PHOTOGRAPHING A STREAK OF LIGHTNING. 753 



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 



Fio. 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 ob- 

 tained 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- 

 VOL. xsiv. — 48 



