=Wheatstone’s Experiments.=--These results are extremely interesting; but we can hardly regard them as finally established, until they have been confirmed by other observers. I may remark, however, that they fit in very well with the experiments made by Professor Wheatstone, many years ago, on the duration of the electric spark, which, as I told you, is a miniature flash of lightning. In these classical experiments, which leave nothing to be desired in point of accuracy, Professor Wheatstone showed that a spark taken directly from a Leyden jar, or a spark taken from the conductor of a powerful electric machine, that is, just such a spark as you have seen here to-day, lasts for less than the millionth of a second. But he also showed that the duration of the spark is greatly increased, when a resisting wire is introduced into the path of the discharge. Thus, for example, when the discharge from a Leyden jar was made to pass through half a mile of copper wire, with breaks at intervals, the sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀ of a second.[7] Hence we should naturally expect that the period of illumination would be still further increased, in the case of a flash of lightning, where the resistance interposed is enormously greater than in either of the experiments made by Wheatstone.[8] =Experiment of the Rotating Disk.=--It would be tedious, on an occasion like the present, to enter into an account of Wheatstone’s beautiful and ingenious method of investigation, by which the above facts have been established; but I will show you a much more simple experiment which brings home very forcibly to the mind how exceedingly short must be the duration of the electric spark. Here is a circular disk of cardboard, the outer part of which, as you see, is divided into sectors, black and white alternately, while the space about the centre is entirely white. The disk is mounted on a stand, by means of which I can make it rotate with great velocity. When it is put in rotation, the effect on the eye is very striking--the central space remains white as before, but in the outer rim the distinction of black and white absolutely disappears and gives place to a uniform gray. This color is due to the blending together of black and white in equal proportions; the blending being effected, not on the cardboard disk, but on the retina of the eye. [Illustration: CARDBOARD DISK AS SEEN WHEN AT REST.] [Illustration: SAME DISK AS SEEN WHEN IN RAPID ROTATION.]