OF THE SENSE OF SEEING.
Having described the parts of which the human body consists, let us
now proceed to examine its principal organs; the expansion of the
senses, and their several functions; and at the same time, point out
the errors to which, through them, we are in some measure subjected by
Nature.
The eyes seem to be formed very early in the human embryo. In the
chicken also, of all the double organs they are the soonest produced.
I have observed in the eggs of several sorts of birds, and in those of
lizards, that the eyes were more large, and early in their expansion,
than any other parts of a two-fold growth. Though, in viviparous
animals, and particularly in man, they are, at first, by no means so
large in proportion as in the oviparous, yet they obtain their due
formation sooner than any other parts of the body. It is the same with
the organ of hearing; the small bones of the ear are entirely formed
before any of the other bones have acquired any part of their growth or
solidity. Hence it is evident, that the parts of the body, which are
furnished with the greatest quantity of nerves, as the ears and eyes,
are those which first appear, and which are the soonest brought to
perfection.
If we examine the eyes of an infant, a few hours after its birth, we
shall discern that it cannot make the smallest use of them; the organ
not having acquired a sufficient consistency, the rays of light strike
but confusedly upon the retina. Before the sixth week, children turn
their eyes indiscriminately upon every object, without appearing to be
affected by any, but at about this time they begin to fix them upon
the most brilliant colours, and seem peculiarly desirous of turning
them towards the light; this exercise does not give any exact notion of
objects, but strengthens the eye, and qualifies it for future vision.
The first great error in the sense of seeing, is the inverted
representation of objects upon the retina, and, till the sense of
feeling has served to undeceive it, the child actually beholds every
thing upside down. The second error in early vision is, that every
object appears double; from the same object being formed distinctly
upon each eye. This illusion like the other, can only be corrected
by children from their being in the practice of handling different
objects, and from which practice alone it is that they learn things are
neither inverted nor double, and custom induces them to believe they
see objects in the position the touch represents them to the mind; and
therefore, were we denied the sense of feeling, that of seeing would
not only deceive us as to the situation, but as to the number of every
object around us.
We may easily be convinced that objects appear inverted, (which arises
from the structure of the eye) by admitting the light to pass into a
darkened room through a small aperture, when the images of the objects
without will be represented upon the wall in an inverted position; for,
as all the rays which issue from their different points cannot enter
the hole in the same extent and position which they had in leaving the
object, unless the aperture was as large as the object itself; as every
part of the object sends forth its image on all sides; as the rays
which from those images flow from all points of the object, as from so
many centres, those only can pass through the small aperture which come
in opposite directions. Thus the hole becomes a centre for the entire
object, through which the rays from the upper, as well as from the
lower parts of it, pass in converging directions; and, of consequence,
they must cross each other at this centre, and thus represent the
objects upon the wall, in an inverted position.
That we, in reality, see all objects double, is also evident; for
example, if we hold up a finger, and look with the right eye at an
object, it will appear against one particular part of the room,
shutting that and looking with the left, it will seem to be on a
different part, and if we open both eyes, the object will appear to
be placed between the two extremes. But the truth is, the image of
the object is formed in both eyes, one of which appears to the right,
and the other to the left; and it is from the habit of touching that
we suppose we see but one image placed between them both. From which
it is clear that we see all objects double, although our imagination
forms them single; and that, in fact, we see things where they are not,
notwithstanding we have a pretty exact idea of their situation and
position; and thus it is that till the sense of feeling has corrected
the errors of sight, if instead of two eyes, we had an hundred, we
should still fancy the objects single, although they were multiplied an
hundred times.
In each eye, therefore, is formed a separate image of every object;
and when the two images strike the correspondent parts of the retina,
that is, the parts which are always affected at the same time, the
object appears single: but, when the images strike the parts of the
retina, which are not usually affected together, then it appears
double, because we are not habitual to this unusual sensation, and are
then somewhat in the situation of infants just beginning to exert the
faculty of vision.
M. Chesselden relates the case of a man, who, in consequence of a blow
on the head, became squint-eyed, and saw objects double for a long
time: but who was at length enabled, by slow and gradual steps, to see
them singly as he had formerly done, notwithstanding the squinting
remained. Is not this a proof, still more evident, that in reality
we see things double, and that it is by habit alone we conceive them
to be single? Should it be asked why children require less time, in
order to see things single, than persons more advanced in years,
whose eyes may have been affected by accident? it might be answered,
that the sensations of children, being unopposed by any contradictory
habit, these errors are rectified with ease; but that persons who have
for many years seen objects single, because they affected the two
correspondent parts of the retina, and who now see them double, labour
under the disadvantage of having a contrary habit to oppose, and must
therefore be a considerable time before it is entirely obliterated.
By the sense of seeing we can form no idea of distances; without aided
by the touch, every object would appear to be within our eyes; and
an infant, that is as yet a stranger to the sense of feeling, must
conceive that every thing it sees exists within itself. The objects
only appear to be more or less bulky as they approach to, or recede
from the eye; insomuch, that a a fly near the eye will appear larger
than an ox at a distance. It is experience alone that can rectify this
mistake; and it is by constantly measuring with the hand, and removing
from one place to another, that children obtain ideas of distance and
magnitude. They have no conception of size but from the extreme rays
reflected from the object, of course every thing near appears large,
and those at a distance small. The last man in a file of soldiers
appears much more diminutive than the one who is nearest to us. We do
not, however, perceive this difference, but continue to think him of
equal stature; for the number of objects we have seen thus lessened by
distance, and found by repeated experience to be of the natural size
when we come closer, instantly correct the sense, and therefore we
perceive every object nearly in its natural proportion, unless when
we observe them in such situations as have not allowed us sufficient
experience to correct the illusions of the eye. If, for example,
we view men upon the ground, from a lofty tower, or look up to any
object upon the top of a steeple, as we have not been in the habit of
correcting the sense in that position, they appear to us exceedingly
diminished, much more so than if we saw them at the same distance in an
horizontal direction.
Though a small degree of reflection may serve to convince us of the
truth of these positions, yet it may not be amiss to corroborate them
by facts which cannot be disputed. M. Chesselden, having couched for a
cataract a lad of thirteen years of age, who had from his birth been
blind, and thus communicated to him the sense of seeing, was at great
pains to mark the progress of his visual powers; his observations were
afterwards published in the Philosophical Transactions. This youth
was not absolutely and entirely blind: Like every other person, whose
vision is obstructed by a cataract, he could distinguish day from
night and even black from white, but of the figure of bodies he had
no idea. At first the operation was performed only upon one of his
eyes, and when he saw for the first time, so far was he from judging
of distances, that he supposed (as he himself expressed it) every
thing he saw touched his eyes, in the same manner as every thing he
felt touched his skin. The objects that pleased him most were those
whose surfaces were plain and the figures regular, though he could in
no degree judge of their different forms, or assign why some were more
agreeable to him than others. His ideas of colours during his former
dark state were so imperfect, that when he saw them in reality he could
hardly be persuaded they were the same. When such objects were shewn
him as he had been formerly familiar with by the touch, he beheld them
with earnestness, in order to know them again, but as he had too many
to retain at once the greatest number were forgotten, and for one
thing which he knew, after seeing it, there were a thousand, according
to his own declaration, of which he no longer possessed the smallest
remembrance. He was very much surprised to find that those persons, and
those things, which he had loved best, were not the most pleasing to
his sight; nor could he help testifying his disappointment in finding
his parents less handsome than he had conceived them to be. Before he
could distinguish that pictures resembled solid bodies above two months
elapsed; till then he only considered them as surfaces diversified by a
variety of colours; but when he began to perceive that these shadings
actually represented human beings, he expected also to find their
inequalities; and great was his surprise to find smooth and even what
he had supposed a very unequal surface; and he inquired whether the
deception existed in feeling or seeing. He was then shewn a miniature
portrait of his father, which was contained in his mother's watch-case,
and though he readily perceived the resemblance, yet he expressed his
amazement how so large a face could be comprised in so small a compass;
to him it appeared as strange as that a pint vessel should contain a
bushel. At first he could bear but a very small quantity of light, and
every object appeared larger than the life; but in proportion as he
observed objects that were in reality large, he conceived the others
to be equally diminished. Beyond the limits of what he saw he had no
conception of any thing. He knew that the apartment he occupied was
only a part of the house, and yet he could not imagine how the latter
should be larger than the former. Before the operation he formed no
great expectations of the pleasure he should receive from the new sense
he was promised, excepting that thereby he might be enabled to read
and write. He said among other things, that he could enjoy no greater
delight from walking in the garden, because there he already walked at
his ease, and was acquainted with every part of it. He also remarked
that his blindness gave him one advantage over the rest of mankind,
namely, that of being able to walk in the night with more confidence
and security. No sooner, however, had he began to enjoy his new sense
than he was transported beyond measure; he declared that every new
object was a new source of delight, and that his pleasure was so great
he had not language to express it. About a year after, he was carried
to Epsom, where there is a beautiful and extensive prospect; with this
he seemed greatly charmed; and the landscape before him he called a
new method of seeing. He was couched in the other eye a year after
the former, and the success was equally great. Every object appeared
larger when he looked at it with the second eye to what it did with the
other; and when he looked at any thing with both eyes it appeared twice
as large as when he saw but with one, though he did not see double, or
at least he shewed no marks from which any such conclusion might be
drawn.
Mr. Chesselden instances several other persons who were in the same
situation with this lad, and on whom he performed the same operation;
and he assures us, that on first obtaining the use of their eyes they
expressed their perceptions in the same manner, though less minutely;
and that he particularly observed of them all, that as they had never
had any occasion to move their eyes while deprived of sight, they were
exceedingly embarrassed in learning how to direct them to the objects
they wished to observe.
As from particular circumstances we cannot from a just idea of
distance, and as we cannot judge of the magnitude of objects but by the
largeness of the angle, or rather the image, which they form in our
eyes, we are necessarily deceived as to the size of such objects. Every
man knows how liable we are, in travelling by night, to mistake a bush
which is at hand for a tree at a distance, or a tree at a distance
for a bush which is at hand. In like manner, if we cannot distinguish
objects by their figure we cannot judge of distance or size. In this
case a fly, passing with rapidity before our eyes, will appear to be a
bird at a considerable distance; and an horse standing in the middle of
a plain will appear no bigger than a sheep till we have discovered that
it is a horse, and then we shall recognize it to be as large as life.
Whenever, therefore, we find ourselves benighted in an unknown place,
where upon account of the darkness no judgment is to be formed of
distance, or figures of the objects that may present themselves, we
are every moment in danger of being misled with respect to our ideas
of such objects. Hence proceeds that internal fear and dread which
most men experience from the obscurity of night, and of those strange
and hideous spectres and gigantic figures which so many persons tell
us they have seen. Though such figures, it is commonly asserted, exist
solely in the imagination, yet they may appear literally to the eye,
and be in every respect seen as described to us; for when we reflect
that whenever we cannot judge of an unknown object but by the angle
which it forms in the eye, this object is magnified in proportion to
its propinquity; and that if it appears at the distance of twenty or
thirty paces to be only a few feet high, when advanced within a short
space of it, it will seem to be of considerable magnitude. At this
the spectator must naturally be astonished and terrified, till he
approaches and knows it by feeling; for in the very instant that he
has an actual perception of what it is, the tremendous form it assumed
to the eye will diminish, and it will appear in no other than its real
and absolute form. If, on the other hand, he is afraid to approach it,
and flies from the spot with precipitation, the only idea he can have
of it will be that of the image which had been formed in his eye; the
image of a figure he had seen, gigantic in its size, and horrible in
its form. The prejudice with respect to spectres, therefore, originates
from Nature, and depend not, as some philosophers have supposed, solely
upon the imagination.
When we cannot form an idea of distance, by the knowledge of the
intermediate space between us and any particular object, we endeavour
to distinguish the form of that object, in order to judge of its
size; but when we cannot perfectly distinguish the figures, and at the
same time behold a number of objects, whose forms are correspondent,
we conceive those which are most brilliant are most proximate, and
those most obscure are most remote; a notion which is not unoften the
source of very singular mistakes. In a multitude of objects disposed
in a right line, as the lamps upon the road from Versailles to Paris,
of which, as we cannot judge of the proximity or remoteness but by the
quantity of light they transmit to the eye, it often happens that when
examined at the distance of the eighth of a league, we see all the
lamps situated on the right hand instead of the left, on which they
are in reality situated. This fallacious appearance is produced from
the above-mentioned cause, for as the spectator has no evidence of the
distance he is from the lamps, but by the quantity of light they emit,
so he conceives that the most brilliant lamps are those which are the
first and the nearest to him. Now if some of the first lamps happen to
be dull and obscure, and any one of the others particularly bright,
that one would appear to be first and the rest behind, whatever was
their real situation; and this seeming transposition would be solely
owing to the supposed change of their situation from the left hand to
the right; for to conceive to be before what is actually behind in a
long file, is to see on the right what is situated on the left, or on
the left what is situated on the right.
We may fairly consider sight as a species of touching, though very
different from what we commonly understand by that sense; for in order
to exercise the latter we must be near the object, whereas we can
touch with the eye as far as the light the object contains will make
an impression, or its figure form an angle therein. This angle, when
the object is viewed at the greatest distance, is about the 3436th
part of its diameter, therefore an object of a foot square is not
visible beyond 3436 feet, or a man of five feet high at a greater
distance than 17180 feet. But the extent of vision is in some measure
influenced by the light which surrounds us, and we should be enabled to
see any object in the night at 100 times greater distance than in the
day, provided it was equally illuminated; thus, for instance, we can
perceive a lighted candle at full two leagues in the night, supposing
the diameter of the luminary to be one inch, whereas in the day we
should not be able to discern it beyond the proportion of the above
ratio; and as this is a circumstance which attends all objects when
viewed at those different periods, we may conclude that one principal
reason for our not being able to discern things at a greater distance,
is the brilliancy of the light which fills up the intermediate space,
and so destroys the reflected rays from those still more distant
objects. When we are surrounded with strength of light the objects near
make a forcible impression on the retina of the eye, and obliterate
those far off, which are weak and faint; and, on the contrary, if we
view a luminous body in the night, even at a considerable distance,
that becomes perfectly visible, while those which are near are scarcely
discernible. From these reasons it is, that a man at the bottom of a
deep pit can see the stars, or, by employing a long tube in a dark
room, may obtain some effects from the telescope in the middle of the
day. From this it is evident, that if bodies were furnished with more
strength of light they would be visible at greater distances, although
the angle was not increased, for a small candle, which burns bright,
is seen much farther off than a flambeau that is dim. Of these facts,
relative to the influence of light, we have a still stronger proof
in the variation between a microscope and telescope, both of them
instruments of the same kind, increasing the visible angles of objects,
whether they be really minute or rendered so by distance, and yet the
latter does not magnify beyond a thousand times, whereas the former
will exceed a million, and this difference plainly arises solely from
the degree of light, for could the distant object be additionally
illuminated, telescopes would have the same effect upon distant objects
as microscopes have upon small bodies. But it is only by comparing
the size of the angle formed in the retina of the eye, the degree of
light which illuminates the adjacent and intermediate objects, and the
strength of light which proceeds from, or is reflected by the object
itself, that we can conclude upon the distance at which any particular
body will be visible.
The power of seeing objects at a distance is very rarely equal in
both eyes. When this inequality is great, the person so circumstanced
generally shuts that eye with which he sees the least, and employs the
other with all its power, and which is one cause of squinting. The
object does not appear doubly distinct, by both eyes being placed upon
it although they are equally strong, but has frequently been proved
not to exceed a 13th part more than if beheld with one; and this is
supposed to arise from the two optic nerves uniting near the place
they came out of the skull and then separating by an obtuse angle
before they enter the eyes; but as the motion made by the impression
of objects cannot pass to the brain without passing this united part,
the two motions must therefore be combined, and, consequently, cannot
act with that force as though they were distinct; but from repeated
experiments seem to bear the proportion above stated.
There are many reasons to suppose that short-sighted persons see
objects larger than others; and it is a certain truth that they see
them less. I am myself short-sighted, and my left-eye is stronger than
my right. A thousand times have I experienced, upon looking at any
object, as the letters of a book, that they appear least to the weakest
eye; and that, when I place the book so that the letters appear double,
the images of the left-eye, are greater than those of the right.
Several others, I have examined, who were in similar circumstances,
and I have always found, that the eye which saw every object best,
saw it also largest. This may be ascribed to particular habits; for
near-sighted people being accustomed to approach close to the object,
and to view but a small part of it at a time, they acquire a small
standard for magnitude, and when the whole of the object is seen, it
necessarily appears smaller to them than to others, whose vision is
more enlarged.
There have been many instances of persons becoming short-sighted on a
sudden, therefore attributing it to the roundness or prominence of the
eye is by no means certain. Mr. Smith, in his Optics, speaks of a young
man that became short-sighted as he quitted a cold bath, and who was
under the necessity of using a concave glass all his life after; and it
cannot be supposed that the vitreous humours were instantly inflated so
as to cause this difference in vision. Short-sightedness may arise from
the position of the various parts of the eye, especially the retina,
from a less degree of sensibility in the retina, or the smallness
of the pupil. In the two first cases a concave glass may be used to
advantage, but yet objects will not be seen so far, or so distinct,
through these glasses as others will perceive with the eye alone, for
as short-sighted persons see objects in a diminished form, the concave
glass diminishes them still farther.
Infants having their eyes smaller than those of adults, must of
consequence, see objects smaller also. For as the image formed on
the back of the eye must be large, as the eye is capacious, so
infants, having it not so great, cannot have so large a picture of
the object. This may likewise be a reason, why they are unable to see
so distinctly, or at such distances, as persons who have attained the
years of maturity, for as objects appear less they must sooner become
invisible.
Old people see bodies close to them very indistinctly, but bodies at
a great distance from them with more precision, than young ones. This
may happen from an alteration in the coats, or perhaps the humours
of the eye; and not, as is supposed, entirely from their diminution.
The cornea, for instance, may become too rigid to adapt itself, and
take a proper convexity for seeing near objects, as a flatness must be
occasioned by drying that will be sufficient of itself to render their
eyes more calculated for distant vision. Although clear and distinct
are frequently confounded by writers on optics, yet they are very
different; for we may be said, for instance, to clearly see a tower,
as soon as we get a view of it, but we must approach near enough to
distinguish its component parts before we see it distinctly. Men in
years see clearly, but not distinctly; they can discern large bodies
at a distance, but cannot distinguish small objects, as the characters
in a book, without the help of magnifying glasses. On the contrary,
short-sighted people see small objects distinctly, but need the aid of
concave glasses to reduce large ones. Much light is also necessary for
clear sight, while a small quantity is sufficient for distinct vision.
When an object is extremely brilliant, or we fix our eyes too long
upon the same object, the organ is hurt and fatigued, vision becomes
indistinct, and the image of the object, having made too violent an
impression, appears painted on every thing we look at, and mixes with
every object that occurs. How dangerous the looking upon bright and
luminous objects is to the sight, is evident from the effect it has on
the inhabitants of countries which are covered for the greatest part
of the year with snow; and travellers, who cross those countries, are
obliged to cover their eyes with crape. In the sandy plains of Africa,
the reflection of the light is so strong, that it is impossible for the
eye to sustain the effects of it. Such persons therefore, as write, or
read for any continuance, should chase a moderate light, for though
it may seem insufficient at first, yet the eye will gradually become
accustomed to the shade; and at any rate, it will be less injured by
too little light than by too much.