It is obvious that if a discharge of lightning is not instantaneous, but has a sensible duration, the rotary movement of the camera, arranged as just mentioned, will spread out the image of the flash, on the photographic plate, into a more or less broad band or ribbon. Most photographs of ribbonlike streaks of lightning made with ordinary cameras are, in fact, due to accidental movements of the apparatus during exposure--such as an involuntary start of the operator, in case the camera is held in the hands--though a certain amount of spreading of the image is sometimes caused by what photographers call “halation.” Pictures taken with the revolving camera show that some flashes are practically instantaneous while others may last as long as half a second or more. Those of the latter class nearly always show several parallel streams of light, more or less distinctly separated by darker spaces. Each of these bright streams represents a separate discharge along the common path. As the speed with which the camera turns is known, it is possible to determine the intervals of time between the discharges of a multiple flash. These intervals may vary from a few thousandths to one or two-tenths of a second, while the duration of each of the consecutive discharges is probably not more than two or three hundred-thousandths of a second in most cases. Sometimes the path of the lightning flash is shifted by the wind while the picture is being taken. In one case this shift was estimated at 36 feet. Photography is also applied to determining the distance of a lightning flash and hence the dimensions of any of its features. For this purpose a stereoscopic method is used, two cameras being mounted side by side and exposed at the same time. Sometimes one of the cameras is made to revolve, while the other remains stationary. The stationary camera will then show the relative positions of the flashes occurring during exposure, while the moving camera will indicate the times at which they occurred. Streaks of “black lightning” and black borders of the white flashes, both often seen in photographs, are a trick of the camera and are due to what is called the “Clayden effect.” Some kinds of plates are much more susceptible to this effect than others. When a flash of lightning has registered its impression on such a plate, and, before the shutter is closed, another flash occurs, the general illumination of the field by light reflected from clouds, etc., often “reverses” the original image, and consequently it prints black.