Yet, in spite of the extreme rapidity of the lightning, it has been possible to determine that these meteors do not last the thousandth part of a second. To prove this, we take a circle of cardboard, divided from the middle into black and white sections. This circle can be turned like a wheel almost as quickly as one can wish. We know that luminous impressions remain on the retina the tenth part of a second; thus, if we imitate the childish game of turning a lighted coal--if the turn is made in the tenth of a second, each successive position of the coal remaining impressed on the retina for the same length of time--we have a continuous circle. In turning our cardboard wheel with the black and white spokes, if each spoke passes before our eyes in less than the tenth of a second, we can no longer distinguish between the sections, but can only see a grey circle. But we can make it rotate a hundred turns or more in a second; this being done, if we continue to observe the circle, we can no longer see the lines, they succeed each other more quickly in our eyes than the impression they produce. But if the circle turns before us in the darkness, and it is suddenly lighted up and as suddenly darkened, the impression produced on our eyes by each of the sections would last less than the tenth of a second, and the circle would appear to us as if it were stationary. In applying a calculated rotation to this contrivance, Charles Wheatstone has proved that some lightning does not last the thousandth part of a second. This measure is probably a minimum; in the majority of cases the duration of lightning is longer than this.
Often during the hot, transparent summer nights, we see a considerable number of flashes, which furrow the firmament with their gentle, bluish light. These fugitive gleams remind us in the sky of the will-o'-the-wisps, which come forth silently from marshy ground. The atmosphere is pure; there are no apparent traces of a storm, and yet the sky is glistening with thousands of small flames. The flashes succeed one another almost without interruption. These electric sparks are known as heat-lightning, but this is quite inaccurate, and has no meaning in the language of modern science.