For the sake of clearness let us first confine our attention to a definite colour--say red. An absolutely pure red is one that is entirely free from any admixture of white; in proportion as it contains more and more white, the more impure, or in other words, the more pale does it become, until at last all trace of perceptible redness is lost and the colour is indistinguishable from white.
[Ill.u.s.tration: _Fig. 35.--Illusion of Colour._]
A convenient way of picturing the scale of purity is shown in Fig 35. The shaded oblong may be supposed to represent a painted strip of cardboard or paper. At the extreme right hand end the colour is supposed to be absolutely pure red; towards the left the red gradually becomes paler or more dilute, and at the middle of the diagram it has merged into perfect whiteness. The figures 0 to 100 from left to right denote the percentage of free red contained in the mixture at different parts of the scale; the luminosity is supposed to be uniform throughout.
Now the white light with which the red is diluted may be regarded as consisting of two parts, one of which is of exactly the same hue as the pure red itself, and the other an equivalent proportion of the complementary colour, which in the present case will be greenish-blue. The fact therefore really is that, as we pa.s.s along the scale from 100 to 0, the _total_ quant.i.ty of red in the mixture is not reduced to nothing, but only to one half, while at the same time greenish-blue is added in proportions increasing from nought at the extreme right to 50 per cent. of the whole at the middle of the card. The ordinates of the quadrilateral figure E D B F show the proportion of red, and those of the triangle E F B the proportion of greenish-blue, at different parts of the scale.
Regarding the portion of the strip which lies above the point marked 0, as representing the zero of colour--that is, whiteness or greyness, which is essentially the same as whiteness--let us continue the diagram in the negative direction, gradually reducing the quant.i.ty of red until it falls from 50 per cent. of the whole at F to nothing at A, and at the same time increasing that of the greenish-blue from 50 per cent. at F to 100 per cent. at A. The resultant hue in the portion of the card between F and A will be greenish-blue, which begins to be perceptible as a very pale tint just to the left of F, and increases in purity as A is approached, at which point the colour will be entirely free from any admixture with white.
We have in the scale thus presented to our imagination a pair of colours, each occupying one-half of the scale, and gradually diminishing in purity towards the middle line; here only, just at the stage where one colour merges into the other, is there no colour at all, and this region represents the fixed physical zero or standard from which is reckoned the purity of a colour corresponding to any other portion of the scale. The completed scale, it will be observed, though originally intended only for the case of red, turns out to be equally serviceable for greenish-blue: if we consider greenish-blue as positive, then the red, being on the other side of zero, must be regarded as negative. Any other possible pairs of complementary colours may be similarly treated.
This device enables us at once to understand the consequence of mentally displacing the zero, while physically the scale remains unchanged. When red is the prevailing colour in the field of vision, we are inclined to consider it unduly pale; in other words we imagine it to be nearer the zero of the scale than is actually the case, and so are led to shift our standard of whiteness from the middle slightly towards the red end of the scale. The new position a.s.signed to white, being a little to the right of the point marked 0 in Fig. 35, is one where, under customary circ.u.mstances, the colour would be called pale red. At the same time, an object which is normally white, and is exactly matched at the middle of the scale, would be a little to the left of the imaginary zero, and would consequently appear to be of a greenish-blue tint.
This apparent transformation of white or grey into a decided colour is most striking when the inducing colour is considerably diluted with white or is of feeble luminosity. A small fragment of neutral grey paper, placed upon a much larger piece of a bright red hue, generally appears at the first glance[11] to be greenish-blue, but if the light is at all strong, only slightly so. If, however, a sheet of white tissue paper is laid over the whole, the greenish-blue tint immediately becomes startlingly distinct, and may even appear more decided than the red itself as seen through the tissue. The same piece of grey paper, when placed upon a green ground, appears rose-coloured, and upon a blue ground, yellow, the effect being always greatly increased by the diluent action of superposed tissue paper.
There seem to be several reasons, partly physical and partly psychological, why these contrast colours, as they are called, are more p.r.o.nounced when the colour that calls them into existence either has a somewhat pale tint or is feebly illuminated. Probably the most important is of a purely physical character. The refracting media of the eye are much less perfectly transparent than a good gla.s.s lens is; they are sensibly turbid or opalescent, and in consequence of this defect some of the light which falls upon them is irregularly scattered over the retina.
If we look at a bright red object with a small white patch upon it, the image of the patch as formed upon the retina is not, physically speaking, perfectly white, but slightly coloured by diffused red light; owing however to the psychological influence to which our attention has been directed, the faint red coloration is not consciously perceived; the same mental displacement of the zero which, when the exciting colour was feeble, led us to regard white (or grey) as bluish-green, now causes what is actually pale red to appear white.
There is no need whatever to a.s.sume that the contrast colours with which we have been dealing are of physiological origin and due to an inductive action excited in portions of the retina adjacent to those upon which coloured light falls. On the contrary, it would be a matter for surprise if the case in question presented an exception to the comprehensive law which governs the fluctuation of the mental judgment.
Of the operation of this law I have quoted several very diverse instances, and the number might easily have been increased. Nor is it only in relation to optical phenomena that the law holds good; in its most general form, supplemented it may be in some instances by obvious corollaries, it is applicable to almost every case in which physical attributes of whatever kind are the subject of una.s.sisted mental judgment.
CHAPTER V.
CURIOSITIES OF VISION.
The function of the eye, regarded as an optical instrument, is limited to the formation of luminous images upon the retina. From a purely physical point of view it is a simple enough piece of apparatus, and, as was forcibly pointed out by Helmholtz, it is subject to a number of defects which can be demonstrated by the simplest tests, and which, if they occurred in a shop-bought instrument, would be considered intolerable.
What takes place in the retina itself under luminous excitation, and how the sensation of sight is produced, are questions which belong to the sciences of physiology and psychology; and in the physiological and psychological departments of the visual machinery we meet with an additional host of objectionable peculiarities from which any humanly-constructed apparatus is by the nature of the case free.
Yet in spite of all these drawbacks our eyes do us excellent service, and provided that they are free from actual malformation and have not suffered from injury or disease, we do not often find fault with them. This, however, is not because they are as good as they might be, but because with incessant practice we have acquired a very high degree of skill in their use. If anything is more remarkable than the ease and certainty with which we have learnt to interpret ocular indications, when they are in some sort of conformity with external objects, it is the pertinacity with which we refuse to be misled when our eyes are doing their best to deceive us. In our earliest years we began to find out that we must not believe all we saw; experience gradually taught us that on certain points and under certain circ.u.mstances the indications of our organs of vision were uniformly meaningless or fallacious, and we soon discovered that it would save us trouble and add to the comfort of life if we cultivated a habit of completely ignoring all such visual sensations as were of no practical value. In this most of us have been remarkably successful; so much so, that if, from motives of curiosity, or for the sake of scientific experiment, we wish to direct our attention to the sensations in question, and to see things as they actually appear, we can only do so with the greatest difficulty; sometimes, indeed, not at all, unless with the a.s.sistance of some specially contrived artifice.
In the present chapter it is proposed to discuss a few of the less familiar vagaries of the visual organs, and to show how they may be demonstrated. Some of the experiments may, it is to be feared, be found rather difficult; success will depend mainly upon the experimentalist"s ability to lay aside habit and prejudice, and give close attention to his visual sensations; but it is hardly to be expected that an unskilled person will at the first attempt observe all the phenomena which will be referred to.
Among the most annoying of the eccentricities which characterise the sense of vision is that known as the persistence of impressions. The sensation of sight which is produced by an illuminated object does not cease at the moment when the exciting cause is removed or changed in position; it continues for a period which is generally said to be about a tenth of a second, but may sometimes be much more or less. It is for this reason that we cannot see the details of anything which is in rapid motion, but only an indistinct blur, resulting from the confusion of successive impressions. If a cardboard disk, which is painted in conspicuous black and white sectors is caused to rotate at a sufficiently high speed, the divisions are completely lost sight of, and the whole surface appears to be of a uniformly grey hue. But if the rapidly rotating disk is illuminated by a properly timed series of electric flashes, it looks as if it were at rest, and in spite of the intermittent nature of the light, the black and white sectors can be seen quite continuously, though as a matter of fact the intervals of darkness are very much longer than those of illumination. Persistent impressions of this kind are often spoken of as positive after-images.
There is a very remarkable phenomenon accompanying the formation of positive after-images, especially those following brief illumination, which seems, until comparatively recent times, to have entirely escaped the notice of the most acute observers. It was first observed accidentally by Professor C. A. Young, when he was experimenting with a large electrical machine which had been newly acquired for his laboratory.
He noticed that when a powerful Leyden jar discharge took place in a darkened room, any conspicuous object was seen twice at least, with an interval of a trifle less than a quarter of a second, the first time vividly, the second time faintly. Often it was seen a third time, and sometimes, but only with great difficulty, even a fourth time. He gave to this phenomenon the name of recurrent vision; it may perhaps be more appropriately denominated the Young effect.
By means of the powerful machine presented to the Royal Inst.i.tution by Mr.
Wimshurst, used in conjunction with a battery of Leyden jars, the Young effect has been successfully shown to a large a.s.sembly. But it is quite easy to demonstrate it on a small scale with any influence machine which will give a spark about an inch long. One of the terminals of the machine should be connected by a wire with the inner coating of a half-pint Leyden jar, the other with the outer coating, and the discharging b.a.l.l.s should be set a quarter of an inch apart. The observer"s eyes must be shielded from the direct light of the spark by any convenient screen, such as a large book set on end. The best object for the experiment is a sheet of white paper, placed in an upright position a few inches away from the terminals of the machine and exposed to the full light of the discharge.
The room being darkened, let the machine be worked slowly, while the eyes are turned towards the white paper. This will be seen for a moment when the spark pa.s.ses, and, after a dark interval of about one-fifth of a second, it will make another brief appearance. After a further short interval of darkness, a second recurrent image will often be seen. It may be remarked that the effect is most striking when the eyes are not directed exactly upon the white paper, but above or on one side of it; the proper distance of the paper from the spark-gap should be found by trial.
Under favourable conditions I have observed as many as six or seven reappearances of an object which was illuminated by a single discharge.
These followed one another at the usual rate--about five in a second--and produced a twinkling or quivering effect, closely resembling that attending a flash of lightning which is not directly seen. There can indeed be little doubt that the proverbial quiver of the lightning-flash is in many cases merely an effect of recurrent vision, though sometimes, of course, as has been shown by photographs, the discharge is really multiple.
Some years ago I called attention to a very different method of exhibiting a recurrent image. The apparatus used for the purpose consists of a vacuum tube mounted in the usual way upon a horizontal axis capable of rotation.
When the tube is illuminated by a rapid succession of discharges from an induction coil, and is made to rotate very slowly by clockwork (turning once in every two or three seconds), a very curious phenomenon may be noticed. At a distance of a few degrees behind the tube and separated from it by an interval of perfect darkness, comes a ghost. This ghost is in form an exact reproduction of the tube; it is very clearly defined, and though its apparent luminosity is somewhat feeble, it can in most cases be seen without difficulty. The varied colours of the original are, however, absent, the whole of the phantom tube being of a uniform bluish or violet tint. If the rotation is suddenly stopped the ghost still moves steadily on until it reaches the luminous tube, with which it coalesces and so disappears. (See Fig. 36, where the recurrent image is represented by dotted lines.)
[Ill.u.s.tration: _Fig. 36.--Recurrent Vision demonstrated with a Vacuum Tube._]
More recently a fresh series of experiments were undertaken in connection with the Young effect and certain allied matters, the results being embodied in a communication to the Royal Society (Proc. Roy. Soc., 1894, vol. 56, p. 132). Among other things an attempt was made to ascertain how far a recurrent image was affected by the colour of the exciting light.
With this object two methods of experimenting were employed. In the first, coloured light was obtained by pa.s.sing white light through coloured gla.s.ses; in the second and more perfect series of experiments, the pure coloured light of the spectrum was used. Among other results it was found that, _caeteris paribus_, the recurrent image was much stronger with green light than with any other, and that when the excitation was produced by pure red light, however intense, there was no recurrent image at all.
[Ill.u.s.tration: _Fig. 37.--Recurrent Vision with Rotating Disk._]
For a repet.i.tion of my first experiment a mechanical lantern slide is required containing a metal disk about three inches in diameter which can be caused to rotate slowly and steadily about its centre. Near the edge of the disk is a small circular aperture. The slide is placed in a limelight lantern, and a bright image of the hole is focussed upon a distant screen, all other light being carefully shut off. When the disk is turned slowly, the spot of light upon the screen goes round and round, and it is generally possible to see at once that the bright primary spot appears to be followed at a short distance by a much feebler spot of a violet colour, which is the recurrent image of the first. (See Fig. 37.) It is essential to keep the direction of the eyes perfectly steady, which is not a very easy thing to do without practice.
If a green gla.s.s is placed before the lens, the ghost will be at its best, and should be seen quite clearly and easily, provided that no attempt is made to follow it with the eyes. With an orange gla.s.s the ghost becomes less distinctly visible, and its colour generally appears to be greenish-blue, instead of violet as before. When a red gla.s.s is subst.i.tuted, the ghost completely disappears. If the speed of rotation is sufficiently high, the red spot is considerably elongated during its revolution, and its colour ceases to be uniform, the tail a.s.suming a light bluish-pink tint. But however great the speed, no complete separation of the spot into red and pink portions can be effected, and no recurrent image is ever found.
The spectrum method of observation can only be carried out on a small scale, and is not suited for exhibition to an audience. It, however, affords the best means of ascertaining how far the apparent colour of the recurrent image depends upon that of the primary, a matter of some theoretical interest.
[Ill.u.s.tration: _Fig. 38.--Recurrent Vision with Spectrum._]
The arrangement adopted is shown in the annexed diagram (Fig. 38). L is a lantern containing an oxyhydrogen light or an electric arc lamp, S is an adjustable slit, M a projection lens, P a bisulphide of carbon prism, D a metal plate in the middle of which is a circular aperture 2 millimetres (1/12 inch) in diameter. A bright spectrum, 6 or 7 centimetres in length (about 3 inches), is projected upon this metal plate, and a small selected portion of it pa.s.ses through the round hole; thence the coloured light goes through the lens N to the little mirror Q, which reflects it upon the white screen R. By properly adjusting the position of the lens N a sharp monochromatic image of the round hole in the plate D is focussed upon the screen R. To the back of the mirror Q is attached a horizontal arm which is not quite perpendicular to the mirror, its inclination being capable of adjustment. The arm is turned slowly by clock-work, thus causing the coloured spot on the screen to revolve in a circular orbit about 30 centimetres (1 foot) in diameter, its recurrent image following at a short distance behind it. When the mirror turns once in 1-1/2 seconds, this image appears about 50 behind the coloured spot, the corresponding time-interval being about one-fifth of a second.
Using this apparatus, it was found that white light was followed by a violet recurrent image; after blue and green, when the image was brightest, its colour was also violet; after yellow and orange it appeared blue or greenish blue. On the other hand, when a complete spectrum was caused to revolve upon the screen, the whole of its recurrent image from end to end appeared violet; there was no suspicion of blue or greenish-blue at the less refrangible end. For this and other reasons given in the paper it was concluded that the true colour was in all cases really violet, the blue and greenish-blue apparently seen in conjunction with the much brighter yellow and orange of the primary being merely an illusory effect of contrast.
It seems likely, then, that the phenomenon which has been spoken of as recurrent vision, is due princ.i.p.ally, if not entirely, to an action of the violet nerve-fibres.
Recurrent vision is, no doubt, generally most conspicuous after a very brief period of retinal illumination, such as was employed in the experiments which we have been discussing; this is evidently due to the fact that the effect is most easily perceived when the sensibility of the retina has not been impaired by fatigue. But by a little effort it may be detected even after very prolonged illumination, and a practised observer can hardly avoid noticing a short flash of bluish light which manifests itself about a quarter of a second after the lights in a room have been suddenly extinguished; the phenomenon forces itself upon my attention almost every night when I turn off the electric lights. It need hardly be pointed out that it represents only a transient phase of the well known positive after-image, and it had even been observed in a vague and uncertain sort of way long before the date of Professor Young"s experiment. Helmholtz, for example, mentions the case of a positive after-image which seemed to disappear and then to brighten up again, but he goes on to explain--erroneously, as it turns out--that the seeming disappearance was illusory.
M. Charpentier, of Nancy, whose work in physiological optics is well known, was the first to notice and record a remarkable phenomenon which, in some form or other, must present itself many times daily to every person who is not blind, but which until about seven years ago had been absolutely and universally ignored. The law which is a.s.sociated with Charpentier"s name is this:--When darkness is succeeded by light, the stimulus which the retina at first receives, and which causes the sensation of luminosity, is followed by a brief period of insensibility, resulting in the sensation of momentary darkness. It appears that the dark period begins about one sixtieth of a second after the light has first been admitted to the eye, and lasts for about an equal time. The whole alternation from light to darkness and back again to light is performed so rapidly, that except under certain conditions, which, however, occur frequently enough, it cannot be detected.
[Ill.u.s.tration: _Fig. 39.--Charpentier"s Dark Band._]
The apparatus which Charpentier employed for demonstrating and measuring the duration of this effect is very simple. It consists of a blackened disk with a white sector, mounted upon an axis. When the disk is illuminated by sunlight and turned rather slowly, the direction of the gaze being fixed upon the centre, there appears upon the white sector, close behind its leading edge, a narrow but quite conspicuous dark band.
(See Fig. 39.) The portion of the retina which at any moment is apparently occupied by the dark band, is that upon which the light reflected by the leading edge of the white sector impinged one sixtieth of a second previously.
But no special apparatus is required to show the dark reaction. In Fig. 40 an attempt has been made to ill.u.s.trate what any one may see if he simply moves his hand between his eyes and the sky or any strongly illuminated white surface. The hand appears to be followed by a dark outline separated from it by a bright interval. The same kind of thing happens, in a more or less marked degree, whenever a dark object moves across a bright background, or a bright object across a dark background.
[Ill.u.s.tration: _Fig. 40.--Charpentier"s Effect shown with the Hand._]
In order to see the effect distinctly by Charpentier"s original method, the illumination must be strong. If, howover, the arrangement is slightly varied, so that transmitted instead of reflected light is made use of, comparatively feeble illumination is sufficient. A very effective way is to turn a small metal disk, having an open sector of about 60, in front of a sheet of ground or opal gla.s.s behind which is a lamp. By an arrangement of this kind upon a larger scale, the effect may easily be rendered visible to an audience. The eyes should not be allowed to follow the disk in its rotation, but should be directed steadily upon the centre.
The acute and educated vision of Charpentier enabled him, even when working with his black and white disk, to detect the existence, under favourable conditions, of a second, and sometimes a third, band of greatly diminished intensity, though he remarks that the observation is a very difficult one. What is probably the same effect can, however, as pointed out in my paper of 1894, be shown quite easily in a different manner. If a disk with a narrow radial slit, about half a millimetre (1/50 inch) wide, is caused to rotate at the rate of about one turn per second in front of a bright background, such as a sheet of ground gla.s.s with a lamp behind it, the moving slit a.s.sumes the appearance of a fan-shaped luminous patch, the brightness of which diminishes with the distance from the leading edge. And if the eyes are steadily fixed upon the centre of the disk, it will be noticed that this bright image is streaked with a number of dark radial bands, suggestive of the ribs or sticks of a fan. Near the circ.u.mference as many as four or five such dark streaks can be distinguished without difficulty; towards the centre they are less conspicuous, owing to the overlapping of the successive images of the slit. The effect is roughly indicated in Fig. 41.
[Ill.u.s.tration: _Fig. 41.--Multiple Dark Bands._]
The dark reaction known as the Charpentier effect occurs at the beginning of a period of illumination. There is also a dark reaction of very short duration at the end of a period of illumination. It should be explained that, owing to what is called the proper light of the retina, ordinary darkness does not appear absolutely black: even in a dark room on a dark night with the eyes carefully covered, there is always some sensation of luminosity which would be sufficient to show up a really black image if one could be produced. Now the darkness which is experienced after the extinction of a light is for a small fraction of a second more intense than common darkness.
The first mention of this dark reaction perhaps occurs in an article contributed to _Nature_ in 1885, in which it was stated that when the current was cut off from an illuminated vacuum tube "the luminous image was almost instantly replaced by a corresponding image which seemed to be intensely black upon a less dark background," and which was estimated to last from a-quarter to a-half second. "Abnormal darkness," it was added, "follows as a reaction after luminosity."
[Ill.u.s.tration: _Fig. 42.--Temporary Insensitiveness of the Eye._]