I mentioned just now that an impression made on the retina lasts for the tenth of a second after the cause of it has been removed. Now, when this disk is in rotation, the sectors follow one another so rapidly that the particular part of space occupied at any moment by a white sector will be occupied by a black sector within a time much less than the tenth of a second. It follows that the impression made by each white sector remains on the retina until the following black sector comes into the same position; and, in like manner, the impression made by each black sector remains until the following white sector takes up the position of the black. Therefore, the impression made by the whole outer rim is the impression of black and white combined--that is, the impression of gray. So far, I dare say, the phenomenon is already familiar to you all. But I propose now to show you the revolving disk illuminated by the electric spark; and you will observe that, at the moment of illumination, the black and white sectors come out as clearly and distinctly as if the disk were standing still. For the success of this experiment it is desirable, not only to have a brilliant spark in order to secure a good illumination of the disk, but also to have a succession of such sparks, that you may see the phenomenon frequently repeated, and thus be able to observe it at your leisure. To attain these two objects, I have made the arrangement which is here before you. In front of the disk is a large and very powerful Leyden jar. The rod connected with the inner coating rises well above the mouth of the jar, and ends in a brass ball nearly opposite the centre of the disk. Connected with the outer coating of the jar is another rod which likewise ends in a brass ball, and which is so adjusted that the distance between the two balls is about an inch. The two rods are connected respectively with the two conductors of a Holtz machine, so that, when the machine is worked, the jar is first quickly charged, and then it discharges itself, with a brilliant spark, between the two brass balls. Thus, by continuing to work the machine, we can get, as long as we choose, a succession of sparks following one another at short and regular intervals right in front of the disk. Everything being now ready, and the room partially darkened, the disk is put in rapid rotation; and you can see, by the twilight that remains, the outer rim a uniform gray, and the central space white. But when my assistant begins to turn the Holtz machine, and brilliant sparks leap out at intervals, the revolving disk, illuminated for a moment at each discharge, seems to be standing still, and shows the black and white sectors distinctly visible.