VISION AND OPTICAL ILLUSIONS—THE EYE DESCRIBED—ACCOMMODATION OF THE
EYE—CHROMATIC ABERRATION—SPINNING TOPS.
The eye is an optical instrument that may be compared with those
constructed by physicists themselves; the _media_ of which it is
composed have surfaces like those which enter into the construction of
optical instruments. It was Kepler who at the end of the eighteenth
century discovered the passage of light into the eye. Soon after the
discovery of the inner chamber he found that the eye realized the
conditions that Porta had combined to obtain the reflection of external
objects.
We will now briefly state that the coats of this organ are constituted
of a fibrous membrane, T (fig. 95), termed _sclerotic_, which
is opaque, except in the anterior portion of the eye, where it forms
the transparent _cornea_. The crystalline, C, enshrined
behind the cornea, is the convergent lens of the inner chamber; it is
covered with a transparent membrane, or _capsule_, and is bathed in two
fluids, the _aqueous humour_, between the crystalline humour and the
cornea, and the _vitreous body_, a gelatinous humour lodged between
the crystalline and the back of the eye. The image of exterior objects
which is produced by the passage of light through these refracting
surfaces, is received by a nervous membrane, the _retina_, B,
formed by an expansion of the optic nerve, N. We must also
mention the _choroid_, a membrane lined with a dark pigment, which
absorbs the light, and prevents interior reflections, and in front of
the crystalline lens, a curtain with an opening, H, called the
_iris_, which gives to the eyes their colour of blue, grey, or black.
The opening in the centre of the iris is called the _pupil_.
[Illustration: Fig. 95.—Structure of the eye.]
The penetration of light through the surfaces of the eye is easily
demonstrated. An object throws divergent rays on the cornea, a part
penetrates into the eye and falls upon the retina, leaving a perfectly
retained image of the object. Magendie has proved in the following
manner the truth of this mathematical deduction. The eye of a rabbit
is very similar to an albino’s; that is to say, the choroid contains
no black pigment, but a transparent matter, and when placed before a
brilliant object, the image can be seen inverted on the retina. The
experiment succeeds also with the eye of a sheep or a cow, if the
sclerotic has been lessened. The _optic centre_ of the eye is the point
where the secondary axes cross; the _optic axis_ passes through the
geometrical axis of the organ, and directs itself spontaneously towards
the point that attracts the eye.
[Illustration: Fig. 96.—Diagram of mode of vision.]
We will now point out in what distinct vision consists. A screen
placed behind a lens will only receive the image of a lighted object,
A B, if placed in a position, R R (fig. 96). If placed nearer at R´´
R´´, or further off at R´ R´, the light from the object is thrown on
the screen, and the image is confused. To prove the imperfection of
sight which is shown by the application of these theoretic rules, MM.
Boutan and d’Alméïda[10] cite the following experiment:—If the head of
a pin is placed from one to two inches from the eye, nothing will be
perceived but a confused haziness of vague outline. The distance of
distinct vision is that at which an object of small dimensions may be
placed to be plainly perceived. This distance, which averages fifteen
inches, varies with different individuals. It can be determined for
different sights by means of an apparatus constructed by Lepot. A white
thread, _a_, is stretched horizontally on a dark board (fig. 97). We
look at it by placing our eye at one end behind a little screen pierced
with an aperture, O; it then appears much reduced in length, but
either nearer or farther off it seems to enlarge and swell, having the
appearance of a white surface, becoming larger and larger in proportion
as we move away from the point at which it is seen most distinctly. In
this manner we can easily obtain a measure of the distance of distinct
vision. One of the most remarkable properties of the eye consists in
the faculty which this organ possesses of seeing different distances.
If we consider it as a dark chamber, there is but one distance at which
an object will be perfectly visible; nevertheless a metal wire, for
example, can be seen as well at a distance of seven, as ten, fifteen,
or twenty inches by good sights.
[Illustration: Fig. 97.—Experiment for sight.]
This faculty of accommodation in the eye is thus demonstrated: we place
two pins, one in front of the other, one eye only being open; we first
look at the nearest pin, which appears confused if it is near the eye,
but by an effort of will the image becomes clear. If, while preserving
the clearness of the image, we then carry our attention to the second
pin, we find that it, too, presents a confused appearance. If we make
an effort to distinguish the contour of the second pin, we at last
succeed, and the first once more appears ill-defined. It is only since
the experiments of M. Cramer and M. Helmholtz that the explanation
of this phenomenon could have been given. M. Cramer has succeeded in
determining on the living eye the curved ray of the cornea, and of the
two surfaces of the crystalline lens. In so doing he followed Samson’s
method, and observed the images thrown by a luminous object, whose
rays strike the different refracting surfaces of the eye. A candle,
L (fig. 98), is placed before the eye, O, and throws as in a convex
mirror a straight image of the flame, A (fig. 99). The other portion
of the light, which has penetrated the pupil, falls on the crystalline
lens, and produces likewise a second straight image, B. Then the light
refracted by the lens reaches the posterior surface; a portion is
reflected on a concave mirror, and gives the inverted image, C, very
small and brilliant. M. Cramer observed it through a microscope, and
studied the variations in the size of images when the eye passed from
the observation of adjacent to distant objects. He stated:—
1. That the image, A, formed on the surface of the cornea,
remains the same size in both cases; the form of the cornea therefore
remains unaltered.
[Illustration: Fig. 98.—M. Cramer’s experiment.]
[Illustration: Fig. 99.—Images in the eye.]
2. That the image, B, formed on the upper surface of the
lens, diminishes in proportion as the eye is nearer the object; the
surface therefore becoming more and more convex, as the focal distance
diminishes—a result indicated by the theory that it is possible in the
vision of near objects to receive the image on the retina.
3. That the third image, C, produced on the posterior surface
of the lens, remains nearly invariable.
We may confirm Cramer’s statements by an easy experiment. We place
ourselves in front of the eye of someone who looks in turn at two
objects placed on the same black line at unequal distances from
him, and are able to distinguish by the dimension of the images of
the candle, which object it is that he is regarding. M. Helmholtz
has carried M. Cramer’s methods to perfection, and has been able to
formulate a complete theory of all the phenomena of accommodation.
The laws of optics show that the rays emitted by a luminous point may
unite at another point by the action of the refracting surfaces of the
eye. Nevertheless, a white light being composed of rays of diverse
refrangibility, particular effects, known under the name of _chromatic
aberration_, are produced through the decomposition of light, which
we will proceed to study, under M. Helmholtz’s auspices[11]. We make
a narrow opening in a screen, and fix behind this opening a violet
glass, penetrable only by red and violet rays. We then place a light,
the red rays of which reach the eye of the observer after having passed
through the glass and the opening in the screen. If the eye is adapted
to the red rays, the violet rays will form a circle of diffusion, and
a red point encircled with a violet aureola is seen. The eye may also
be brought to a state of refraction, so that the point of convergence
of the violet rays is in front, and that of the red rays behind the
retina, the diameters of the red and violet circles of diffusion being
equal. It is then only that the luminous point appears monochromatic.
When the eye is in this state of refraction, the simple rays, whose
refrangibility is maintained between the red and the violet rays, unite
on the retina.
There is another kind of aberration of luminous rays of one colour
emitted through a hole, which generally only approach approximately to
a mathematical focus, in consequence of the properties of refracting
surfaces; it is called _aberration of sphericity_. The phenomena are as
follows:—
[Illustration: Fig. 100.]
1. We take for our object a very small luminous point (the hole
made by a pin in some black paper, through which the light passes),
and having also placed before the eye a convex glass, if we are not
near-sighted, we fix it a little beyond the point of accommodation, so
that it produces on the retina a little circle of diffusion. We then
see, instead of the luminous point, a figure representing from four
to eight irregular rays, which generally differ with both eyes, and
also with different people. We have given the result of M. Helmholtz’s
observations in fig. 100; _a_ corresponds to the right eye, and _b_ to
the left. The outer edges of the luminous parts of an image, produced
in this way by a white light, are bordered with blue; the edges towards
the centre are of a reddish yellow. The writer adds that the figure
appears to him to have greater length than breadth. If the light is
feeble, only the most brilliant parts of the figure can be seen, and
several images of the luminous point are visible, of which one is
generally more brilliant than the others. If, on the other hand, the
light is very intense,—if, for example, the direct light of the sun
passes through a small opening,—the rays mingle with each other, and
are surrounded by aureola of rays, composed of numberless extremely
fine lines, of all colours, possessing a much larger diameter, and
which we distinguish by the name of the aureola of capillary rays.
[Illustration: Fig. 101.]
The radiating form of stars, and the distant light of street-lamps
belong to the preceding phenomena. If the eye is accommodated to a
greater distance than that of the luminous point,—and for this purpose,
if the luminous point itself is distant, we place before the eye a
slightly convex lens,—we see another radiating image appear, which M.
Helmholtz represents thus (fig. 101): at _c_ as it is presented to the
right eye, and at _d_ as seen by the left.
If the pupil is covered on one side, the side opposite to the image
of diffusion disappears; that is to say, that part of the retinal
image situated on the same side as the covered half of the pupil. This
figure, then, is formed by rays which have not yet crossed the axis
of the eye. If we place the luminous point at a distance to which
the eye can accommodate itself, we see, through a moderate light, a
small, round, luminous spot, without any irregularities. If the light,
on the contrary, is intense, the image is radiated in every position
of accommodation, and we merely find that on approaching nearer, the
figure which was elongated, answering to a distant accommodation,
gradually diminishes, grows rounder, and gives place to the vertically
elongated figure, which belongs to the accommodation of a nearer point.
When we examine a slender, luminous line, we behold images developed,
which are easily foreseen, if for every point of the line we suppose
radiating images of diffusion, which encroach on each other. The
clearest portions of these images of diffusion mingle together and form
distinct lines, which show multiplied images of the luminous line. Most
persons will see two of these images; some, with the eyes in certain
positions, will see five or six.
[Illustration: Fig. 102.]
To show clearly by experiment the connection existing between double
images and radiated images from points, it is sufficient to make in a
dark sheet of paper a small rectilinear slit, and at a little distance
from one end, on a line with the slit, a small round hole, as shown
at _a_ in fig. 102. Looking at it from a distance we shall see that
the double images of the line have exactly the same distance between
them that the most brilliant parts of the starred figure of diffusion
have from the point, and that the latter are in a line with the first,
as will be seen at _b_ (fig. 102), where in the image of diffusion of
the luminous point, we only see the clearest parts of star _a_ of the
figure.
On lighted surfaces, to which the eye is not exactly accommodated,
multiplied images are often remarked through the passage from light to
darkness being made by two or three successive steps.
A series of facts which have been collected under the title of
_irradiation_, and which show that brightly-lighted surfaces
appear larger than they are in reality, and that the dark surfaces
which surround them appear diminished to a corresponding degree,
explains this by the circumstance that the luminous sensation is not
proportional to the intensity of the objective light. These phenomena
affect very various appearances, according to the form of respective
figures; they are generally seen with the greatest ease and intensity
when the eye is not exactly accommodated to the object examined, either
by the eye being too near or too far off, or by using a concave or
convex lens, which prevents the object being seen clearly. Irradiation
is not completely wanting, even when the accommodation is exact, and
we notice it clearly in very luminous objects, above all when they are
small; small circles of diffusion increase relatively the dimensions
of small objects much more than of large ones, with regard to which,
the dimensions of the small circles of diffusion which the eye
furnishes, when properly accommodated, become insensible.
[Illustration: Fig. 103.—Experiment 1.]
1. _Luminous surfaces appear larger._ We can never judge exactly of
the dimensions of a slit or small hole through which a bright light
escapes; it always appears to us larger than it really is, even with
the most exact accommodation. Similarly, the fixed stars appear in the
form of small luminous surfaces, even when we make use of a glass which
allows of perfect accommodation. If a gridiron with narrow bars—the
spaces intervening being exactly equal to the thickness of the bars—is
held over a light surface, the spaces will always appear wider than the
bars. With an inexact accommodation, these phenomena are still more
remarkable. Fig. 103 exhibits a white square on a black foundation, and
a black square on a white foundation. Although the two squares have
exactly the same dimensions, the white appears larger than the black,
unless with an intense light and an inexact accommodation.
[Illustration: Fig. 104.—Experiment 2.]
2. _Two adjacent luminous surfaces mingle together._ If we hold a fine
metallic wire between the eye and the sun, or the light of a powerful
lamp, we shall cease to see it; the lighted surfaces on all sides
of the wire in the visual range pass one into the other, and become
mingled. In objects composed of black and white squares, like those of
a draught-board (fig. 104), the angles of the white squares join by
irradiation, and separate the black squares.
3. _Straight lines appear interrupted._ If a ruler is held between
the eye and the light of a bright lamp or the sun, we perceive a very
distinct hollow on the edge of the ruler in the part corresponding to
the light. When one point of the retina is affected by a light which
undergoes periodical and regular variations, the duration of the period
being sufficiently short, there results a continuous impression, like
that which would be produced if the light given during each period
were distributed in an equal manner throughout the whole duration of
the period. To verify the truth of this law, we will make use of some
discs, such as that represented in fig. 105. The innermost circle is
half white and half black; the middle circle has two quarters, or half
its periphery, white, and the outer circle has four eighths’ white, the
rest being black. If such a disc is turned round, its entire surface
will appear grey; only it is necessary to turn it with sufficient force
to produce a continuous effect. The white may also be distributed in
other ways, and provided only that on all the circles of the disc the
proportion of the angles covered with white is the same, they will
always exhibit the same grey colour. Instead of black and white we may
make use of different colours, and obtain the same resultant colour
from all the circles, when the proportion of the angles occupied by
each of the colours in the different circles is the same.
[Illustration: Fig. 105.—Disc which appears uniformly grey by reason of
its rotation.]
If we paint on a disc a coloured star, which is detached from a
foundation of another colour (fig. 106), during the rapid rotation of
the disc the centre affects the colour of the star; the outer circle
assumes that of the background, and the intermediate parts of the disc
present the continuous series of the resultant colours. These results
are in accordance with the theory of the mixture of colours.
[Illustration: Fig. 106.—Disc with a star painted on the background of
another colour.]
Rotative discs, which are so much used in experiments in optical
physiology, were employed for the first time by Müsschenbroeck;
the most simple is the top. M. Helmholtz ordinarily uses a brass
spinning-top, which fig. 107 represents at a third the natural size.
It is set in motion by the hand, and its quickness may be increased
or moderated at will; but it cannot be made to spin quicker than six
rounds in a second; this motion will be kept up for three or four
minutes. Thus, with a feeble movement of rotation, a uniform luminous
impression can only be obtained by dividing the disc into four or six
sections, on each of which we repeat the same arrangement of colours,
light, and shade. If the number of repetitions of the design is less,
we obtain, with a bright light, a more or less shot-coloured disc.
[Illustration: Fig. 107.—M. Helmholtz’s top for studying the impression
of light on the retina.]
It is easy to place designs on the disc, even when in motion, or to
make any desired modification, by superposing on the first disc another
disc with sectors, of which we can vary the position by slightly
touching it, or even blowing on it, thus producing during the rotation
of the disc very varied modifications. If, for instance, we place on
a disc covered with blue and red sectors of equal size, a black disc,
of which the sectors are alternately filled in or empty, the disc, as
it turns round, will appear blue if the black sectors of the upper
disc exactly cover the [red] sectors of the lower disc; and it appears
red, if, on the contrary, the blue sectors are covered with the black;
while in the intermediate positions we obtain different mixtures of
red and white, and during the rotation of the disc may vary the colour
insensibly by a gentle touch. By dividing the different sectors with
broken or curved lines, instead of straight ones, we can produce an
arrangement of coloured rings of great variety and beauty. To give
the top greater speed, we set it in motion by drawing a string twined
round its stem. The simplest method, as shown in fig. 108, consists in
the employment of a handle similar to that of the German top. It is a
hollow cylinder of wood set into a handle with two circular holes; and
at right angles with these is a groove for the passage of the string.
The stem of the top is passed through the holes of the cylinder, one
end of the string is fixed in the small hole in the stem, and is rolled
round by turning the top in the hand. The part of the stem on which
the string is twisted becomes sufficiently thick for the top to remain
suspended to the handle; then holding it a little above the table, and
giving the string a powerful pull, we set the top in motion, and as
the string unrolls it falls on the table, where it will continue its
rotation for some time. The top represented in fig. 109 is constructed
so that the discs may be firmly pressed by the stem, which is necessary
in experiments for demonstrating Newton’s theory of the mingling of
colours. We make use for this purpose of a variety of discs, made of
strong paper of different sizes, having an opening in the centre and
a slit, as in fig. 110; each of the discs is covered uniformly with
a single colour; and if two or more are superposed, with their slits
placed one over the other, we obtain sectors, the size of which we
may vary at will, so that we can modify in a continuous manner the
proportions of the colours. The most perfect construction is that
of Busold’s chromatic top (fig. 111), which should only be employed
for very rapid rotations. The disc, which weighs 5 lbs., is made of
an alloy of zinc and lead, about an inch and a quarter in diameter.
The brass axis terminates at its lower end with a blunt point of
untempered steel; the cylindrical part of the axis is roughened to
encourage the adherence of the string; the axis is placed between the
clamps of a vice, and a plate is put underneath; we then pull the
string firmly with the right hand, and when the top is in motion it is
separated from the clamps. By pulling the string very powerfully it is
possible to obtain a speed of sixty turns in a second, and the movement
will be kept up for three quarters of an hour.
[Illustration: Fig. 108.—Spinning a top with coloured discs.]
[Illustration: Fig. 109.—Top for experiments demonstrating Newton’s
theory of the mingling of colours.]
[Illustration: Fig. 110.—Disc.]
[Illustration: Fig. 111.—Busold’s chromatic top.]
Besides tops, we may make use of different kinds of discs, with an
axis rotating between two clamps; they are moved either by a kind of
clock-work, or by the unrolling of a string, like the tops. Generally,
however, these contrivances have this inconvenience, that the discs
cannot be changed without stopping the instrument, and partly taking
it to pieces. On the other hand, we have the advantage of being able
to turn them on a vertical plane, so that we can conveniently carry on
our experiments before a numerous auditory, which is a more difficult
matter with tops. Montigny contrived to obtain the mingling of colours
by means of a turning prism, which he caused to throw its shadow on a
white screen. The Thaumatrope is a small rectangle of cardboard, which
is made to rotate on an axis passing through the centres of the longest
sides. We shall describe it at greater length when we come to consider
a new apparatus known under the name of the _Praxinoscope_.
More complicated contrivances have also been constructed on the same
principle, by which one may perceive the rotating disc through slits
which turn at the same time. We will now describe the construction of
some discs invented by Plateau under the name of the Phenakistoscope.
These discs are made of strong cardboard, from six to ten inches in
diameter (fig. 112), on which a certain number of figures (eight to
twelve) are placed in circles at an equal distance from each other,
presenting the successive phases of a periodical movement. This disc
is placed on another opaque circle of rather larger diameter, which
has on its margin as many openings as the first disc has figures. The
two discs are placed one on the other, and are fixed in the centre by
means of a screw at the anterior extremity of a small iron axis, the
other end being fitted into a handle. To make use of this contrivance
we place ourselves in front of the glass, towards which we turn the
disc with the figures, placing the eye so as to see the figures through
one of the holes of the large disc. Directly the apparatus begins
to turn round, the figures seen in the glass appear to execute the
particular movements which they represent in different positions. Let
us designate by means of the figures 1, 2, 3, the different openings
through which the eye successively looks, and point out by the same
numbers the figures in the radiuses thus numbered. If the experimenter
looks in the glass through opening 1, he will see first figure 1,
which appears in the glass to pass before his eyes; then the rotation
of the disc displaces opening 1, and the cardboard intervenes, until
opening 2 appears; then figure 2 takes the place of figure 1, until it
in turn disappears, and opening 3 presents figure 3 to view. If these
figures were all similar, the spectator would have but a series of
visual impressions, separate but alike, which by a sufficiently rapid
rotation mingle together in one durable impression like a perfectly
immovable object. If, on the contrary, the figures differ slightly
from each other, the luminous sensations will also mingle in a single
object, which will however appear to be modified in a continuous
manner, conformably with the differences of successive images. With a
difference of speed, we obtain a new series of phenomena. A most simple
contrivance of this kind is a top of C. B. Dancer, of Manchester (fig.
113). It will be seen that the axis carries another disc, pierced
with openings of different shapes, to the edge of which a thread is
attached. This second disc is carried along by the friction of the
axis, but its rotation is less rapid because of the great resistance
offered by the air to the piece of thread which participates in the
movement. If the lower disc has several differently-coloured sectors,
they produce a very motley appearance, which seems to move sometimes by
leaps, and sometimes by continuous motion. We must distinguish between
the phenomena of successive contrast and simultaneous contrast.
[Illustration: Fig. 112.—Rotating disc.]
[Illustration: Fig. 113.—Mr. Dancer’s top.]
Phenomena of _successive contrast_ develop what are called _accidental
images_. If we fix our eyes for a considerable time on a coloured
object, and then suddenly direct them towards a uniform white surface,
we experience the sensation of the object as it is, but it appears
coloured with a complementary tint; that is to say, it has the colour
which, superposed on the genuine tint, we obtain from pure white. Thus
a red object produces a consecutive green object. The experiment can be
tried by gazing at the sun when it is setting, and then directing one’s
eyes towards a white wall in the same direction.
Phenomena of _simultaneous contrast_ arise from the influence
exercised over each other by different shades and colours which we see
_simultaneously_. That we may be certain that we have really obtained
phenomena of this kind, the experiments must be arranged in such a
manner that accidental images are not produced, and that the part of
the retina affected by the sensation of colour does not receive, even
momentarily, a passing image.
[Illustration: Fig. 114.—Disc, which exhibits, when in rotation, a
series of concentric rings.]
The phenomena of simultaneous contrast appear with the greatest
clearness with slight differences of colour, and are therefore exactly
the contrary of phenomena of successive contrast, which are favoured
by strong oppositions of colour and light. We can, in general,
characterise phenomena of simultaneous contrast as governed by this
law, common to all perceptions of the senses: _the differences clearly
perceived appear greater than the differences equal to them, but
perceived with greater difficulty, either because they only affect
the observation in an uncertain manner, or that the memory fails to
judge of them_. A man of middle height appears small beside a tall
man, because at the moment it is forcibly impressed on us that there
are taller men than he, and we lose sight of the fact that there are
smaller. The same man of medium height appears tall beside a man of
small stature. We can easily make experiments on simultaneous contrast
with a sheet of transparent paper. We fasten together a sheet of green
and a sheet of rose-coloured paper, so as to obtain a sheet half red
and half green. On the line of separation between the two colours we
place a strip of grey paper, and cover the whole with a sheet of thin
letter-paper of the same size. The grey strip will then appear red at
the edge touching the green, and green at the edge touching the red;
the centre presenting an intermediate shade. It presents a still more
decided appearance if the grey strip is perpendicular with the line
of separation of the two colours; the piece of grey then stretching
into the green will present as deep a red as the red foundation on
the other side. If the line of grey colour exactly covers the line of
separation between the two colours, the contrasting colour is more
feeble; the edges of the grey paper then present complementary strips
of colour. Similar effects may be obtained by superposing, in gradually
diminishing layers, strips of thin paper, so as to form successive
bands of different thicknesses. If it is then lit up from behind, the
objective intensity is evidently constant through the extent of each
layer; nevertheless every strip appears darker at the edge touching
a more transparent layer, and lighter at the edge in contact with a
thicker layer. The dull tints of China ink, superposed in layers,
will produce a similar effect. The phenomena are produced by means of
rotative discs of most beautiful and delicate gradations of colour. Let
us give the sectors of the disc the form represented by fig. 114, and
make them black and white; and when in rotation we shall see a series
of concentric rings of a shade that becomes darker and darker towards
the centre. The angular surface of the dark portions is constant in
each of these rings. The intensity, therefore, of each ring is uniform
during rapid rotation; it is only between one ring and another that
the intensity varies. Each ring also appears lighter on its inner
side when it borders on a darker ring, and darker on its outer side
when in contact with a lighter ring. If the differences of intensity
in the rings are very slight, one can scarcely judge sometimes if the
inner rings are darker than the outer; the eye is only struck by the
periodical alternations of light and shade presented by the edges
of the rings. If, instead of white and black, we take two different
colours, each ring will present two colours on its two edges, although
the colour of the rest of the ring will be uniform. Each of the
constituent colours presents itself with more intensity on that edge of
the ring which borders on another ring containing a smaller quantity of
the colour. Thus, if we mix blue and yellow, and the blue predominates
in the exterior and the yellow in the interior, every ring will appear
yellow at its outer, and blue at its inner edge; and if the colours
present together very slight differences, we may fall into the illusion
which causes the differences really existing between the colours of the
different rings to disappear, leaving instead, on a uniformly coloured
background, the contrasting blue and yellow of the edges of the rings.
It is very characteristic that in these cases we do not see the mixed
colours, but seem to see the constituent colours separately, one beside
the other, and one through the other.
All the experiments we have described afford great interest to the
student; they can easily be performed by those of our readers who are
particularly interested in these little-known subjects. Any one may
construct the greater part of the appliances we have enumerated, and
others can be obtained at an optician’s. The discs in particular are
extensively manufactured, and with great success.
FOOTNOTES:
[10] _Traité de Physique_, Paris 1874.
[11] _Traité d’optique Physiologique._ French translation by MM. Javal
and Klein.