A simpler apparatus is, however, quite sufficient for showing the phenomenon,[12] and with practice one can even acquire the power of seeing it without any artificial aid at all. I have many times noticed flashes of red upon the black letters of a book that I was reading, or upon the edges of the page: bright metallic, or polished objects often show it when they pass across the field of vision in consequence of a movement of the eyes, and it was an accidental observation of this kind which suggested the following easy way of exhibiting the effect experimentally.
An incandescent electric lamp was fixed behind a round hole in a sheet of metal which was attached to a board. The hole was covered with two or three thicknesses of writing paper, making a bright disk of nearly uniform luminosity. When this arrangement was moved rather quickly either backwards and forwards or round and round in a small circle, the edge of the streak of light thus formed appeared to be bordered with red.
If this experiment is performed with a strong light behind the paper, the streak becomes bordered with greenish-blue instead of red. With an intermediate degree of illumination, both blue and red may be seen together.
Most of the effects that have so far been described were produced by transmitted light, but reflected light will show them equally well. If you place a printed book in front of you near a good lamp and interpose a dark screen before your eyes, then, when the screen is suddenly withdrawn, the printed letters will for a moment appear red, quickly changing to black. Some practice is required before this observation can be made satisfactorily, but by a simple device it is possible to obliterate the image of the letters before the redness has had time to disappear; the colour then becomes quite easily perceptible.
Hold two screens together side by side, a black one and a white one, in such a manner that an open space is left between them. (See [Fig. 46].) In the first place let the black screen cover the printing; then quickly move the screens sideways so that the printed letters may be for a moment exposed to view through the gap, stopping the movement as soon as the page is covered by the white screen. During the brief glimpse that will be had of the black letters while the gap is passing over them, they will, if the illumination is suitable, appear to be bright red.
Fig. 46.—Black and White Screens.
