Three coloured wafers were then placed, at equal distances from each other, on the disc, and the instrument having been arranged before a looking-glass, the party were desired to observe the reflected image as the circle revolved. “The wafers are blended into one continuous zone,” observed Mrs. Seymour. “To be sure,” said Louisa; “upon the same principle that the ignited stick appears as a fiery circle.” “It would be very strange, after the different experiments we have seen, if we were not able to explain the present appearance,” observed Tom. “The fiery circle produced by the revolving stick is a much better illustration of the principle; I do not see what object Mr. Seymour has in thus multiplying his experiments,” said the vicar. “Gently, if you please, Mr. Twaddleton, and, before you favour us with your criticism, wait until I have concluded my experiment. You have seen that the reflected image of the revolving wafers appears as a continuous zone, and you have very correctly explained the reason of such an appearance; but I must now request you to inspect the reflected image through the slit in the disc, as it revolves, and say what new effect you observe.” “How very strange!” exclaimed Tom; “I see the three wafers very distinctly, and perfectly at rest.” “Impossible,” exclaimed the vicar--“let me have a peep. Why, I declare, they appear, as you say, stationary, although I know them to be in rapid motion; as sure as fate I shall become a Cartesian.” The Major, Louisa, and Mrs. Seymour, were all equally surprised, and incapable of giving any explanation of the phenomenon they had witnessed. “Let us remember,” said Mr. Seymour, “that in viewing the image through the slit in the revolving disc, we catch but a momentary glance as it passes before the eye, and that the image thus produced on the retina is retained until the next revolution again brings the slit into the same position. Now it is evident, that before the eye can ascertain a body to be in motion, it must observe it in two successive portions of time, in order to compare its change of place;[68] but in the experiment under consideration, the glance is momentary, the wafer is no sooner seen than it passes away; its figure alone is impressed upon the retina, and this impression is continued without any change, until the circle completes its round, and consequently the image must appear at rest.” “I understand you; the figure, but not the motion, of the wafer, is discernible in the short period during which it is visible through the slit,” observed the vicar.