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XII. Note on some Photographs of Lightning and of ' " Black " 

 Electric Sparks. By A. W. Clayden*. 



DURING the thunderstorm on the night of June 6 I ex- 

 posed several plates in the hope of securing photographs 

 of lightning. Three of these gave results. 



One was exposed to two flashes, not counting such as did 

 not cross the field of view. These two flashes show compli- 

 cated and beautiful structure. One of them is a multiple flash, 

 distinctly seen as double by both my wife and myself. An 

 enlargement of this shows curious flame-like appendages 

 pointing upwards from every angle. The other flash is a 

 broad ribbon. The images of the masonry in the left-hand 

 corner (which are necessarily slightly out of focus) show 

 three positions of the camera. They are sharp, hence the 

 camera did not move during the existence of a flash ; and the 

 directions of those movements which did occur do not in any 

 way correspond to the movements (if such there were) which 

 would have been required to produce the ribbon-like effect 

 from a linear flash. 



A second plate shows four flashes, and the camera moved 

 much more than in the first case. None of these flashes are 

 ribbons. Development showed the plate to be overexposed. 



The third was exposed to six flashes ; that is to say, I 

 judged that six of them crossed the field of view. There 

 were many others between times, which were either in the 

 clouds or occurred in other parts of the sky. One flash, I 

 remarked at the time, must be " right down the middle of the 

 plate.'''' Development showed this plate to be very much 

 overexposed, and the image required careful nursing. I was 

 much surprised to see nothing but one triple flash in the 

 corner. I supposed that I must have mistaken the plate, and 

 was about to throw it away, but on carefully searching for 

 the above-mentioned vertical flash, I found its image was 

 reversed, printing as a black flash with a white core. Sub- 

 sequent observation showed other dark flashes;, and the enlarge- 

 ment of part of the plate shows that there are indications of 

 white cores to each of them. 



Now the connexion between this reversal and overexposure 

 was very striking. Hence it occurred to me that the black 

 flashes might be due to a sort of cumulative action. Not to 

 the excessive brightness of the individual flashes, but rather 

 to the excessive action produced by the superposition of the 



* Communicated by the Physical Society : read June 22, 1889. 



