The Thaumatrope; being _Rounds_ of Amusement, or How to please and surprise By _Turns_. Mr. Seymour now proceeded to explain more fully the optical theory of the instrument, which neither Louisa nor Tom could, as yet, thoroughly understand. He told them that an object was seen by the eye, in consequence of its image being delineated on the retina, or optic nerve, which is situated on the back part of the eye; and that it had been ascertained, by experiment, that the impression which the mind thus receives, lasts for about the eighth part of a second after the image is removed. “It is, therefore, sufficiently evident,” said Mr. Seymour, “that if any point, as a lighted stick, be made to revolve, so as to complete the circle in that period, we shall not see a fiery point, but a fiery circle; because the impression made by it in every point of its circuit will remain until it comes round again to the spot from which it set out;--but we will, at once, exemplify this fact by an experiment.” Tom was accordingly directed to procure a piece of stick and a candle; and as soon as they were brought into the room, Mr. Seymour ignited the end of the stick and whirled it round, when a bright circle, without any intervals of darkness, was seen by the whole party. “Never until this instant,” exclaimed the vicar, with an expression of high satisfaction, “did I fully appreciate the beauty of that passage in Milton, wherein the poet evidently describes the rapidity of Satan’s flight, as well as the refulgence of his appearance-- “‘Sprung upward like a pyramid of fire.’ “Now to take in the full meaning of this figure,” continued Mr. Twaddleton, “we must imagine ourselves in chaos, and that a vast luminous body is rising near the spot where we may be supposed to be standing, so swiftly as to appear a continued track of light, and lessening to the view, according to the increase of distance, until it ends in a point, and then disappears; and all this must be supposed to strike our eye at one instant.” “It is very probable,” said Mr. Seymour, “that the poet had such an idea in view, and that he intended by it to convey the immense rapidity of Satan’s flight. Homer makes use of the same figure to express the velocity of the javelin, δολιχοσκιον εγχος, the ‘_long shadowed_’ javelin. We shall have ample proof of the effect of this power in the eye of retaining impressions, and of thus converting points into lines and circles, during the exhibition of your fire-works; and which, in fact, derive the greater part of their magical effect from it.” “The pin wheel is certainly nothing more than a fiery circle produced by the rapid revolution of a jet of flame,” said the vicar.