“Impossible,” exclaimed the vicar--“let me have a peep. Why, I declare, they appear, as you say, stationary, although I know them to be in rapid motion; as sure as fate I shall become a Cartesian.”
The Major, Louisa, and Mrs. Seymour, were all equally surprised, and incapable of giving any explanation of the phenomenon they had witnessed.
“Let us remember,” said Mr. Seymour, “that in viewing the image through the slit in the revolving disc, we catch but a momentary glance as it passes before the eye, and that the image thus produced on the retina is retained until the next revolution again brings the slit into the same position. Now it is evident, that before the eye can ascertain a body to be in motion, it must observe it in two successive portions of time, in order to compare its change of place;[[68]] but in the experiment under consideration, the glance is momentary, the wafer is no sooner seen than it passes away; its figure alone is impressed upon the retina, and this impression is continued without any change, until the circle completes its round, and consequently the image must appear at rest.”
“I understand you; the figure, but not the motion, of the wafer, is discernible in the short period during which it is visible through the slit,” observed the vicar.
“I lately witnessed a beautiful illustration of this subject at the Royal Institution,” said Mr. Seymour. “A number of cogged wheels, cut out of pasteboard, were set in motion in a perfectly dark room, when occasional flashes of light from an electric battery, displayed their forms most distinctly, and yet although whirling round at the time, they appeared to the spectator as motionless as so many solid blocks of marble. In like manner, in a storm during the darkness of midnight, the rolling ship and waves, when rendered visible by flashes of lightning, will appear as completely at rest, as a representation of them upon the canvass.[68a] I may at some future time extend this interesting subject, by exhibiting some optical illusions produced by the revolution of wheels in different directions, and at different velocities, for the knowledge[[69]] of which we are indebted to Mr. Faraday, and in mentioning that distinguished philosopher,” added Mr. Seymour, addressing himself more particularly to the vicar, “I cannot avoid remarking, that if Philosophy in Sport can be made Science in Earnest, the juvenile lectures delivered by that professor have established the converse proposition, that the sternness of Science may be relaxed into the engaging aspect of Sport.”
“Before quitting this subject,” continued he, “I have yet another toy in store for your amusement; it is founded upon the optical principle which I have every reason to believe you now thoroughly understand.” A square box was then produced containing a number of card discs, the edges of which exhibited a series of notches corresponding with the figures delineated on their margins. That the reader may better understand their construction, as well as the explanation of Mr. Seymour, the following representation has been introduced.
“To exhibit the magical effects of this toy,” said Mr. Seymour, “I will, by means of the spindle to which it is attached, cause it to revolve rapidly before the looking-glass, and you shall view the reflection through the openings.”
Each member of the party obeyed in succession the direction thus given, and severally expressed the great astonishment they felt, at observing the figures in constant motion, and exhibiting the most grotesque attitudes.
“Now,” said Mr. Seymour, “attend to my explanation. Each figure is seen through the aperture, and as it passes and is succeeded in rapid succession by another and another, differing from the former only in attitude, the eye is cheated into the belief of its being the same object successively changing the position of its body. Consider what takes place in an image on the retina when we actually witness a man in motion; for instance, a man jumping over a gate, in the first moment he appears on the ground, in the next his legs are a few inches above it, in the third they are nearly on a level with the rail, in the fourth he is above it, and then in the successive moments he is seen descending as he had previously risen. A precisely similar effect is produced on the retina, by the successive substitution of figures in corresponding attitudes, as seen through the orifices of the revolving disc; each figure remaining on the retina long enough to allow its successor to take its place without an interval that would destroy the illusion.”