Page:Popular Science Monthly Volume 49.djvu/324

308 round to avoid doing this. The arrangement adopted, when it is desired to obtain both positive and negative discharges simultaneously



on the same plate, and when connecting sparks from positive to negative are desired, is similar, only in these cases the two discharging points are placed on the film side of the plate. Many very beautiful photographs of lightning flashes have been made. They are not at all difficult to get. It is only necessary to wait for a suitable stormy night, point the camera containing the sensitive plate in the direction of the storm, remove the cap, and await the flash. The photographer of lightning flashes is sometimes startled to obtain in his finished picture streaks of lightning, some white and others black, the latter being produced by the flashes which were so actinic as to produce upon the sensitive film the phenomenon known as "reversal." One of the most interesting photographs of lightning flashes that I have seen is that reproduced above. It has been kindly sent to me by Prof. Ch. von Zenger, the renowned meteorologist. It was made on May 20, 1894, when a terrific storm, of short duration fortunately, broke over the town of Prague. The lightning depicted in the photograph struck four houses at once, doing considerable damage. It will be noticed that descending from the clouds are six discharges; one flash can