the same figure to express the velocity of the javelin, dxxxIov EYxos, 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 fireworks; 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.

"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major: "that as the image remains the eighth of a second on the retina, after it has been withdrawn from the eye, a revolution of eight times in a second will secure its uninterrupted continuance."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*, 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.'Ӡ

"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 + Ibid. p. 100.

* Vol. I. p. 275.

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 eye-balls 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."

"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, pro

"Ocular

* Those who are desirous of gaining farther information upon this subject may consult the chapter on Spectra," in Dr. Darwin's Zoonomia.

bably, acquainted with the opinions of Eusebius, Fabricius, and Dr. Lardner, upon this alleged miracle."

"I admire the ingenuity of your theory," said the vicar," and am not prepared to question its truth. But see, here comes Mrs. Sey

"If I shall not interrupt your discussions," said the lady, "I will now exhibit my proposed improvements."

"Believe me, madam, we are all impatient to witness your classical suggestions," replied the vicar.

"Behold, then, the Trojan ships!"

66

Ay, ay, sure enough; but let me see, are their forms according to ancient authority? Very well, indeed, Mrs. Seymour. Very well; the poops have the bend so accurately described by Ovid and Virgil—'Puppesque recurvæ,' as the poet has it. And there is the triton; but is its size in proportion to the vessel? Yes, madam, you are doubtless correct, the figure is generally represented of considerable magnitude on ancient medals; and Silius Italicus, if my memory serves me, alludes to the weight of the image having