The reason of this is clear: So brief is the moment for which the spark endures, that the disk, though in rapid motion, makes no sensible advance during that small fraction of time; therefore, in the image on the retina, the impression made by the white sectors remains distinct from the impression made by the black, and the eye sees the disk as it really is. I may notice, in passing, a very interesting consideration, suggested by this experiment. A cannon ball is now commonly discharged with a velocity of about 1,600 feet a second. Moving with this velocity it is, as you know, under ordinary circumstances, altogether invisible to the eye. But suppose it were illuminated, in the darkness of night, by this electric spark, which lasts, we will say, for the millionth of a second. During the moment of illumination, the cannon ball moves through the millionth part of 1,600 feet, which is a little less than the fiftieth of an inch. Practically, we may say that the cannon ball does not sensibly change its place while the spark lasts. Further, the impression it makes on the eye, from the position it occupies at the moment of illumination, remains on the retina for at least the tenth of a second. Therefore, if we are looking toward that particular part of space where the cannon ball happens to be at the moment the spark passes, we must see the cannon ball hanging motionless in the air, though we know it is traveling at the rate of 1,600 feet a second, or about 1,000 miles an hour. =Brightness of a Flash of Lightning.=--I should like to say one word about the brightness of a flash of lightning. Somewhat more than thirty years ago, Professor Swan, of Edinburgh, showed that the eye requires a sensible time--about the tenth of a second--to perceive the full brightness of a luminous object. Further, he proved, by a series of interesting experiments, that when a flash of light lasts for less than the tenth of a second, its apparent brilliancy to the eye is proportional to the time of its duration.[9] Now consider the consequence of these facts in reference to the brightness of our electric spark. If the spark lasted for the tenth of a second, we should perceive its full brightness; if it lasted for the tenth part of that time, we should see only the tenth part of its brightness; if it lasted for the hundredth part, we should see only the hundredth part of its brightness; and so on. But we know, in point of fact, that it lasts for less than the millionth of a second, that is, less than the hundred-thousandth part of the tenth of a second. Therefore we see only the hundred-thousandth part of its real brightness.