Page:Popular Science Monthly Volume 24.djvu/773

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given for estimating the length of the luminous train, which, in one instance, is calculated to be seventeen hundred metres, or more than a mile.

Wheatstone demonstrated by direct experiments of great ingenuity that single flashes of lightning do not last more than a millionth of a second. We may judge from this of the wonderful sensibility of the new gelatine-bromide plates which permit the taking of correct views under these conditions.

M. Haensel has given a short account of the circumstances under which his photographs were taken and of the processes he employed. On the 6th of July, 1883, during a storm, when the sky was traversed

PSM V24 D773 Overlapping photo exposures of two lightnings.jpg

Fig. 2.

by frequent flashes of lightning, he turned his instrument at about ten o'clock in the evening toward that point whence the strongest flashes seemed to issue. The apparatus was furnished with the most sensitive gelatine-bromide plates, and the flash left its own impression upon them as it was formed. Out of ten plates that were exposed, he obtained only four photographs, of two of which we here give exact copies, taken from heliographic reproductions by M. Gillot, of Paris. The first figure represents two flashes. In the left one will be observed a double spark, which also appears triple in the middle. Simulta-