of the clear and well-defined picture, for it calls into action one of
the most sensitive of all the impressions of which we are capable, and
one which possesses in so high a degree the power of uniting with our
other sensations, that this sixth sense has been, as already stated,
utterly overlooked, except by the more modern students of the nature of
our sensations. It is usually termed the _muscular sense_, and to it are
due some of the nicest distinctions of impressions of which we are
capable. The muscles of every part of our frame take their part in
producing impressions in our minds, and those of the eyeballs have a
very large share in furnishing us with ideas of forms and motions. Fig.
237 is a diagram showing the general arrangement of these muscles; and
their anatomical designations, which need not much concern us at
present, are given beneath the figure. The wonder is, that the
sensations arising from the relative conditions of parts so few, should
afford us the immense variety of notions referrible for their origin to
these muscles only. We take one example in illustration. Suppose we
watch the flight of a bird, at such an elevation that no part of the
landscape comes into the field of view at all; and that, again, we
follow with the eye, under similar circumstances, the path of a rocket.
We can unhesitatingly pronounce the motions unlike, and yet in each case
there was no visual impression present but that of the object focussed
upon the yellow spot. But the movement of _the muscles in one case_ was
different from that in the other. Nay more, we can form such a judgment
of the motion as to pronounce that the object followed such and such a
curve—we may recognize the parabola in one path, and the circle,
perhaps, in the other. And this kind of discrimination arises from the
fact, that when we have, maybe times without number, previously looked
at parabolas and circles, in diagrams perhaps, the muscles of the
eyeballs have performed just the same series of movements, as point
after point of the line was made to form its image on the yellow spot.
This is not the only class of impressions that these muscles are capable
of affording; there is, for example, little doubt that they aid us in
estimating distance. But space will not permit further discussion of
this subject.
Although the blurred and indefinite retinal picture may be compensated,
and perhaps more than compensated, by the readiness with which the eyes
move, it is, of course, possible that greater precision and delicacy of
visual impression over the whole surface of the retina might be
consistent with a still greater increase of our powers of perception.
There are instances in which the absence of finish, as it may be termed,
in all but one little spot in the picture, proves a real inconvenience
and a sensible deprivation. Perhaps a friend calls our attention to the
fact that a balloon is sailing through the air, or some fine morning,
hearing in the fields the blithe song of the sky-lark, we look up and
vainly try to bring the small image upon the place of distinct vision.
Now, if an image which falls upon any other part of the retina is
perceived, even indistinctly, an instant suffices to direct the eyes
into the exact position requisite for clear vision—an example of the
marvellous precision with which impressions are put in relation to each
other by the unconscious action of the brain. But while an image on the
fovea, only 1/6000th of an inch diameter, produces a distinct sensation,
it is found that if the image falls on the retina at a point some
distance from the yellow spot, the image must be 150 times larger in
order to produce any impression; and it is in consequence of the image
of balloon or bird not having the requisite size to give any impression
to the less sensitive portion of the retina, that we grope blindly, as
it were, until by chance the image falls near the yellow spot, when the
tentative motion of the eyeballs is instantly arrested, and the image
fixed. On the other hand, the field of indistinct vision which the eye
takes in is extremely wide, for bright objects are thus perceived, even
when their direction forms an angle laterally of nearly 90° with the
axis of the eye; and, if the object be not only bright, but in motion,
its presence is noticed under such circumstances with still greater
ease. Thus, an observer scanning the heavens would have a perception of
a shooting star anywhere within nearly half the hemisphere. The range
is, however, less than 90° in a vertical direction.
We have said that the fibres of the optic nerve, entering the back part
of the eyeball, at K, Fig. 234, ramify over the anterior surface of the
retina in fibres which form a layer of considerable relative thickness.
The light, therefore, first encounters these nerves, and only after
traversing their transparent substance does it reach the deeper seated
layer of rods and cones, where it excites some action that is capable of
stimulating the optic nerve. These rods and cones might naturally be
supposed to be merely accessory to the fibres of the optic nerve, had we
not the following conclusive evidence that the cones play a necessary
part in the action, and that it is only through them that light acts
upon the optic nerve:
[Illustration:
FIG. 238.
]
1. The cones are more developed and more numerous in the spot where
vision is most distinct.
2. The “blind spot” is full of fibres of the optic nerve, but is
absolutely insensible to light, and is without rods or cones.
3. We can distinguish an image on the fovea, having only 1/6000th of an
inch diameter; but on the other parts of the retina the images must have
larger dimensions. It is found that the size of the smallest
distinguishable images agrees nearly with the diameters of the cones at
the respective parts.
To some readers the fact will doubtless be new, that a considerable
portion of the eye is quite insensible to light, namely, that portion
already designated as the “blind spot.” A simple experiment, made by
help of Fig. 238, will prove this. Place the book so that the length of
the figure may be parallel to the line joining the eyes, and let the
right eye be exactly opposite the white cross, and at a distance from it
of about 11 in. If the left eye be now closed, while with the right the
cross is steadily viewed so that it is _always_ clear and distinct, the
white circle will completely disappear, and the ground will appear of a
uniform black colour. In order to insure success, the observer must be
careful not to _look at_ the white circle, but at the cross, and some
persons find this more difficult than others. The position of the blind
spot in the eye has been already mentioned, and its significance in
showing the insensibility to light of the fibres of the optic nerve has
been pointed out. In the table of the dimensions of some parts of the
eye, which, for convenience of reference, is given together below, it
will be seen that the diameter of the blind spot is considerable
compared with the size of the retina, its greatest diameter being about
8/100 in. The length on the retina of the image of a man at a distance
of 6 ft. or 7 ft. is not greater than this, so that in a certain
position with regard to the eye a person would, like the white circle,
be quite invisible. In like manner, by looking steadily in a certain
direction with one eye, the image of the full moon may be made to fall
upon the blind spot, and the luminary then becomes invisible, and would
be so even if its apparent diameter were eleven times greater; so that
if we suppose eleven full moons ranged in a line, the whole would be
quite invisible to a person looking towards a certain point of the sky
at no great angular distance from them.
The following are the dimensions in English inches of some parts of the
eye:
In.
Diameter of the entrance of the optic nerve 0·08
Distance of centre of optic nerve from centre of yellow spot 0·138
Diameter of _fovea centralis_ 0·008
Diameter of the nerve-cells of the retina 0·0005
Diameter of the _nuclei_ 0·00003
Diameter of the rods 0·00004
Diameter of the cones in yellow spot 0·00018
Length of rods 0·0016
Length of cones in yellow spot 0·0008
Thickness of retina at the back of the eye 0·0058
By means of an instrument to be presently described, the ophthalmoscope,
it is possible to view directly the whole surface of the retina, and to
observe the inverted images of the objects there depicted. It is thus
observed that it is only on the parts near the yellow spot that the
images are formed with clear and sharp definition. Away from this the
definition is less perfect; and besides the diminished sensitiveness of
the retina, this circumstance contributes to the vagueness of the visual
picture, although the falling off in clearness of vision at a very
little distance from the yellow spot is far more marked than the loss of
definition in the image there formed.
Until within the last few years it has been most confidently asserted by
many authors that the eye, considered as an _optical instrument_, is
absolutely perfect, and entirely free from certain defects to which
artificial instruments are liable. Thus Dr. W. B. Carpenter states, in
his “Animal Physiology” (1859): “The eye is much more remarkable for its
perfection as an optical instrument than we might be led to suppose from
the cursory view we have hitherto taken of its functions; for, by the
peculiarities of its construction, certain faults and defects are
avoided, to which all ordinary optical instruments are liable.” Among
the imperfections which are completely corrected in the eye, he names
“spherical aberration” and “chromatic aberration”—both of which give
rise to certain defects in optical instruments. But by recent careful
investigations it has been conclusively shown that the eye is not free
from chromatic aberration; that it has defects analogous to spherical
aberration; and that there are, besides, certain optical imperfections
in its structure, which are avoided in the artificial instruments.
Professor Helmholtz, one of the most distinguished of German
mathematicians, physicists, and physiologists, whose great work on
“Physiological Optics” is the most complete treatise on the subject
which has ever appeared, is so far from considering the eye as possessed
of all optical perfections that he remarks that, should an optician send
him an instrument having like _optical_ defects, he would feel justified
in sending it back. The defects which may be traced in the eye,
_considered as an optical instrument_, do not, however, he admits,
detract from the excellence of the eye _considered as the organ of
vision_.
When we find that Sir Isaac Newton pointed out the chromatic aberration
of the eye two centuries ago—when we find that D’Alembert, in 1767,
proved that the lenses of the eye might have as great a dispersive power
as glass without the want of achromatism necessarily becoming
noticeable—when we find that the celebrated optician Dolland, the
inventor of the achromatic lens, showed that the refractions which take
place in the eye all tend to bring the violet rays towards the axis more
than the red—when we find that Maskelyne the astronomer, Wollaston the
physicist, Fraunhofer the optician, and other scarcely less
distinguished men of science, have made actual measurements of the
distances of the _foci_ in the human eye for the different rays of the
spectrum—when we find how these defects have so long ago been observed,
examined, and measured as to their amount—the persistence with which
writer after writer has asserted the achromatism of the human eye
appears so extraordinary, that it can only be accounted for by the
prevalence of the preconceived notion that the eye is absolutely
perfect—a notion not without its reason and grounds, in the fact of the
exquisite adaptation of the organ of sight to the needs of humanity.
Although the want of achromatism in the eye thus escapes ordinary
notice, it is, on the other hand, easy to render it evident by simple
experiments. If, for example, we view from a certain distance the solar
spectrum projected on a white screen, it will be found that, when we see
the red end quite distinctly, the violet end will, at the same time,
appear vague and confused, and _vice versâ_. The author believes that
the following very simple experiment will at once convince any person
that the fact is as stated. Procure a small piece of blue or _violet_
stained glass, and another piece of _red_ glass, and, having cut out of
an opaque screen a rectangular opening, say ½ in. long and ¼ in. wide,
place the glasses close to it, so that one-half the opening is covered
by the red glass and the other half by the violet glass, the two being
placed so that, on looking through the screen, a violet square and a red
square are visible. The opaque screen may be made of black paper,
cardboard, or tinfoil, and the edges of the opening must be cut
perfectly even. On looking through this arrangement, held at a distance
of about two feet from the eye, both squares may be seen distinctly by a
person of ordinary vision; but, at a distance of five inches from the
eye, he will find it impossible to see the squares otherwise than with
vague and ill-defined edges. This is because the crystalline lens cannot
adapt its curvature so as to bring the rays from the object to a focus
on the retina. Now, by trial, the nearest distance at which each of the
coloured squares becomes visible may be found, and it will be observed,
that the violet square is first sharply defined at a less distance than
the red, whereas, if the eye brought the red and violet rays to a focus
at the same point, the smallest distance of distinct vision would
coincide in both cases.
The reader may observe the same fact for himself, in even a still
simpler manner, by turning to Fig. 238, page 461. When the white circle
is viewed by one eye, at a distance of about a foot, and an opaque
screen, such as a coin, is held close to the eye, so that the pupil is
half covered by it, the one side of the white circle will appear
bordered by a narrow fringe of blue, and the other side by a narrow
fringe of orange. If the opaque screen be shifted from one side of the
pupil to the other, the colours will change places, the orange appearing
always on the same side of the white circle as the screen is held before
the eye. The same appearances are presented in a still more marked
degree when the full moon is made the subject of the experiment.
The diagram, Fig. 239, shows the course of the red and violet rays from
a luminous point, A, the refraction being supposed to take place at
B_{1} B_{2}. The violet rays after refraction form the cone, B_{1}, E,
B_{2}, and E is their focus; the red rays form the cone, B_{1}, F,
B_{2}, and have a focus at F. The position of the retina would be
intermediate between E and F, and is indicated by C_{1}, C_{2}. It will
be noticed that the violet rays cross, and are received on the retina in
the same circle, G G, so that the colours, then blended, would be
separately imperceptible; but the point would produce a diffused
circular image of the blended colours.
In viewing an object—the moon, for example—the accommodation of the eye
is like that indicated in the diagram. The distinct image due to the red
rays would be formed behind the retina, and that due to the violet rays
would be in front of it. In the image on the retina the most intense
rays—such as the orange, yellow, and green—are those which are blended
by the adjustment of the eye, and the red and violet form images more
out of focus (to use a common expression), and a very little larger than
the more intense image. We might expect that a white disc would
therefore appear with a fringe of colour, resulting from a mixture of
red and violet; but the fringe is too narrow, and the colour itself too
feeble, to become perceptible. When, however, the pupil of the eye is
half covered, the red and violet images are displaced in different
directions, the position of the retina being too far forward for the
one, and too far back for the other. The coincidence therefore ceasing,
the colours show themselves at the margins of the image.
[Illustration:
FIG. 239.
]
The non-perception under ordinary circumstances of the chromatic
aberration of the eye is largely due to the greater intensity of the
colours which differ least in their refrangibilities. The clearness of
our vision does not, therefore, practically suffer from this defect of
the eye. Professor Helmholtz constructed lenses which rendered his eyes
really achromatic, and looking through these when the pupil was half
covered, no coloured fringes were seen at the edges of dark or light
objects, or when the objects were looked at with an imperfect
accommodation of the eye. He was, however, unable to detect any increase
of clearness or distinctness of vision by the correction.
The eye is also subject to other aberrations and irregular refractions,
which are special to itself; for example, with moderately illuminated
objects the crystalline lens produces images apparently well defined,
and nothing is visible to suggest the absence of uniformity in its
structure. But when the light is intense, and concentrated in a small
object surrounded by a dark field, the irregular structure of the
crystalline lens shows itself in the most marked manner. Every one must
have noticed the appearance presented by the distant street-lamps on a
dark night, and by the stars. The latter we know to be for us mere
points of light, and their images produced by perfect lenses would also
be mere points; instead of which we see what seem to be rays issuing
from the star, an appearance which has given rise to the ordinary
representation of a star as a figure having several rays. That no such
rays actually do emanate from the real star may be easily proved: first,
by concealing the luminous point from view, by means of a small object
held up as a screen. If the rays had any existence outside of the eye,
they would still be seen; instead of which, the whole of them disappear
when the luminous point, or, in the case of the street-lamp, when the
flame, is covered by the screen. A second proof that the origin of the
phenomenon is in the eye, and not in the object, is afforded by the fact
that if, while attentively observing the rays, we incline the head, the
rays turn with the eyes, so that when the head is resting on the
shoulder the ray which appeared vertical becomes horizontal. The cause
of these divergences from the regular image lies in the fact of the
crystalline lens being built up of fibres which have refractive powers
somewhat different from that of the intermediate substance. These fibres
are arranged in layers parallel to the surfaces of the crystalline lens,
and the direction of the fibres in each layer is generally from the
centre to the circumference; but towards the axis they form, by bending,
a kind of six-rayed figure, as shown in Fig. 240, which represents the
arrangement of the fibres of the external layers of the lens. In the
outermost layers the branches of the star-shaped figure are subdivided
into secondary branches, which give rise to more complicated figures.
When we view by night a very brilliant but small light, even these
subdivisions may be traced in the radiating figure.
[Illustration:
FIG. 240.
]
The light which enters the eye is partly absorbed by the black pigment
of the choroid, and partly sent back by diffused reflection from the
retina through the crystalline lens and pupil. The image of a luminous
body as depicted on the retina of another person cannot be seen by us
under ordinary circumstances, because, by the principle of reversibility
already mentioned as of universal application in optics, the rays which
issue from the retinal images are refracted on leaving the eye, and
follow the same paths by which they entered it, so that they are sent
back to the object. An observer cannot see the retinal image of a candle
in another person’s eye, unless he allows the rays to enter his own, and
this cannot be done directly, because the head of the observer would be
interposed between the candle and the eye observed, and the light would
then be intercepted. By holding a piece of unsilvered plate glass
vertically, we may reflect the light of a candle into the eye of another
person, and then the light thrown out from the retinal image of the
candle will, on again meeting the surface of the glass, be in part
reflected to its source, and in part pass through the glass, on the
other side of which it may be received into the eye of an observer. The
positions of the observed and observing eye may be described as exactly
opposite to and near each other, while the candle is placed to one side
in the plane separating the two eyes, and the glass is held so that it
forms an angle of 45° with the line joining the pupils. Under these
circumstances the observer may see the light at the back of the eye, but
he will not be able to distinguish anything clearly, because his own eye
cannot accommodate itself so as to bring to a focus the rays coming from
the retina of the other, since these rays are refracted by the media
through which they emerge. But, by means of suitable lenses interposed
between the two eyes, the retina and all its details may be distinctly
seen and examined. Such an arrangement of lenses and a reflecting
surface constitute the instrument called the _ophthalmoscope_ (οφθαλμος,
_the eye_) of which there are many forms, but all constructed on the
principle just indicated. This principle was first pointed out by
Helmholtz, who described the first ophthalmoscope in 1851.
[Illustration:
FIG. 241.—_Ruete’s Ophthalmoscope._
]
Ruete’s ophthalmoscope is represented in Fig. 241. The parts of the
instrument are supported on a stand, C, and about the vertical axis of
this the column, D, and the arms, H and K, can turn freely and
independently; E is a concave metallic mirror, about 3 in. in diameter,
and having an aperture in its centre through which the observer, B,
looks. The arm, H, merely carries a black opaque screen, which serves to
shield the eye of B from the light of the lamp, and to reduce, if
required, the amount of light passing through the aperture in the
mirror. The arm, K, which is about a foot in length, carries two
uprights which slide along it, and in each of these slides a rod bearing
a lens, which can thus be adjusted into any required position. The
instrument is used in an apartment where all light but that of the lamp
can be excluded. In the instrument just described an inverted image is
obtained, which is sufficient for ordinary medical purposes, but this
construction does not allow of the examination of retinal images, which
is best performed with an instrument having a plane mirror.
The appearance presented by the back of the eye when viewed in the
ophthalmoscope is represented in Fig. 242. The retina appears red,
except at the place where the optic nerve enters, which is white. On the
reddish ground the retinal blood-vessels can be distinguished; A, A, A,
branches of the retinal artery, have a brighter red colour, and more
strongly reflect the light than the branches, B, B, B, of the retinal
vein. Among these, and especially towards the margin, are seen, more or
less distinctly, the broader vessels of the choroid. Above the optic
nerve and a little to the right may be observed the _fovea centralis_.
[Illustration:
FIG. 242.
]
During the last twenty years the ophthalmoscope has been the chief means
of extending the knowledge of oculists regarding the diseased and
healthy conditions of the eye. In this way the substance of the lens and
the state of the humours can be directly seen, the causes of impaired
vision can be discovered, and the nature of many maladies made out with
certainty. This modern invention, by which the interesting spectacle of
the interior of the living eye can be observed, has therefore been far
from proving a barren triumph of science. Many insidious maladies can
thus be detected, and may be successfully treated before the organ has
become hopelessly diseased. In some cases the ophthalmoscope gives the
most certain evidence of the existence of obscure and unsuspected
diseases of other parts of the body.
_VISUAL IMPRESSIONS_.