of their being “ultra-violet” rays, which was suggested by some. The
meaning of this term is seen when it is understood that a great distance
beyond the violet end of the visible spectrum there are radiations,
revealed by their photographic impressions, so that the whole spectrum
is really some eight times as long as the visible part. In consequence
of these ultra-violet rays acting on the photographic plate, it is
possible, as has long been known, to take a photograph in the dark. The
eye is quite insensible to the X rays also, and although these, as we
have seen, readily pass through the bodily tissues, it may be placed
quite near the discharge tube, the latter being enveloped in black
paper, without causing any sensation. That the new rays are in some way
allied to light is the opinion held by Dr. Roentgen, and he is inclined
to consider them as due to _longitudinal_ vibrations in the ether; that
is, instead of the transverse waves to which light is attributed, these
resemble the waves of sound, in so far that they move in the direction
of propagation. This would account for the absence of any distinct
refraction, or polarisation, which seems to characterise the X rays.
Their connection with certain electric Maxwell-Hertz waves (see p. 541)
is more problematical, as the mathematical formulæ for these admit only
transverse oscillation. But on the assumption of certain conditions, due
to the action of electricity, etc., on highly rarefied air, the possible
existence of longitudinal vibrations has been deduced by admitting a
certain variation in some of the factors of the Maxwell formulæ.
Other suggestions have been advanced in order to make the observed facts
concerning the X rays fit into established theories, but so far these
attempts have been unsuccessful. It would seem as if our present
conceptions of light, electricity, the ether and matter, will have to be
profoundly modified and enlarged in order to bring these and other
recently discovered phenomena within their scope. Since the publication
of Dr. Roentgen’s paper, his results have received confirmation in every
quarter, and many new observations have been added, some of which seem
to tend not so much to elucidate the phenomena, as to prove them even
more complicated than was at first supposed. Such was the announcement
in June, 1896, of the discovery of several varieties of X rays.
[Illustration:
FIG. 232_c_.—_Metal Objects taken through Calico and Sheet of
Aluminium._
]
In the meantime, various modifications have been made in the forms of
the tubes and electrodes, and divers arrangements have been used for the
exciting electrical apparatus. Thus it has been found that the X rays
are given off from platinum more copiously than from glass, aluminium,
or any other substance, and by using a tube closed by a “platinum
window,” on which the cathode rays impinge, Mr. Gifford has been able to
reduce the time of exposure for obtaining a skiagraph of the bones on
the hand to half a minute, whereas twenty times that period was formerly
required. Another form of tube is advertised by Brady & Martin of
Newcastle, with which, in conjunction with a new screen, a coil giving a
5–inch spark will, it is stated, yield a good skiagraph of the hand in
_two seconds_, which appears to be the shortest time yet attained.
Another firm of tube-makers, Newton & Co., London, state that their
special form of tube, excited by a coil giving a 6–inch spark, and used
with their fluorescent screen, “will work right through the human body,
showing the heart, liver, spine, ribs, the movements of the heart and of
the diaphragm, etc.” It has recently been observed that the best results
are obtained when there is a certain, but as yet undefined, relation
between the degree of rarefaction of the residual gas in the tube, and
the intensity or frequency of the electric discharges, and that these
should be accommodated to the work required. Thus, for example, if a
skiagraph of the hand be attempted with an apparatus in which these
factors are carried to too high a degree, the resulting X rays will pass
through the bones almost as freely as through the surrounding tissues,
and their shadows will therefore not appear. If, on the other hand, the
contrary conditions hold, an incomplete or maybe no result will be
found. This seems to explain the failures that have sometimes occurred
when tubes of apparently identical construction have been used in the
hands of the same, or of different, observers. Perhaps more depends also
on the time of exposure. For instance, if a short exposure be given in
the case of the hand, the photograph will be merely a silhouette of that
member; with a little longer exposure, this will show the nails; with
still longer time, the shadow of the fleshy parts begins to grow faint
and the skeleton to appear. With yet more prolonged exposure only the
bones will show, in their various degrees of opacity, and the shadows of
these will gradually disappear as the time of exposure is increased,
until at length the image will be entirely effaced. The considerable
differences as to distinctness of the various tissues, which are
exhibited by the published prints of hand shadows, are thus explicable.
[Illustration:
FIG. 232_d_.—_A Skiagraph of Layers of Various Substances._
]
[Illustration:
FIG. 233.—_Portrait of Professor Helmholtz._
]
SIGHT.
The investigations of modern science have borne rich fruit, not only by
vastly extending our knowledge of the universe of things around us, but
also making us acquainted with the mode in which certain agents act upon
our bodily organs, and by revealing, up to a certain point, what may be
termed the mechanism of that most wonderful thing—the human mind—or, at
least, that part which is immediately concerned in the perceptions of an
external world. Of all the physical influences which affect the human
mind, those due to light are the most powerful and the most agreeable.
One of the most ancient of philosophers says, in the simple words which
are appropriate to the expression of an undeniable truth, “Truly the
light is sweet, and a pleasant thing it is for the eyes to behold the
sun.” The impression produced by light alone is a source of pleasure—a
cheering influence of the highest order; and there is a special
character in the pleasing effects of light, from the circumstance that
they do not exhaust the sense so quickly as do even pleasurable
impressions on other organs—such as sweet tastes, fragrant odours, or
agreeable sounds. Sight is not liable to that satiety which soon
overtakes the enjoyment of sensations arising from the other senses; it
possesses, therefore, a refinement of quality of which the rest are
devoid. Sight converses with its objects at a greater distance than does
any other sense, and it furnishes our minds with a greater variety of
ideas. Indeed, our mental imagery is most largely made up of
reminiscences of visual impressions; for when the idea of anything is
brought up in our minds by a word, for example, there arises, in most
cases, a more or less vivid presentation of some visible appearance. Our
visual impressions are also longer retained in memory or idea than any
other class of sensations.
The nature of the impressions we receive through the eye is extremely
varied; for we thus perceive not only the difference between light and
darkness, but in the sensations of colour we have quite another class of
effects, while the lustre and sparkle of polished and brilliant objects
add new elements of beauty and variety. We find examples of the latter
qualities in the verdant sheen of the smooth leaf, in the splendid
reflections of burnished gold, in the bright radiance of glittering
gems, and “in gloss of satin and glimmer of pearls.” The eye is also the
organ which conveys to our minds the impressions of visible motion, with
all those pleasures of exciting spectacle which enter so largely into
our enjoyment of life. It likewise discriminates the forms, sizes, and
distances of objects; but by a process long misunderstood, and dependent
upon a set of perceptions which, although precisely those whence we
derive our most fundamental notions of the objects around us, have been
completely overlooked in that time-honoured enumeration of the senses
which recognizes only five.
If such be the extent to which our minds are dependent upon the
wonderful apparatus of the eye, it may easily be imagined what must be
the comparative narrowness of mental development in those who have never
enjoyed this precious sense, and the feeling of deprivation in those,
who, having enjoyed it, have unfortunately lost it. Well may our sublime
poet despairingly ask—
“Since light so necessary is to life,
And almost life itself—if it be true
That light is in the soul—
The all in every part: why was the sight
To such a tender ball as the eye confined,
So obvious and so easy to be quenched?”
—for he himself, in his own person, experienced this deprivation, and he
thus touchingly, in his great work, laments his loss:
“Thus with the year
Seasons return; but not to me returns
Day, or the sweet approach of even or morn,
Or sight of vernal bloom, or summer’s rose,
Or flocks, or herds, or human face divine;
But cloud instead, and ever-during dark
Surround me; from the cheerful ways of men
Cut off; and for the book of knowledge fair
Presented with a universal blank
Of Nature’s works—to me expunged and rased,
And wisdom at one entrance quite shut out.”
An organ which is the instrument of so many nice discriminations as is
the eye must, of course, present the most delicate adjustment in its
parts. So much has in recent times been learnt of the nature of its
mechanism; of the relation between the impressions made upon it and the
judgments formed by the mind therefrom; of the illusions which its very
structure produces; of the defects to which it is liable; and of its
wonderfully refined physiological elements—that a branch of science
sufficiently extensive to require a large part of a studious lifetime
for its complete mastery has grown up under the hands of modern
physiologists, physicists, and psychologists. To some of the results of
their labour we would invite the reader’s attention; and in order to
render the account of them intelligible, we must, to a certain extent,
describe “things new and old.”
_THE EYE._
[Illustration:
FIG. 234.—_Vertical Section of the Eye._
]
The form of the human eye and the general arrangement of its parts may
be understood by referring to Fig. 234, which is a section of the
eyeball. It has a form nearly globular, and is covered on the outside by
a tough firm case, A, named the _sclerotic coat_, which is, for the most
part, white and opaque. This covering it is which forms what is commonly
termed the “white of the eye;” but in the front part of the eyeball it
loses its opacity, and merges into a transparent substance, termed the
_cornea_, B. The cornea has a greater convexity than the rest of the
exterior of the eyeball, so that it causes the front part of the eye to
have a somewhat greater projection than would result from its general
globular form. This sclerotic coat—with its continuation, the
cornea—serves to support and protect the more delicate parts within, and
is itself kept in shape by the _humours_, which fill the whole of the
interior. The greater space is occupied by the _vitreous humour_, C; but
the space immediately behind the transparent cornea is filled with the
_aqueous humour_, D. The latter is little else than pure water, and the
former is like thin transparent jelly. The cavities containing these two
humours are separated by the transparent double convex lens, E, called
the _crystalline lens_, which, in consistence, resembles very thick
jelly or soft gristle. The outward surface of this lens has a flatter
curvature than the inner surface. Immediately in front of the
crystalline lens is found the _iris_, F, which may be described as a
curtain having in the middle a round hole. The iris is the part which
varies in colour from one individual to another—being blue, brown, grey,
&c.; and the aperture in its centre is the dark circular spot termed the
_pupil_.
The general disposition of the parts of the eye with regard to light
will be most easily understood by comparing it with an optical
instrument, to which it bears no little resemblance, namely, the _camera
obscura_, so well known in connection with photography. We may picture
to ourselves a still more complete resemblance, by imagining that the
lens of the camera is single, that we have fixed in front of it a
watch-glass, with the convex side outwards, and that we have filled with
water the whole of the interior of the camera, including the space
between the watch-glass and the lens. The _focussing-screen_ of the
camera corresponds with the inner surface of the back of the eyeball,
about which we shall presently have more to say. Now, even if the camera
had no lens, but were simply a box filled with water, and having in
front the watch-glass, fixed in the manner just mentioned, we could
obtain the images of objects on the screen, as a consequence of the
curvature of the watch-glass. It would, however, in this case, be
necessary to have the camera much longer, or, in other words, the rays
would be brought to a focus at a greater distance than if we put in the
glass lens, which would, thus placed in the water, cause the rays to
converge to a focus at a much shorter distance, although its effect when
surrounded by water would be less powerful than in the air. There we see
the effect of the crystalline lens of the eye in bringing the rays to a
focus within a much shorter distance than that which would be required
had there been present only the curved cornea, and the aqueous and
vitreous humours of the eye, which are but little different from pure
water in their optical properties.
If we _focus_ the camera by adjusting the distance between the lens and
the screen so as to get a distinct image of a near object, we should
find, on directing the instrument to a distant one, that the image would
be blurred and indistinct, and the lens would have to be moved nearer to
the screen; or we could get the image of the distant object distinct by
replacing the lens by another lens in the same position, but having some
flatter curvature. It is plain that the same object would be gained if
our lens could be made of some elastic material, which, on being pulled
out radially at its edges, could be made to assume the required degree
of flatness without losing its lenticular form. Now, it is precisely
with an automatic adjustment of this kind that the _crystalline lens_ of
the eye is provided, for the lens is suspended by an elastic ligament,
G, by the tension of which its surfaces are more flattened than they
would otherwise be; but when the tension of this ligament is relaxed, by
the action of certain delicate muscles which draw it down, the
elasticity of the lens causes it to assume a more convex form.
[Illustration:
FIG. 235. _Section of Retina._
]
These optical adjustments give, on the inner surface of the coats of the
eye, a more or less perfect real image of the objects to which the eye
is directed, and it is on the back part of this inner surface that the
network of nerves, called the _retina_, H, is spread out. The sclerotic
coat, already spoken of, is lined internally with another, named the
_choroid_, which is composed of delicate blood-vessels, intermingled
with a tissue of cells filled with a substance of an intensely black
colour. It is upon this last layer that the delicate membrane of the
retina is spread out between the choroid and the vitreous humour.
The retina is, in part, an expansion of the fibres of the optic nerve
over the back part of the eyeball. If we suppose the globe of this cut
vertically into two portions, and so divide the front from the back part
of the eye, the retina would be seen spread out on the concave surface
of the back part, and in the middle of this part, opposite the
crystalline lens, would be seen a spot in which the retina assumes a
yellowish colour, and in the centre of this, a little round pit or
depression. The spot is called the _macula lutea_, or _yellow spot_, and
the little central pit, which is of the highest importance in vision, is
termed the _fovea centralis_. A little way from the yellow spot, and
nearer the nose, is a point from which a number of fibres are seen to
radiate, and this is, in fact, the part at which the optic nerve enters
the eyeball, and from which it sends out its ramifications over the
retina. This part, for a reason which will shortly appear, is called the
_blind spot_.
When the minute structure of the retina is examined by the microscope,
its physiological elements are found to undergo very remarkable
modifications at the yellow spot. In the retina, although the total
thickness does not exceed the 1/80th part of an inch, no fewer than
eight or ten different essential or nervous layers have been
distinguished. Fig. 235 rudely represents a section. The lowest stratum,
A, which is next the choroid, and forms about a quarter of the total
thickness, is formed of a multitude of little rod-shaped bodies, _a_,
ranged side by side, and among these are the conical or bottle-shaped
bodies, _b_. This lowest stratum of the retina is called the _layer of
rods and cones_. At their front extremities the rods and cones pass into
very delicate fibres, which, going through an extremely fine layer of
fibres, B, are connected with a series of small rounded bodies, which
form the layer of _nuclei_, C, separated by a layer of nervous fibres,
D, from a granular layer, E, in front of which is a stratum of still
finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a
larger size than any of the other elements, and these ganglionic cells
send out numerous branching nerve-fibres, forming the layer H. Finally,
on the front surface of the retina there is a thin stratum formed of
fibres, which issue from the optic nerve, K, Fig. 234, and in fact
constitute the expansion of this nerve on the inner surface of the
eyeball. The terminations of some, at least, of these nerve-fibres have
been traced, and have been found to form junctions with those branching
from the ganglionic cells.
[Illustration:
FIG. 236.
]
Of the part played by each of these delicate structures in exciting
visual impressions little is yet known. How light, or the pulsations of
ether, if such there be, is ultimately converted into sensation will
probably for ever remain a mystery, although it is quite likely that the
kind of visual impression which is conveyed by each part of the
elaborate structure of the retina may ultimately be distinguished. One
curious result of modern investigation is that _light falling directly
upon fibres of the optic nerve is quite incapable of exciting any
sensation whatever_. Light has no more effect on this nerve and its
fibres than it would have on any other nerve of the body if exposed to
its action. The apparatus of rods, cones, and other structures are
absolutely essential to enable light to give that stimulus to the optic
nerve which, conveyed to the brain, is converted into visual sensations.
So if this apparatus were absent in our organs of vision, in vain would
the optic nerve proper be spread out over the interior of the eyeball:
we should be no more able to _see_ with such eyes than we are able to
see with our hands.
We now invite the reader’s careful consideration to the diagram, Fig.
236, which is a section of the retina through the yellow spot. The upper
part of the figure is the front, and the deep depression is the little
pit already spoken of—the _fovea centralis_. The lowest dark line
represents the basement membrane of the retina, and immediately above is
seen the layer of rods and cones, and the various strata already spoken
of are represented in their due order in the marginal parts of the
diagram. Now observe the remarkable modifications of the nervous
structures in the neighbourhood of the _fovea centralis_, some of which
are visible in the diagram. In the first place, the cones are there much
longer, more slender, and more closely set, so that there is a far
greater number of them on a given surface; but the rods are
comparatively few, and are, in fact, not found at all under the floor of
the little pit. The layer of _nuclei_, into which the cones extend, is
thinner, and is found almost immediately below the anterior surface, for
all the other layers thin out in the fovea in a very curious manner. It
is, however on the margin of the fovea that the stratum of ganglionic
cells, G, Fig. 235, attains its greatest thickness, for there it is
formed by the superposition of eight or ten cells, being here thicker
than any other layer, while it is so thinned off towards the margin of
the retina that it no longer forms even a continuous stratum. This
layer, however, becomes much thinner _in the fovea_, which contains, in
fact, but few superposed cells. The tint of the yellow spot is said to
be derived from a colouring matter, which affects all the layers except
that of the cones. The centre of the yellow spot, where the _fovea
centralis_ is situated, is extremely transparent, and is so delicate
that it is very easily ruptured, and has frequently been taken for an
aperture.
We should not have risked wearying the reader with these details
concerning the little pit in the centre of the retina had it not
possessed an extreme importance in the mechanism of the eye, a fact
which he will at once appreciate when we say that _of the whole surface
of the retina, the only spot where the image of an object can produce
distinct vision is the fovea centralis_. Since this is undoubtedly true,
it follows that the physiological elements which we there find are
precisely those which are most essential for producing this effect. The
case may be exemplified by recurring to the comparison of the eye with a
photographer’s camera, by supposing his screen to be of such a nature
that only on one _very small spot_ near its centre could a distinct
image be possibly obtained of just one point of an object. Such a defect
in his camera would render the photographer’s art impossible, and this
defect (if it may be so called) in the eye would render it almost
equally useless, had not an adjustment, which more than compensates for
it, been afforded in the extreme _mobility_ of our organs of vision.
This adjustment is so perfect that people in general do not even suspect
that the image of _each point_ of an object which they distinctly see
must be formed on one particular spot on the retina—a spot about
one-tenth of the diameter of an ordinary pin-head! We may venture,
without any disrespect to the reader, to assume that the chances are
that it is new to him to learn how each letter in the lines beneath his
eye must successively, but momentarily, form its image in the very
little pit in the centre of his retina; and the chances are at least a
hundred to one that, even if aware of this, he has passively received
the statement, and that he has not made the least attempt to _realize
the truth for himself_. Yet nothing is easier. Let him request a friend
to slowly peruse some printed page, while he meanwhile intently watches
his friend’s eyes. He will then perceive that before a single word can
be read there is a _movement_ of the eyeballs, which are, quite
unconsciously to the person reading, so directed that the image of each
letter (for the area of distinct vision is incapable of receiving more
than this at once) shall fall upon the only parts of the retinæ from
which a distinct impression can be conveyed along the optic nerve. Thus
it is that the eye, without any conscious effort of the observer, is
directed in succession to the various points of an object, and it is
only by an effort of will in fixing the eyes upon one spot that one
becomes aware of the blurred and confused forms of all the rest of the
visual picture. Yet so readily do the eyeballs turn to any part of the
indistinct picture on which the attention is fixed, that it is not
improbable a person unversed in such experiments, wishing to verify our
conclusions by looking, say, at one spot on the opposite wall, will be
very apt, in thinking of the features of the rest of the picture, to
direct his eyes there, and then declare that he, at least, sees no such
vague forms. If such be his experience, the correction is easy. He has
only to ask some one to watch closely his eyes while he repeats the
experiment, and after a few trials he will succeed in maintaining the
requisite immobility of the eyeballs—a condition upon which the success
of many such experiments depends.
[Illustration:
FIG. 237.—_Muscles of Eyes._
The muscles of the eyeballs viewed from above:—B, the internal rectus;
E, the external rectus; S, the superior rectus; T, the superior
oblique, passing through a loop of ligament at U, and turning
outwards and downwards to its insertion at C. The inferior rectus
and the inferior oblique are not visible in the figure: the superior
rectus is removed from the right eyeball in order to show the optic
nerve N.
]