1878.] the Illumination of Lines of Molecular Pressure. 105 



from this focus shows a sharply- defined spot of greenish-yellow light. 

 On still further exhaustion, and especially if the cup is made positive, 

 the bulb becomes beautifully illuminated with greenish-yellow phos- 

 phorescent light. 



This greenish-yellow phosphorescence, characteristic of high ex- 

 haustions, is frequently spoken of in the paper. It must be remem- 

 bered, however, that the particular colour is due to the special kind of 

 soft German glass used. Other kinds of glass phosphoresce in a dif- 

 ferent colour. The phosphorescence takes place only under the in- 

 fluence of the rays from the negative pole. At an exhaustion of 4 M*, 

 no light other than this is seen in the apparatus. At *9 M the phosphor- 

 escence is about at its maximum. When the exhaustion reaches '15 M 

 the spark has a difficulty in passing, and the green light appears occa- 

 sionally in flashes only. At '06 M the vacuum is almost non-con- 

 ductive, and a spark can be forced through only by increasing the 

 intensity of the coil, and well insulating the tube and wires leading 

 to it. Beyond that exhaustion nothing has been observed. 



Focus of Molecular Energy. 



In an apparatus specially constructed for observing the position of 

 the focus, the author found that the focal point of the green phos- 

 phorescent light was at the centre of curvature, showing that the mole- 

 cules by which it is produced are projected in a direction normal to the 

 surface of the pole. Before reaching the best exhaustion for the green 

 light, another focus of blue- violet light is observed ; this varies in posi- 

 tion, getting further from the pole as the exhaustion increases. In the 

 apparatus described, at an exhaustion of 19 - 3 M, these two foci are seen 

 simultaneously, the green being at the centre of curvature, while the 

 blue focus is at nearly twice the distance. 



Nature of the Green Phosphorescent Light. 



The author adduces the following characteristics of the green 

 phosphorescent light, as distinguishing it from the ordinary light 

 observed in vacuum-tubes at lower exhaustions : — 



a. The green focus cannot be seen in the space of the tube, but 

 where the projected beam strikes the glass only. 



b. The position of the positive pole in the tube makes scarcely any 

 difference to the direction and intensity of the lines of force which 

 produce the green light. The positive pole may be placed in the tube 

 either at the extremity opposite the negative pole, or below it, or by 

 its side. 



c. The spectrum of the green light is a continuous one, most of the 

 red and the higher blue rays being absent ; while the spectrum of the 

 light observed in the tube at lower exhaustions is characteristic of the 



* M signifies the millionth of an atmosphere. 



