“‘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.
“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put it in this way,” said the major: “that as the image remains the eighth of a second on the retina, after it has been withdrawn from the eye, a revolution of eight times in a second will secure its uninterrupted continuance.”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]]