“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.” “Sir David Brewster, in his late work on Natural Magic, has given us a beautiful illustration of the same principle; it is as follows,” said Mr. Seymour. “A figure dressed in _black_ and mounted upon a _white_ horse, was riding along exposed to the bright rays of the sun, which through a small opening in the clouds was throwing its light only upon that part of the landscape. The _black_ figure was projected against a white cloud, and the white horse shone with particular brilliancy by its contrast with the dark soil against which it was seen. A person interested in the arrival of such a stranger, had been for some time following his movements with intense anxiety; but upon his disappearance behind a wood, was surprised to observe the spectre of the mounted stranger in the form of a _white_ rider upon a _black_ steed, and this spectre was seen for some time in the sky, or upon any pale ground to which the eye was directed.” “I cannot understand why the spectre should be opposite in colour to the real image,” observed Tom. “It is a law of vision which you must for the present take for granted,” answered his father; “that such is the fact, however,” continued he, “I will satisfy you by a very simple experiment.” Mr. Seymour here placed three different coloured wafers in a triangular form on a piece of white paper, and instructed his son to fix his eyes steadily upon them for a minute; this having been performed, he next directed him to turn them from the wafers to a blank part of the paper, and to tell him what he saw. “I see,” exclaimed Tom, “three spectral wafers, but the colours are different; the red wafer is represented by a _green_, the violet by a _yellow_, and the orange by a _blue_.”