upon the Electrical Discharge in Rarefied Gases. 127 



23. After the excitation of the magnet, the light diffused in 

 the bulb of the warmth-pole, as it gradually accumulates to a 

 disc, rotates around the axial line. The direction of this rota- 

 tion is the then direction of the Ampereian currents in the 

 excited magnetic poles. When the magnetism ceases, the light 

 rotates back again in the opposite direction. This rotatory motion 

 is stronger on inversion of the poles. The direction of the 

 Ampereian currents in the newly-caused magnetic excitation 

 determines the direction of the rotation : on inversion this 

 direction is reversed. In fig. 7 the arrow shows this direction, 

 N and S marking the north and south magnetism of the 

 armatures. 



24. Finally, the tube and bulbs remaining in the position 

 last described, the armatm-e was removed from one of the two 

 limbs of the magnet (fig. 8). The horizontal disc of light 

 remained, as before, bounded by the axial diameter of the bulb ; 

 but the violet light, in the neighbourhood of the remaining pole- 

 point, was more intense, and the bright green strip enveloping 

 the disc became somewhat broader towards the opposite ex- 

 tremity of the diameter, and diminished in the intensity of its 

 light in a corresponding degree. 



25. It follows as a matter of course, that in all the cases 

 hitherto described, if the tube be turned about the axial line, the 

 plane of the disc moves with the tube also about the same axial line. 



26. The tube still remaining in its horizontally equatorial 

 position, on bringing the bulb down upon the two pole-points, 

 that which was before a disc, assumed the form of a dome- 

 shaped surface which descended from the platinum wire on both 

 sides. By a greater approximation of the pole-points, the cur- 

 vature of the surface is increased. 



27. If we consider the contents of paragraphs 20 to 26 as a 

 whole, the idea presents itself forcibly to us, that the several 

 planes or curved surfaces, in which the diffused light spread 

 around the warmth-pole becomes concentrated, are formed of 

 lines of light which, proceeding from the separate points of the 

 positive electrode, coincide with magnetic curves. 



28. This view is supported also by the fact that the arched 

 concave or convex border of the disc of light becomes moi-e 

 strongly curved in the two cases first described, if, instead of 

 the armatures with conoidal points, parallelopipedal armatures 

 (190 millims. long, 68 millims. broad, 20 millims. thick) be 

 laid upon the magnet, and then the tube so brought between the 

 approximated armatures that in one case the bulb with the 

 warmth-electrode rests upon them from the outside, in the 

 other the surfaces of the armatures meet the tube with the bulb 

 exteriorally. 



