It consists of a disc of blackened tin plate, which is made to revolve on its axis in the manner above exhibited. A narrow opening extends from the circumference to the centre as seen at A. If a device of any kind, as a star (which for increasing the beauty of the experiment ought to be transparent, and illuminated with a lamp) be placed behind the disc, it is evident that, as long as the circle remains at rest, no other part of the figure can be visible than that which is immediately behind the slit A, but the instant it is put into rapid motion, the whole of the star will be seen, as exhibited in figure B.
Mr. Seymour observed that the explanation of the phenomenon was obvious;--each successive portion of the figure seen through the opening remains on the eye, until the circle has completed its entire revolution.
“This experiment,” continued Mr. Seymour, “admits of a very curious modification, which I shall now proceed to exhibit.”
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.”