“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 I well remember seeing the flame of a candle for several seconds after it had been suddenly withdrawn from the apartment.”
“I admit that strong lights frequently continue for some time thus visible in the ‘mind’s eye;’ and it is well known that such impressions are often followed by images of similar shape, but of various colours. In passing from sunshine to a dark room, we frequently witness the appearance of stars and circles of vari-coloured light; but this phenomenon is very distinct from that of the Thaumatrope, and is to be explained upon very different principles.”[[67]]
“I know exactly to what you allude,” said the major: “and I do not doubt but that many of those illusive appearances, which have been described, might be referred to the operation of the same natural cause. It is easy to imagine that a person who has steadfastly fixed his eyes upon an illuminated object, may, for some minutes afterwards, see the same figure in shade; it was from such a cause, no doubt, that Constantine saw the image of a cross in the sky. You are, probably, acquainted with the opinions of Eusebius, Fabricius, and Dr. Lardner, upon this alleged miracle.”
“Yes,” exclaimed the vicar, “and I also know, that this miracle for the conversion of Constantine gave origin to the Catholic custom of illuminating the cross at St. Peter’s in Rome.”