The observations were first made in the following manner. A blackened zinc plate, in which is a small round hole covered with a piece of thin writing-paper, is fixed over a larger opening in a wooden board; thus we are furnished with a sharply-defined translucent disk, which is surrounded by a perfectly opaque substance. An arrangement is provided for covering the translucent disk with a shutter, which can be opened very rapidly by releasing a strong spring. If this apparatus is held between the eyes and a lamp, and the translucent disk is suddenly disclosed by working the shutter, the disk appears for a short time to be surrounded by a narrow red border. The width of the border is perhaps a millimetre (1/25 inch), and the appearance lasts for something like a tenth of a second. Most people are at first quite unable to recognise this effect, the difficulty being, not to see it, but to know that one sees it. Those who have been accustomed to visual observations generally perceive it without any difficulty when they know what to look for, and no doubt it would be very evident to a baby which had not advanced very far in the education of its eyes. The observation is made rather less difficult by a further device. If the disk is divided into two parts by an opaque strip across the middle, it is clear that each half disk will have its red border, and if the strip is made sufficiently narrow, the red borders along its edges will meet or perhaps overlap, and the whole strip will, for a moment after the shutter is opened, appear red. A disk was thus prepared by gumming across the paper a very narrow strip of tinfoil. The effect produced when such a disk is suddenly exposed is indicated in Fig. 45, the red colour being represented by shading. [Illustration: _Fig. 45.--Demonstration of Red Borders._] 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.