192 J. Trowbridge — Electrical Discharges in Air. 



photograph of a spark taken with a Euryscope lens, such as is 

 commonly employed for landscape work. This does not show 

 any detail. Figs. 3, 4 and 5 are photographs taken with a 

 Dallmeyer portrait lens, without a diaphragm, and show on the 

 negatives what may be considered an aureole accompanying 

 the spark its entire length. Furthermore, the oscillatory 

 nature of the sparks is shown by forked discharges which 



diverge from the main path of the spark and which point in 

 opposite directions on the same spark. If a photograph of 

 lightning could be obtained which would show a similar phe- 

 nomenon, there could be no doubt of the oscillatory nature of 

 lightning. 



Since one can, with a large number of Plante cells in con- 

 nection with a rheostatic machine, control the sign of the elec- 

 tric charges on the spark terminals, I was interested to test the 

 question whether the eye can detect any direction in electric 

 sparks. One observer, looking through an opening'which con- 

 cealed the spark terminals and only revealed the central portion 



