122 PHOTOGRAPHING LIGHTNING WITH MOVING CAMERA. 



rate rushes or oscillations very close together and one black dis- 

 charge. It is this dark discharge which makes this flash interesting, 

 and the photograph shows it running ^^arallel and on both sides of 

 the first bright rush, extending 0.0 millimeter on one side and 0.1 

 millimeter on the other, the boundary line on the latter side not being 

 very marked. From this black discharge issue several side branches 

 on both sides, a large one spreading out over the other rushes quite 

 prominently. These side branches all pointing dowuAvard indicate 

 that the black flash was a downward stroke, and they also tend to 

 l^rove that it must have had a good deal of resistance to overcome. 

 It must have cleared the way for the first bright discharge, which in 

 all probability proceeded from the ground upward. The difference 

 in width of the bright flash, measured at its lower and upper part, 

 would confirm this opinion, being for the lower part 0.38 millimeter 

 and for the upper part 0.22 millimeter. 



An interesting question here presents itself. Have we here two 

 separate discharges with different rates of oscillation traveling the 

 same path? Can such a condition be possible? To the writer's 

 mind the most plausible explanation would be that the two dis- 

 charges occupied two separate paths, one inside of the other, one 

 discharge forming, so to speak, a tul)e through which the other passed. 



It may also be claimed that the bright flash is probably part of the 

 dark discharge for some reason rendered more luminous. This 

 explanation nuiy be the true one, although it appears as if the bright 

 flash is entirely separate. The measurements of the width of the 

 upper and lower parts of both flashes confirm this opinion, the 

 difference in width of the bright flash being 10 per cent and for the 

 dark discharge only 20 per cent. Authorities vary in their opinions 

 as to the probable cause of these dark flaslies. It has been suggested 

 by some that there really are no black discharges, but what appear 

 as such are excessively bright flashes causing a reversal of the image 

 on the plate. This explanation may be the true one if we under- 

 stand the word '' brightness " to mean increased actinic power of the 

 light. In the black flash represented this chemical effect must have 

 been extremely high, owing to the fact that the smallest hair-like 

 extremities of the side branches are well reproduced on the picture 

 as black, in comparison with the broader and to all appearance more 

 powerful discharges which followed after. 



It was at first thought probable that we had to deal with an inter- 

 ference })henomenon, but that idea was discarded. Then it was 

 suggested that the black discharge was probably due to slow oscilla- 

 tions (the width of it would tend to confirm this opinion), and that 

 what appeared as l)lack on the plate would in reality be a dark red 

 discharge on a partially illuminated background. This opinion had 

 to be discarded for the reason that, if such be the case, the side 



