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 circumstances 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 assistance 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 Institution by Mr.
Wimshurst, used in conjunction with a battery of Leyden jars, the Young
effect has been successfully shown to a large assembly. 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 balls 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 passes, 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.)
[Illustration: _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 passing white light through coloured
glasses; 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, _cæteris 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.
[Illustration: _Fig. 37.--Recurrent Vision with Rotating Disk._]
For a repetition 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 glass 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 glass the ghost becomes
less distinctly visible, and its colour generally appears to be
greenish-blue, instead of violet as before. When a red glass is
substituted, 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 assuming 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.
[Illustration: _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 passes 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 principally, 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 associated 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.
[Illustration: _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 illustrate 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.
[Illustration: _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 glass 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 glass with a lamp
behind it, the moving slit assumes 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
circumference 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.
[Illustration: _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."
[Illustration: _Fig. 42.--Temporary Insensitiveness of the Eye._]
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 space 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 illustrated 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
diminishing 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.
[Illustration: _Fig. 43.--Visual Sensations attending a period of
Illumination._]
[Illustration: _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 assume 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 assume 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.
[Illustration: _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 pass 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 space 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 passing over them, they will, if
the illumination is suitable, appear to be bright red.
[Illustration: _Fig. 46.--Black and White Screens._]
[Illustration: _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.
[Illustration: _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.
[Illustration: _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 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 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 analogous 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 peacock-blue; the petals of the sunflower also become
yellow, and its foliage green. Other designs show equally remarkable
transformations of colour.
[Illustration: _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.
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
really _four_ fundamental 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] See _Nature_, vol. 55, p. 367 (Feb. 18th, 1897).
[13] Or, for best results, use a balanced metal disk covered with black
velvet and white paper.
End of Project Gutenberg's Curiosities of Light and Sight, by Shelford Bidwell