of Electric Induction and Conduction. 373 



• be just as much diminished as the velocity in the first case is in- 

 creased, because of the relaxation of tension normal to the lines 

 of force ; but this the model does not show.) Let the real 

 plane of polarization be xy. Then, resolving each oscilla- 

 tion into two equal components parallel to x and y respectively, 

 the y component will be retarded behind the other in their pas- 

 sage through the medium ; and when the beam emerges, it will 

 be no longer plane, but elliptically polarized. The amount of 

 ellipticity depends on the difference of the velocities of the two 

 components (which depends on the square of the electromotive 

 force), and on the thickness of strained medium through which 

 the light has passed. If one component were retarded a quarter 

 wave-length behind the other, the emergent light would be 

 "circularly polarized ; so the actual retardation must be ex- 

 . tremely small, as the change in the light is, even under favour- 

 able circumstances, barely perceptible. This is the pheno- 

 menon long looked for by Faraday, but first observed by Dr. 

 Kerr*, of Glasgow, last year. 

 University College, London, 



Note added October 14, 1876. 



Since the above was in print I have seen a paper of Sir William 

 Thomson's in tie Philosophical Magazine for June 1853, "On 

 Transient Electric Currents," from which I gather that Weber had 

 already applied his electrodynamometer to the investigation of 

 transient currents — in which, moreover, I imd that Thomson pre- 

 dicted the experiment of Dr. Feddersen, mentioned above (§3 

 and footnote, p. 355), almost exactly, and calculated the value 

 of every oscillation in a discharge. It is also shown that all the 

 oscillations ought to occur at equal intervals of time, whatever be 

 the resistance p of the discharging circuit, provided only that it 

 remains constant. The interval is not indeed the simple harmonic 



semiperiod -— — -, but it is — — — - — — . A being a constant 

 y(iu-) */(AnK — 1(>~) 



which I do not yet understand, but which Thomson calls " the elec- 



trodynamic capacity of the discharger." I imagine, therefore that 



the increasing intervals actually observed were due to the resistance 



of the air, across which the discharge took place, increasing as the 



strength and heat of the successive sparks dimiuished. 



It appears that the model agrees perfectly with Thomson's 



theory, except that it locates the principal cause of the action in the 



* Phil. Mag. November and December 1875. The most singular point 

 in Dr. Kerr's discovery is his observation that bodies may be divided into 

 two classes — those which act as if compressed along the lines of force, and 

 those which act as if extended. I have repeated his experiments with 

 glass, but not without failures sufficient to excite my admiration for the 

 skill and patience involved in the discovery. 



