Intelligence and Miscellaneous Articles. 163 



other pole passed first through the spark-length, then through a 

 discharge-tube of the usual form, or a telephone wrapped with 

 india-rubber, and then to earth. The result was that not only the 

 rhythm of the discharge was entirely different (as shown by the 

 altered note in the telephone), but the whole character of the 

 discharge was altered, as the appearance of the Gi-eissler-tube showed 

 without difficulty. In place of the irregular discharge, upon 

 illumination a steady discharge occurred (also clearly observed 

 directly in the spark length) ; but all this occurred only ivith the 

 negative discharge through the spark-length. With the passage of 

 positive electricity, there is no difference between illumination 

 and non-illumination. In order to obtain the phenomena, sharply 

 disturbing reflexions of the light from one ball upon the other must 

 be avoided. In the same way the phenomena may be disturbed by 

 the induction of the electricity accumulated upon the balls. If we 

 avoid these distm'bances by suitable arrangements, then, with atmo- 

 spheric pz*essure,upon illumination and passage of negative electricity, 

 we hear a note, and upon non-illumination a noise veiling a much 

 deeper note. With positive electricity there is no difference to be 

 observed. The change which the discharge itself suffers is very 

 striking. When the light does not reach it, there are irregular 

 paths in which the electricity passes which change their form and 

 place so often that, in consequence of the persistence of the image 

 upon the retina, there are several to be seen at the same time ; as 

 soon as the first ray of light reaches the place at which the negative 

 electricity enters the spark-distance, all these paths unite into one, 

 in which the electricity passes perfectly steadily and uniformly 

 without noise as at first. At bo ; h balls it is at right angles to their 

 surface. 



(3) The intensity of this action of the rays upon the negative 

 electricity is essentially dependent upon the pressure of the gas in 

 which the experiment is made ; with certain moderate pressures and 

 distances between the balls, it reaches its maximum, from which it 

 diminishes rapidly both with increase and with decrease of 

 pressures. 



(4) Further, the mode of illumination plays an important part ; it 

 is essentially only one point of the electric light (which was the part 

 exclusively employed in the later experiments), and that the hottest 

 part of the carbons, which is especially concerned in producing the 

 phenomenon ; the rays therefrom must accurately strike the point 

 where the discharge strikes the negative ball ; for the reason already 

 given, it is advantageous to shade the positive ball. 



(5) The phenomenon is seen both in dry and in moist atmo- 

 spheric air, in dry and moist hydrogen as well as in an atmosphere 

 of carbonic acid ; in the latter gas remarkably clear and sharp. 

 The maximum intensity of action occurs in different gases at 

 different pressures ; this has apparently an intimate connexion 

 with the development of the kathode-rays. 



(6) The phenomenon is certainly not exclusively associated 

 with the ultra-violet rays ; this is seen most clearly in the experi- 



