Curiosities of Light and Sight Part 6

In the Royal Society paper before referred to the point is further discussed, and a method is described by which the stage of reaction may be easily exhibited and its duration approximately measured. If a translucent disk, made of stout drawing-paper and having an open sector, is caused to rotate slowly in front of a luminous background, a narrow radial dark band, like a streak of black paint, appears upon the paper very near the edge which follows the open sector. From the s.p.a.ce covered by this band when the disk was rotating at a known speed, the duration of the dark reaction was calculated to be about one-fiftieth of a second; my original estimate was therefore an excessive one. The experiment is ill.u.s.trated in Fig. 42.

One more interesting point should be noticed in the train of visual phenomena which attend a period of illumination. The sensation of luminosity which is excited when light first strikes the eye is for about a sixtieth of a second much more intense than it subsequently becomes.

This is shown by the fact, which is obvious enough when once attention has been directed to it, that the bright band, which in the Charpentier disk intervenes between the dark band and the leading edge of the white sector, appears to be much more strongly illuminated than any other portion of the sector.

The complete order of visual phenomena observed when the retina is exposed to the action of light for a limited time may therefore be summed up as follows:--

(1) Immediately upon the impact of the light there is experienced a sensation of luminosity, the intensity of which increases for about one-sixtieth of a second: more rapidly towards the end of that period than at first.

(2) Then ensues a sudden re-action, lasting also for about one-sixtieth of a second, in virtue of which the retina becomes partially insensible to renewed or continued luminous impressions.

These two effects may be repeated in a diminished degree, as often as three or four times.

(3) The stage of fluctuation is succeeded by a sensation of steady luminosity, the intensity of which is, however, considerably below the mean of that experienced during the first one-sixtieth of a second.

(4) After the external light has been shut off, a sensation of diminis.h.i.+ng luminosity continues for a short time, and is succeeded by a brief interval of darkness.

(5) Then follows a sudden and clearly-defined sensation of what may be called abnormal darkness--darker than common darkness--which lasts for about one-sixtieth of a second, and is followed by another interval of ordinary darkness.

(6) Finally, in about a fifth of a second after the extinction of the external light, there occurs another transient impression of luminosity, generally violet coloured, after which the uniformity of the darkness remains undisturbed.

Fig. 43, which is copied from my paper, gives a rough diagrammatic representation of the above described chain of sensations. No account is here taken of the comparatively feeble after-images which succeed the recurrent image, and may last for several seconds.

I propose now to say a few words about a curious phenomenon of vision which a short time ago excited considerable interest.

[Ill.u.s.tration: _Fig. 43.--Visual Sensations attending a period of Illumination._]

[Ill.u.s.tration: _Fig. 44.--Benham's Top._]

In the year 1895 Mr. C. E. Benham brought out a pretty little toy which he called the Artificial Spectrum Top. It consists of a cardboard disk, one half of which is painted black, while on the other half are drawn four successive groups of curved black lines at different distances from the centre, as shown in Fig. 44. When the disk rotates rather slowly, each group of black lines generally appears to a.s.sume a different colour, the nature of which depends upon the speed of the rotation and the intensity and quality of the light. Under the best conditions the inner and outer groups of lines become bright red and dark blue; at the same time the intermediate groups also appear tinted, but the hues which they a.s.sume are rather uncertain and difficult to specify. By far the most striking of the colours exhibited by the top is the red, and next to that the blue; this latter is, however, sometimes described as bluish-green.

Some experiments carried out by myself in 1896 (Proc. Roy. Soc., vol. 60, p. 370) seem to indicate pretty clearly the cause of the remarkable bright red colour, and also that of the blue. The more feeble tints of the two intermediate groups of lines perhaps result from similar causes in a modified form, but these have not yet been investigated.

In the red colour we have another striking example of an exceedingly common phenomenon which is habitually disregarded; indeed I can find no record of its ever having been noticed at all. The fact is that whenever a bright image is suddenly formed upon the retina after a period of comparative darkness, this image appears for a short time to be surrounded by a narrow coloured border, the colour, under ordinary conditions of illumination, being red. If the light is very strong, the transient border is greenish-blue, but this colour, as will be explained later, turned out to be merely an after-effect of red. Sometimes, when the object is in motion, both red and blue are seen together.

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.

[Ill.u.s.tration: _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 pa.s.s 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 s.p.a.ce 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 pa.s.sing over them, they will, if the illumination is suitable, appear to be bright red.

[Ill.u.s.tration: _Fig. 46.--Black and White Screens._]

[Ill.u.s.tration: _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 ill.u.s.trated in Fig. 48, where the shaded band indicates the blue border.

[Ill.u.s.tration: _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 const.i.tuent) 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.

[Ill.u.s.tration: _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 in Fig. 49, and spin it above a page of printing. The letters beneath the zone which is partly black and partly white will, 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 own experiments, 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 circ.u.mstance, 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 diminis.h.i.+ng 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 black and white screens depicted in Fig. 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 a.n.a.logous phenomena have been observed in other branches of physiology; a well-defined reaction 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, and a 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 peac.o.c.k-blue; the petals of the sunflower also become yellow, and its foliage green. Other designs show equally remarkable transformations of colour.

[Ill.u.s.tration: _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 subjective effects produced by intermittent illumination would lead to results tending to clear up several doubtful points in the theory of colour vision.