Part 5 (2/2)
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._]
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