130 Prof. Pliicker on the Action of the Magnet 



The interior one, which continued to pass through the platinum 

 point and remained a magnetic curve, on gradually slipping the 

 tube along, became more and more elongated towards the 

 narrower part of the tube, until, finally, on reaching the upper 

 portion of the tube, it bounded the light around the warmth- 

 pole. By this change a remarkable commixture of the light 

 occurs, and new colours appear when the ring opens and its 

 light approaches that of the other part of the tube. The 

 former of these was violet, the latter reddish ; a beautiful blue 

 light was formed on their approximation. 



34. If, finally (fig. 14), the conical tube be so moved length- 

 ways from its position as to come entirely above one of the two 

 armatures, the ring of light remains quite sharply defined. In 

 this case that arc which was formerly the inside one, has become 

 the exterior one, still passing through the point of the platinum 

 wire, and intercepts the violet light towards the narrower side of 

 the tube. The other arc, which was formerly the outside one, 

 still continues to pass through that point of the platinum 

 electrode where it enters the tube through the glass. This arc 

 has now become the brightly luminous one, and in its neighbour- 

 hood the otherwise uniform light of the ring is brighter. Both 

 limiting arcs follow here also the path of magnetic curves. 



35. The tube employed in the last experiment showed also 

 the beautiful green light, whose appearance is so enigmatical. 

 At first I considered this light to be a subjective '' contrast- 

 colour ;" subsequently, however, when this generally-accepted 

 view was shown to be untenable, the idea forcibly presented 

 itself that it was a fluorescence in the glass itself. Neverthe- 

 less the light in question is in the inside of the tube ; but it is 

 situated so closely to its sides as to follow exactly its inequalities, 

 and thus to give the impression of belonging to the glass itself. 

 This explanation was first given on using the magnet. On 

 inverting the magnetic poles, the light was moved back- 

 wards and forwards in the neighbourhood of the warmth-pole, 

 and collected at the place where the surface formed by the 

 magnetic curves touched the glass. If, in consequence of a 

 movement of the tube, this surface touches the glass in a fresh 

 place, the green light immediately appears at that point. 



The conditions under which this light appears at all in 

 Geissler's tubes, can only be determined by solving the question 

 as to the nature of the dependence of the phsenomena in these 

 tubes upon the chemical nature of the traces of ponderable 

 matter which they contain. 



36. We can only say with certainty that the concentration of 

 the luminous atmosphere around the warmth-pole to a luminous 

 surface formed of magnetic curves, is a general physical fact 



