Q. If separate figures (as a man and a horse) be drawn on separate sides of a card, and the card twisted quickly, the man seems to be seated on the horse,—Why is this?
A. Because the image of the horse remains upon the eye till the man appears.
The Thaumatrope is constructed on this principle.
Q. Why do the stars twinkle?
A. Fixed stars are so far off, that their rays of light do not strike upon the eye in a continuous flow, but at intervals: when their rays reach the eye, the star becomes visible, and then is obscured till the next batch of rays arrive; and this perpetually occurring, makes a kind of twinkling.
Q. If we look at a red-hot fire for a few minutes, why does every thing seem tinged with a bluish green colour?
A. Because bluish green is the “accidental colour” of red: and if we fix our eye upon any colour whatsoever, when we turn aside, we see every object tinged with its accidental colour.
Q. If we wear blue glasses, (when we take them off,) every thing appears tinged with orange,—Why is this?
A. Because orange is the “accidental colour” of blue: and if we look through blue glasses, we shall see its “accidental colour,” when we lay our glasses aside.