By means of the powerful machine presented to the Royal Institution by Mr. Wimshurst, used in conjunction with a battery of Leyden jars, the Young effect has been successfully shown to a large assembly. But it is quite easy to demonstrate it on a small scale with any influence machine which will give a spark about an inch long. One of the terminals of the machine should be connected by a wire with the inner coating of a half-pint Leyden jar, the other with the outer coating, and the discharging balls should be set a quarter of an inch apart. The observer's eyes must be shielded from the direct light of the spark by any convenient screen, such as a large book set on end. The best object for the experiment is a sheet of white paper, placed in an upright position a few inches away from the terminals of the machine and exposed to the full light of the discharge. The room being darkened, let the machine be worked slowly, while the eyes are turned towards the white paper. This will be seen for a moment when the spark passes, and, after a dark interval of about one-fifth of a second, it will make another brief appearance. After a further short interval of darkness, a second recurrent image will often be seen. It may be remarked that the effect is most striking when the eyes are not directed exactly upon the white paper, but above or on one side of it; the proper distance of the paper from the spark-gap should be found by trial. Under favourable conditions I have observed as many as six or seven reappearances of an object which was illuminated by a single discharge. These followed one another at the usual rate--about five in a second--and produced a twinkling or quivering effect, closely resembling that attending a flash of lightning which is not directly seen. There can indeed be little doubt that the proverbial quiver of the lightning-flash is in many cases merely an effect of recurrent vision, though sometimes, of course, as has been shown by photographs, the discharge is really multiple.