Curiosities of Light and Sight

by Shelford Bidwell

Fig. 47.—Disk for Red Borders.

We may go a step further. Cut out a disk of white cardboard, divide it into two equal parts by a straight line through the centre, and paint one half black.[13] At the junction of the black and white portions cut out a gap, which may conveniently be of the form of a sector of 45°. (See [Fig. 47].) Stick a long pin through the centre and hold the arrangement by the pointed end of the pin a few inches above a printed page near a good light. Make the disk spin at the rate of about five or six turns a second by striking the edge with the finger. As before, the letters when seen through the gap will appear red, and persistence will render the repeated impressions almost continuous so long as the rotation is kept up; any one seeing the printing for the first time through the rotating disk would believe that it was done with red ink. Care must be taken that the disk does not cast a shadow upon the page, and that the intensity of the illumination is properly adjusted. I have devised several rather more elaborate contrivances for making the disks rotate at a uniform speed; one of these is shown in [Fig. 50].

In none of these experiments does an extended black surface ever appear red, but only black dots or lines. And the lines must not be too thick; if their thickness is much more than a millimetre (¹⁄25 inch), the lines, as seen by an observer from the usual distance for reading, do not become red throughout, but only along their edges. The red appearance does not in fact originate in the black lines themselves: these serve merely as a background for showing up the red border which fringes externally the white portions of the paper, and the width of this border does not exceed about one-fifth of a degree. But by employing a sufficiently large disk and selecting designs or letters composed of lines of suitable thickness, the colour effect has been shown to a large audience.

When the disk is turned in the opposite direction, so that the gap is preceded by white and followed by black, the lines of the design appear at first sight to become dark blue instead of red. Attentive observation, however, shows that the apparently blue tint is not formed upon the lines themselves, as the red tint was, but upon the white ground just outside them. This introduces to our notice another border phenomenon, which seems to present itself when a dark patch is suddenly formed on a bright ground, for that is essentially what takes place when the disk is turned the reverse way. I made some attempts to obtain more direct evidence that such a dark patch appeared for a moment to have a blue border, and after some trouble succeeded in doing so.

A circular aperture was cut in a wooden board and covered with white paper; a lamp was placed behind the board, and thus a bright disk was obtained, as in the former experiment. An arrangement was prepared by means of which one half of this bright disk could be suddenly covered by a metal shutter, and it was found that when this was done a narrow blue band appeared on the bright ground just beyond and adjoining the edge of the shutter when it had come to rest. The blue band lasted for about a tenth of a second, and it seemed to disappear by retreating into the black edge of the shutter. The phenomenon is illustrated in [Fig. 48], where the shaded band indicates the blue border.

Fig. 48.—Demonstration of Blue Border.

We have then to account, if possible, for the two facts that, in the formation of these transient colour-borders, the red sensation occurs in a portion of the retina which has not itself been exposed to the direct action of light, while the blue occurs in a portion which is steadily illuminated, both colour sensations being referred to localities adjacent to those in which a change of illumination has suddenly taken place. Accepting the Young-Helmholtz theory of colour vision, the effects must, I think, be attributed to a sympathetic affection of the red nerve fibres. When the various nerve fibres occupying a limited portion of the retina are suddenly stimulated by white light (or by any kind of light which contains a red constituent) the immediately surrounding red nerve fibres are for a short period excited sympathetically, while the violet and green fibres are not so excited, or in a much less degree. And again, when light is suddenly cut off from a patch in a bright field, there occurs a sympathetic insensitive reaction in the red fibres just outside the darkened patch, in virtue of which they cease for a moment to respond to the luminous stimulus; the green and violet fibres, by continuing to respond uninterruptedly, give rise to the sensation of a blue border.

It is perhaps desirable to refer briefly to another proposed explanation of the phenomenon, which occurred to myself at an early stage of the investigation, and has since been suggested by many different persons. The explanation in question is of a purely physical character, and depends upon the non-achromatism of the eye.