The following beliefs regarding vision meet with very general acceptance:—

First. Very intense light of any wave-length produces practically the sensation of whiteness.

Second. Very weak light of any wave-length produces practically the sensation of whiteness.

Third. A coloured area of normal brightness retains its colour, though with diminished purity, when gazed at steadily for a long time.

Fourth. Small, faintly luminous, objects are seen more easily by indirect vision than by direct vision.