on the Nature of the Light emitted by a Substance. 235 



manner that right-handed circularly-polarized light was 

 quenched. Now according to the preceding the widened line 

 must at one edge be right-handed circularly-polarized, at the 

 other edge left-handed. By a rotation of the analyser over 

 90° the light that was first extinguished will be transmitted, 

 and vice versa. Or, if first the right edge of the line is visible 

 in the apparatus, a reversal of the direction of the current 

 makes the left edge visible. The cross-wire of the eyepiece 

 was set in the bright line. At the reversal of the current the 

 visible line moved ! This experiment could be repeated any 

 number of times. 



21. A small variation of the preceding experiment is the 

 following. With unchanged position of the quarter-wave 

 plate the analyser is turned round. The widened line is then, 

 during one revolution, twice wide and twice fine. 



22. The electromagnet was turned 90° in a horizontal 

 plane from the position of § 20, the lines of force now being 

 perpendicular to the line joining the slit with the grating. 

 The edges of the widened line now appeared to be plane 

 polarized, at least in so far as the present apparatus permitted 

 to see, the plane of polarization being perpendicular to the 

 line of the spectrum. This phenomenon is at once evident 

 from the consideration § 19. The circular orbits of the ions 

 being perpendicular to the lines of force are now seen on 

 their edges. 



23. The experiments 20 to 22 may be regarded as a proof 

 that the light- vibrations are caused by the motion of ions, as 

 introduced by Prof. Lorentz in his theory of electricity. 

 From the measured widening (§ 14) by means of relation (6), 

 the ratio e /m may now be deduced. It thus appears that e/m 

 is of the order of magnitude 10 7 electromagnetic C.G.S. units. 

 Of course this result from theory is only to be considered as 

 a first approximation. 



24. It may be deduced from the experiment of § 20 whether 

 the positive or the negative ion revolves. 



If the lines of force were running towards the grating, the 

 right-handed circularly-polarized rays appeared to have the 

 smaller period. Hence in connexion with § 18 it follows that 

 the positive ions revolve, or at least describe the greater orbit. 



25. Now that the magnetization of the lines of a spec- 

 trum can be interpreted in the light of the theory of Prof. 

 Lorentz, the further consideration of it becomes specially 

 attractive. A series of further questions already present 

 themselves. It seems very promising to investigate the 

 motion of the ions lor various substances, under varying 

 circumstances of temperature and pressure, with varying 



