarized oppositely to the third or residual constit- 

 uent. The phenomenon has been investigated pho- 

 tographically by Preston in Dublin and by Michelson 

 and Ames in this country. All spectrum lines are not 

 tripled, some being left unchanged, some doubled, 

 and others quadrupled or even sextupled. The 

 polarization is usually such as to indicate that mo- 

 tions of a negative ion or electron constitute the 

 source of light, but a few lines are affected in the 

 opposite way. Cornu (Paris Academy of Sciences, 

 Jan. 17) finds that the action appears to depend not 

 only on the chemical nature of the source of light 

 but also on the nature of the group of spectral rays 

 to which each radiation belongs, and on its function 

 in this group. If the direction of the field is normal 

 to the lines of force the rays are quadrupled, not 

 trebled. The same experimenter (ibid., Jan. 24) has 

 made measurements snowing that the magnitude of 

 the separation produced increases with the refrangi- 

 bility of the ray. Fitzgerald (London Physical So- 

 ciety, Jan. 21) thinks that when the effect appears 

 to be a doubling of the line, a third central line has 

 been produced but absorbed by the surrounding 

 medium. The same authority, in an article on " The 

 Zeeman Effect and Dispersion " (" Science Progress," 

 November), gives the following explanation of the 

 effect : " The effect of magnetic force on the vibra- 

 tions of molecules is a complicated one. . . . 

 Every simple vibration of a point maybe analyzed 

 into two circular rotations, one right-, and the other 

 left-handed, in a plane at right angles to the mag- 

 netic field and into a linear component in the direc- 

 tion of the field, all these being of the same period. 

 The effect of the magnetic force in the simplest and 

 apparently very common case is to leave the com- 

 ponent in' the direction of the field unchanged and 

 to make one of the circular components rotate more 

 rapidly and the other less rapidly than when there 

 is no magnetic force acting. The changes of fre- 

 quency are directly proportional to the strength of 

 tne magnetic field, but are independent of the in- 

 tensity of the vibration of the molecule. This sim- 

 plest action is what we would expect to take place 

 when the magnetic force acts on a moving electron 

 which is equally free to vibrate in every direction. 

 On account of this important condition, however, 

 we need not be surprised to find more complicated 

 effects produced in the case of a large number of 

 spectral lines. It is very improbable that the mole- 

 cules of most gases are so symmetrical that all vi- 

 brations in every direction are equally possible, and, 

 as a matter of fact, very complicated effects have 

 been observed in the case of a large number of lines. 

 The effect of magnetic force in this simplest case is 

 to make the axis of the orbit of an electron rotate 

 round the line of magnetic force. We might expect 

 a disturbing force to produce other changes in the 

 orbit, in general ; such, for example, as causing 

 the inclination of the orbit and its eccentricity to 

 change. Actions such as this would produce com- 

 plications in the spectra, as has some time ago been 

 pointed out by Dr. Stoney. 



"These theories as to light vibrations being due 

 to simple harmonic vibrations of electrons are. how- 

 ever, almost certainly only provisional. They re- 

 quire the forces acting on the electrons t o be directly 



proportional to the distance from their positions of 

 equilibrium. It is unlikely that electrons can be 

 subject to such forces, and their vibrations are much 

 more probably of the nature of perturbations of 

 orbital motions executed under quite other laws of 

 force. For example, the rotation of the lunar nodes 

 is a vibration of a system, the earth, sun, and moon, 

 which is controlled by forces varying inversely as 

 the square of the distance and is one whose period 

 is almost independent of the eccentricity of the 

 lunar orbit, although the amplitude of the radiation 

 such a rotation might emit might be directly pro- 

 portional to this eccentricity. The whole quest inn 

 is very complicated. It practically assumes that 

 the fundamental motions in the molecules are im- 

 mensely more rapid than any of those we deal with 

 in light vibrations, and that these latter are merely 

 perturbations of the fundamental motions. It is 

 well to keep these things in mind, although we are 

 to all appearance so very far from any satisfactory 

 explanation of it all. because it points out the direc- 

 tion in which to look for an advance in our knowl- 

 edge. That we know so little and see so little ahead 

 in these fundamental matters may be disheartening, 

 but it shows how very important any advance in 

 our knowledge of molecular motions is, and should 

 encourage us to a study of even minute effects such 

 as Zeeman has observed." Fitzgerald connects the 

 phenomenon with other electro-optic effects as fol- 

 lows : " It appears probable that the whole cause of 

 the Faraday effect is to be looked for in each sub- 

 stance in a Zeeman change of the free vibrations of 

 the molecules of the substance by the action of 

 magnetic force, this change in the free periods re- 

 acting through their resonances on the rate of prop- 

 agation of the circularly polarized components of 

 light passing through the magnetized medium. A 

 theory on similar lines may be worked out to ex- 

 plain the Kerr effect and the Hall effect, this latter 

 being an action on electrons moving continuously 

 in one direction, not on simply vibrating ones. 

 Becquerel and Deslandres ( Paris Academy of 

 Sciences, July 4) find that in a very intense mag- 

 netic field (35,000 C. G. S. units) the bands of nitro- 

 gen and cyanogen show no signs of doubling. Most 

 of the rays examined broke up into three, but cer- 

 tain ones split into five. The distribution of these, 

 considered as a function of the wave length, shows 

 signs of periodicity. 



Luminescence. Levison (New York Academy of 

 Sciences, Dec. 5) has attempted to classify all phos- 

 phorescent and fluorescent substances, grouping 

 under the former head all those that give out 

 shorter radiations than they receive, and plat-in,' 

 under the latter those that give out longer radia- 

 tions than they receive. The following is his ar- 

 rangement of the former class : 



f Thermo- Heated or cooled. 

 Electro- < Staticall >" electrifU 



Phosphorescent - 



Lumino- 



Tribo- 



etc. 



Exposed to X-ray: 



- ( 'oiii|>ivssed. 

 ( Hammered. 



The fluorescent substances are subdivided simi- 

 larly. Schmidt (Wiedemann's . \nnalen." A|>ri > 

 has performed some experiments to test the theory 

 advanced by E. Wiedemann and himself, that Hi - 

 orescence is due to the recombination of molecules 

 split up into ions by the action of light. If this is 

 true, fluorescent bodies would not be photo-ele<- 

 trically sensitive, but no such connection l>et\V( < i 

 the two phenomena was found in the experiments, 

 which showed, on the contrary, that the most 

 strongly thermo-luminescent bodies are also most 



