288 



Electrical Discharges in Air 



which concealed the spark-terminals, and only revealed the 

 central portion of the sparks, noted down his impression of 



F^ 



bD 



fcc 



the apparent direction of each spark while another observer 

 reversed the poles of the battery which charged the rheostatic 

 machine. On comparing the notes of the observers it was 

 found that there was no agreement in regard to direction. 

 This result was to be expected from the oscillatory nature of 

 the discharges. It may be that in the case of lightning the 

 eye is forcibly impressed by the greater brightness of the 

 positive terminal in the cloud, and the observer concludes that 

 the flash has a unidirectional movement. The phenomenon 

 is psychological rather than physical. 



When oscillatory sparks of the nature represented in 

 figs. 3, 4, and 5 are passed through Crookes tubes of the 

 focus-tube pattern, it was found that the photograph could 

 be taken on plates exposed to the inclined surface of the 

 platinum, both when it was made the anode and when it 



