“And the rocket,” added Mr. Seymour, “is a column of light occasioned by the same rapid movement of a burning body in a rectilinear or curved direction.” “I perfectly understand all that you have said,” observed Tom. “Then you will not have any difficulty in explaining the action of the Thaumatrope, for it depends upon the same optical principle; the impression made on the retina by the image, which is delineated on one side of the card, is not erased before that which is painted on the opposite side is presented to the eye; and the consequence is, that you see both sides at once.” “Or, you might put it in this way,” said the major: “that as the image remains the eighth of a second on the retina, after it has been withdrawn from the eye, a revolution of eight times in a second will secure its uninterrupted continuance.” “On turning round the card,” observed Louisa, “I perceive that every part of the figure is not equally distinct.” “Because every part of the card does not revolve with the same velocity,” said her father; “and this fact offers a good illustration of what I formerly stated,[65] that in circular motion, the parts more remote from the axis of rotation are those which move with the greater velocity. This toy will also be found capable of exemplifying another truth to which I have before alluded, that ‘the axis of motion remains at rest while all the parts revolve round it.’”[66] “I remember that very well,” exclaimed Tom. “Then take the card, and spin it between yourself and the window, and tell me what you observe,” said his father. “I see a dark line across the window; and what is very strange, the other parts of the card appear transparent; for they do not obstruct the view of the window, as they would if the card were at rest.” “The dark line you see is the axis of rotation, which being stationary, necessarily excludes the light; the other parts being in motion do not remain a sufficient time to obliterate the image made on the eye by the window. It is true that the card disc passes between your eye and the light, but as it does not continue at any one point for more than the eighth of a second, there is no more apparent intermission of the light than what occurs during the winking of the eyes.” “You allude to a very curious fact,” observed the vicar, “that, although we are perpetually covering the eyeballs with our eyelids, we are not conscious of the intervals of darkness.” “The reason of which must surely be obvious from the explanation I have just offered,” said Mr. Seymour: “the sensation of light is not exchanged for that of darkness in so short a period as the twinkling of the eye.” “I admit the plausibility of your theory,” said the vicar; “but it appears to me that objects frequently linger on the sight for a longer period than that which you assign to them. I well remember seeing the flame of a candle for several seconds after it had been suddenly withdrawn from the apartment.”