174 Dr. E. Goldstein on the Electric 



believed to be of tho same nature as the production of light, 

 which the luminous strata of the positive discharge caused in * 

 the walls of the tube, or even through the walls upon a quinine- 

 screen, or similar fluorescent body. My researches have 

 shown: — 



(1) The production of light by an electric ray from the nega- 

 tive pole in highly rarefied gas takes place only when the ray 

 strikes upon a solid obstacle. 



(2) It is not the whole length of the ray which produces the 

 light, but only the end of it furthest from the negative pole. 



Both of these statements (whose complete experimental 

 proof cannot be here described) maybe easily verified by cut- 

 ting off a sharply-defined pencil of light from the mass of light 

 from the negative pole by means of a screen with a small open- 

 ing. If a fluorescent screen be caused to approach such a 

 pencil sideways, no fluorescence is observed, however close the 

 approach, whether the end of the pencil strike upon a solid 

 wall and cause fluorescence there or not. 



(3) The cause of the production of light is to be sought in an 

 opticcd action. 



This may be concluded with much probability from the 

 identity of colour given by a series of different substances 

 (fluor-spar, calc-spar, potash-glass, lead-glass, silver chloride, 

 &c.) when exposed to electric radiation and to the sun's rays. 

 It follows with greater certainty from the fact that fluorescent 

 screens are actually excited when they are so placed in the 

 interior of the tubes that they are protected from the rectili- 

 near radiation of rays from the kathode, but are exposed 

 to rectilinear radiation from those portions of the walls 

 upon which the electric rays impinge. Such screens shine 

 out with their own peculiar light when distant about 1 cen- 

 tim. from the ends of the rays, which have of course no 

 measurable length. The molecules at the ends of the rays 

 therefore emit rays in all directions, in the same way as the 

 particles of a glowing body, and to points which could not be 

 reached by the direct electric radiation. (Hence it is neces- 

 sary in the experiment illustrating (1) and (2) to arrange a 

 screen so as to intercept the rays emitted obliquely by the end 

 molecules.) 



I had already found, w r hile occupied with the influence of 

 the surface of the negative pole on the discharge, that when 

 the kathode does not possess a perfectly smooth surface the 

 inequalities are very faithfully reproduced in the light excited 

 by the electric rays on the solid wall. Thus, for example, if a 

 coin be employed as negative pole, there is an appearance of 

 the head on the wall of the containing vessel, 



