Houston.] ^°® [Nov. 20, 



occurred in Philadelphia and vicinity on the 29th of Octoher, 1885. The 

 exposure was made at 7 p. u. The thick clouds produced pronounced 

 darkness. At the same time the rain was heavy, and the wind high. 

 Considering the circumstances, the negatives secured were very good. 



TLe circumstances of the exposure were as follows, viz : the camera 

 was placed in an open window, and pointed towards the stahle, its focus 

 for this point having heen previously obtained. The slide was then drawn 

 and the plate left exposed to the night until a lightning flash came. This 

 occurred in less than one minute, when the slide was instantly closed, the 

 plate holder reversed, and another exposure obtained by means of the 

 illumination of the next flash. 



The plates were developed during the evening. The results obtained 

 were, in Mr. Barker's judgment, about equal to what would have been 

 secured by an exposure of about ^ 5 of a second in bright sunlight at 

 noon. 



The plates used were exceedingly sensitive gelatine films. A compara- 

 tively large diaphragm was employed in these exposures. 



The circumstances under which these exposures were taken were such 

 as thoroughly prevented any illumination of the objects save by the flash 

 itself. The room in which the camera was placed was of course quite 

 dark. 



Apart from the interest attached to Mr. Barker's photographs as 

 evidence of the recent advances made in what is generally called instan- 

 taneous photography, they appear to present considerable value in the 

 light they throw on the question of the duration of the ordinary light- 

 ning flash. 



The views generally held as regards the duration of the lightning flash 

 is, that it is practically, if not actually, instantaneous. From experiments 

 made by Wheatstone and others, the duration of a flash, as deter- 

 mined by means of a rapidly revolving disc, it is generally believed to be 

 from the T oVo> to the T to57 °f a second. Whatever may have been the 

 duration of the flashes thus measured, it would appear probable that 

 flashes of great severity, where the discharge traverses many miles of air, 

 would, under many circumstances, continue for quite an appreciable time. 



Mr. Barker's photographs appear to show that this was the case during 

 the night in which they were taken. While the fixed objects, such for 

 example as the stable, came out quite sharply, the trees show unmistak- 

 able evidences of violent motion. It is true that these trees were not in 

 sharp focus, being nearer the camera than the stable. Though somewhat 

 blurred, they nevertheless exhibit unmistakable signs of having percept- 

 ibly changed their position during the time of exposure. In other words, 

 the plate was illumined for a sufficient length of time to permit the mo- 

 tion to be clearly shown. The lightning flash, therefore, was not instan- 

 taneous in the sense generally attributed to it, but continued to illumine 

 the plate for quite an appreciable time. 



It would be interesting for the photographic experiments of Mr. Barker 



