A. DE MORGAN.
Optical Phenomenon.
—I shall be much obliged to anybody who will explain a phenomenon which I have observed.
Suppose 1. A street from twenty to thirty feet broad.
2. At the open window of a house on one side stands a man looking at the corresponding window of the house on the opposite side; that is, he looks at what was a window, but is now filled up with a large board that is covered with an inscription of short lines, black on white; in short, just such a board as one sees at a turnpike gate.
3. From shortness, or defect, of sight (I cannot say which), the man is unable to read the inscription as he stands at his window.
4. He sits down on a low seat, so as to bring his eye almost close to, and just on a level with, the sill of his own window. He then slowly raises and depresses his head. As he does this, it of course appears to him as if his own window-sill travelled up and down the board opposite.
5. In doing so it comes successively under each line of the inscription.
6. As it does so, that one line becomes perfectly legible.
N. B.