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) theimmediately 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 inquestion is of a purely physical character, and depends upon the non-achromatism of the eye.
Fig. 49.—Disk for experiments on the origin of Colour-borders.
Without going into details, it will suffice to quote a single experiment which is of itself fatal to any such theory. Prepare a disk like that shown inFig. 49, and spin it above a page of printing. The letters beneath the zone which is partly black and partly whitewill, under the usual conditions, turn red, but those beneath the remainder of the disk will retain their blackness. The demarcation is quite definite, and a single printed word may be made to appear red in the middle and black at its two ends. Now it is, of course, impossible that the lenses of the eye should be perfectly accommodated for the letters which appear black, and at the same time seriously out of focus for the others. This explanation, therefore, simple and obvious as it may seem, is altogether untenable.
Whether or not the hypothesis which I have suggested is correct in all its details, it is, I think, sufficiently obvious that the red and blue colours of Benham’s top are due to exactly the same causes as the colours observed in my ownexperiments, for the essential conditions are the same in both cases.
The last curiosity which I will notice is connected with the fact already mentioned, that when the illumination is strong, the transient border-colours are nearly reversed, greenish-blue appearing in place of red, and brick-red in place of blue.
I was for a long time quite unable to imagine any reasonably probable explanation of this circumstance, but a clue was finally obtained from consideration of the fact that greenish-blue is the complementary colour to red, and in a subsequent memoir (Proc. Roy. Soc., vol. 61, p. 269) some experiments were described which show, as I believe conclusively, that the greenish-blue borders seen in a strong light are simply negative after-images of the usual red one.
These negative after-images are of the familiar kind that are observed after one has gazed for some time at a bright coloured object. If a red “wafer” lying upon a sheet of white or grey paper is looked at steadily for about half a minute, and the gaze is then suddenly transferred to some other part of the paper, a greenish-blue ghost of the wafer will be seen. The portion of the retina upon which the red image at first falls becomes fatigued and partially insensible to red light; it is therefore unable to appreciate the red component of the white light afterwards reflected to it by the paper, and the sensation of the complementary colour consequently predominates; hence the greenish-blue ghost, which is called the negative after-image of the wafer.
The new experiments show that, if a certain condition is fulfilled, the usual prolonged stare becomes unnecessary, a momentary glance sufficing to produce a strong but fugitive after-image. The condition is that the part of the retina where the image is to be formed, shall have been darkened immediately before excitation by the bright object. The retinal nerves, when in darkness, rapidly acquire a state of sensitiveness far exceeding the normal average in the light, but quickly diminishing again under the influence of illumination. This peculiar sensitiveness may, indeed, be both gained and lost in a small fraction of a second, and is therefore very favourable for the rapid generation of negative after-images.
Once more making use of the blackand white screens depicted inFig. 46, let the black screen first cover the paper upon which the wafer is lying; this will darken a portion of the retina, and render it sensitive. Then let the screens be quickly moved sideways, so that the wafer, after having been seen for a moment through the opening, may be immediately covered by the white screen. A bright but evanescent greenish-blue ghost will succeed the red impression.
But the most curious thing is that if the illumination is strong, and the screens are moved at the proper speed, no trace of red will be seen at all; it will appear exactly as if the actual colour of the wafer seen through the gap were greenish-blue. I am informed that analogous phenomena have been observed in other branches of physiology; a well-definedreaction sometimes occurs when no direct evidence can be detected of the existence of the excitation to which the reaction must be due.
As in the former experiments, the effect may be shown continuously by means of a rotating disk with an open sector. The annexed diagram (Fig. 50) indicates a convenient apparatus for the purpose. The disk is made of thin metal, and properly balanced; the dark portion of the surface is covered with black velvet, and the light portion with unglazed grey or buff paper. It should turn some six or eight times in a second, while its front is well illuminated either by bright diffused daylight or by a powerful lamp. A red card placed behind the rotating disk is made to appear green, a green card pink, anda blue one yellow, while a black patch painted upon a white ground appears lighter than the ground itself. I have prepared some designs which demonstrate the phenomenon in a very striking manner. One of these is a picture of a lady with indigo-blue hair, an emerald-green face, and a scarlet gown, who is represented as admiring a violet sunflower with purple leaves. Seen through the disk, the lady’s tresses appear flaxen, her complexion a delicate pink, and her dress a light peacock-blue; the petals of the sunflower also become yellow, and its foliage green. Other designs show equally remarkable transformations of colour.
Fig. 50.—Disk for transforming Colours.
I have mentioned only a few among many curious phenomena which have presented themselves in the course of these investigations. It is not improbable that a careful study of the subjectiveeffects produced by intermittent illumination would lead to results tending to clear up several doubtful points in the theory of colour vision.
William Byles & Sons, Printers, 129, Fleet Street, London, and Bradford.
Footnotes:
[1]It should be clearly understood that the length of each wave of a series is measured by the distance between the crests of two successive waves. The length of water-waves which break upon a sea shore is not the length along the crest of a single wave measured in a direction parallel to the shore, as the uninitiated are apt to suppose. The true wave-length, or distance from crest to crest of successive waves, can be well observed from the top of a cliff.
[2]In practice, wave-lengths are expressed in ten-millionths of a millimetre. The wave-lengths of the lines A and H of the solar spectrum, which approximately coincide with the limits of visibility, are 7594 and 3968 ten-millionths of a millimetre.
[3]Possibly the human eye is at present in process of transformation from an inferior type to a different and more perfect one.
[4]It is sometimes necessary to place the lens I on the other side of K.
[5]It is easy to find specimens of red and green glass suitable for this experiment. The proper kind of purple is not so commonly met with.
[6]Some recent experiments on artificial colour-blindness (Proc. Roy. Soc., Feb., 1898) have led Mr. Burch to the conclusion that there are reallyfourfundamental colour-sensations—a red, a green, a blue, and a violet. His results are, however, thought to be capable of a different interpretation.
[7]Or through several pieces superposed.
[8]A violet-coloured haze may sometimes be actually seen around the opal globes of the electric lamps in the streets.
[9]A “focus” electric lamp was used in the lantern.
[10]Proc. Roy. Soc., Jan., 1899.
[11]After a few seconds’ observation the greenish-blue colour often becomes much more intense, but this is an effect of fatigue, with which we are not at present concerned.
[12]SeeNature, vol. 55, p. 367 (Feb. 18th, 1897).
[13]Or, for best results, use a balanced metal disk covered with black velvet and white paper.