Rule a short line upon a slate, and upon another slate rule another line, one-eleventh longer than the first: a person possessing what is called “a true eye,” may perceive the difference in length, even though fifty or sixty seconds elapse between looking at the first and the second lines. If they differ only one-twentieth, then an interval of thirty-five seconds may elapse without destroying the judgment; but, if it be longer, the estimate will be incorrect. When the difference between the lines amounts only to one-fiftieth, an interval of three seconds between the examination of each, is the longest that can be allowed without interfering with the correctness of the comparison.
VISUAL DECEPTION.
Let a room be only lit by the feeble gleam of a fire, almost extinguished, and the eye will see with difficulty the objects in the apartment, from the small degree of light with which they happen to be illuminated. The more exertion is made to ascertain what these objects are, as by fixing the eye more steadily upon them, the greater will be the difficulty in accomplishing it. The eye will be painfully agitated, the object will swell and contract, and partly disappear, but will again become visible when the eye has recovered from its delirium.
HAND-WRITING UPON THE WALL.
Cut the word or words to be shown, out of a thick card or pasteboard, place it before a lighted lamp, and the writing will be distinctly seen upon the wall of the apartment.
IMITATIVE HALOES.
Look at a candle, or any other luminous body, through a plate of glass, covered with vapour, or dust in a finely divided state, and it will be surrounded with a ring of colours, like a halo round the sun or moon. These rings increase with the size of the particles which produce them; and their brilliancy and number depend on the uniform size of these particles.
Or, haloes may be imitated by crystallizing various salts upon thin plates of glass, and looking through the plate at a candle or the sun. For example, spread a few drops of a strong solution of alum over a plate of glass so as to crystallize quickly, and cover it with a crust scarcely visible to the eye. Then place the eye close behind the smooth side of the glass plate, look through it at a candle, and you will perceive three fine haloes at different distances, encircling the flame.