84 



SCIENCE. 



[N. S. Vol. XVI. No. 394. 



thus far used do not give directly the wave- 

 velocity or the velocity of the individual 

 disturbance, but a velocity dependent on 

 that of the group. 



While the agreement between these elec- 

 trical constants and the light constant has 

 perhaps been the strongest factor in the 

 identification of electromagnetic and op- 

 tical phenomena, additional discoveries now 

 give incontrovertible evidence of the com- 

 mon agency of the two classes of phenom- 

 ena, so that these constants may now be 

 considered with good reason to be, not so 

 valuable as evidence of like phenomena, 

 as independent data in determining the 

 true value of the velocity of propagation 

 of the medium for electrical and optical 

 disturbances. It is true that exact quan- 

 titative evidence is lacking. The experi- 

 ment of Rowland is essentially qualitative, 

 and although his results agree approxi- 

 mately with values calculated from theory, 

 more exact results are extremely desirable, 

 although such a possibility seems to trans- 

 cend present mechanical attainm.ents. The 

 futile attempts to definitely establish by 

 direct experiments the electrodynamic re- 

 lations between electric charges and the 

 electromagnetic field do not disturb our 

 confidence in the truth of the theory. 



Experiment still fails to give us a me- 

 chanical reaction on a charged particle 

 moving in a magnetic field. It fails also 

 in giving a positive reaction on a charged 

 particle when the magnetic field is varied. 

 The experiments to detect the electromotive 

 intensity produced by the variation of the 

 velocity of a moving charge have not yet 

 been successful. These are all essential 

 features of the electromagnetic theory 

 and undoubtedly will receive a successful 

 solution in the future. On the other hand 

 the action of a magnetic field in affecting 

 the discharge in a vacuum electrode tube 

 and the celebrated discovery by Hall desig- 



nated as the Hall 'effect,' are evidence of 

 the reality of the mechanical action on a 

 charge moving in a magnetic field. The 

 phenomena of discharge, in electrodeless 

 tubes in the presence of electric oscilla- 

 tions is significant of the mechanical ac- 

 tion on a charged particle in a varying 

 magnetic field. The discoveries by Fara- 

 day and by Zeemann— as we now interpret 

 the association of electrical charges on mat- 

 ter as evidenced by what we know from 

 electrolysis— are a further confirmation of 

 the mechanical reaction of a field of force 

 upon moving charges. The experiments of 

 Lecher on the magnetic action of displace- 

 ment currents in a dielectric also confirm 

 our ideas in regard to the essential char- 

 acteristic of an insulating medium and the 

 electric charges on the ultimate elements 

 of matter. Hence we are with full rea- 

 son bound to identify these constants, and 

 may therefore examine their derivation by 

 a closer analysis of their real significance. 

 If on the optical side the problem of the 

 velocity of propagation of individual dis- 

 turbances has never been attacked directly, 

 there seems to be full reason for doing so 

 in order to complete the evidence from the 

 standpoint of light phenomenon which we 

 already have at hand on the electrical side. 



It would be desirable to determine the 

 velocity of a group of periodic electric dis- 

 turbances under varying conditions in or- 

 der to compare it with the velocity of a 

 single disturbance. 



In the methods of Fizeau and of Fou- 

 cault, which are the only ones used thus 

 far, the time of the 'go' and return of a 

 fiash of light is measured. The relation 

 of this time to that of the time of the 

 'go' and return of a single one of the com- 

 ponent waves is not a relation at once sim- 

 ple and evident. No experiments have 

 been directly carried out to determine this 

 relation in optical media. We have the- 



