Photographs of Lightning and "Black" Electric Sparks. 93 



glare from an illuminated white cloud upon the normal image 

 of the flash. 



To test this I endeavoured to obtain the same effect with 

 the sparks from a small Wimshurst machine ; but, under the 

 conditions in which I worked, I could not get a longer spark 

 than one inch. 



I first photographed a series of brilliant sparks, using two 

 large Ley den jars. These gave normal images, very dense, 

 and shaded off at the margins, although the focus, as shown 

 by the knobs of the machine, was good. 



Next I tried less brilliant sparks from the machine with its 

 ordinary small jars. These gave similar images, but less dense. 

 Then I repeated both experiments, and before developing 

 the plates exposed them to the diffused light from a gas-flame. 

 The brilliant sparks then yielded images which may either be 

 called normal with a reversed margin, or reversed with a 

 normal core. The fainter sparks were completely reversed. 



One plate of bright sparks was exposed to the gas-light, so 

 that different parts were acted upon for different times. The 

 reversal seems to spread inwards as the exposure to diffused 

 light is increased. 



One plate of faint sparks was only half of it exposed to 

 diffused light. The result is that on that part the sparks are 

 reversed, while on the other they are normal. 



Finally I photographed a number of sparks in a series 

 across the plate, and placed a sheet of white cardboard behind 

 them to do duty for the white background of cloud. Some 

 of the first sparks impressed on the plate show reversed 

 images. 



Coupling these experiments with the observations as to the 

 overexposure of the " dark-flash " plate, and with the fact 

 that all dark-flash plates I have seen show symptoms of con- 

 siderable exposure, I submit that there is at least a good case 

 for this theory of cumulative or repeated action producing 

 the reversal. The partial reversal of the bright sparks seems 

 to correspond with the bright core to some dark flashes; and 

 the complete reversal of the less brilliant sparks to the absence 

 of any such core from the le.-s conspicuous portions of a dark 

 flash. 



There is certainly one difficulty yet to be got over, and that 

 is the crossing of a dark flash by a bright one. However, I 

 have some experiments * in view which I hope may throw some 



* Since writing the above communication I have made a number of 

 further experiments, which I hope to describe in detail at some future 

 time. But perhaps I may be allowed to say at once that I have suc- 

 ceeded in imitating the phenomenon of a bright image crossing a dark 



Phil Mag. S. 5. Vol. 28. No. 171. August 1889. I 



