Walk unsustained, and, unsupported, feed.
They live _at once_,--forsake the dam's warm side,--
Take the wide world, with nature for their guide,--
Bound o'er the lawn, or seek the distant glade,
And find a home in each delightful shade.”[121]
This instinctive suggestion, which, however subsequent it may be to the
primary visual sensation, seems like immediate perception in the young
of other races of animals, is a very strong additional proof, if any
such were necessary, that there is no physical impossibility, in the
supposition that a similar original suggestion may take place in man.
The question, as I before said, becomes truly a question of observation
and experiment.
But, in man, there is not that necessity for the instinct, which
exists in the peculiar situation of the other animals; and we find
accordingly, that there is no trace of the instinct in _him_. It is
long before the little nurseling shews, that his eye has distinguished
objects from each other, so as to fix their place. We are able
almost to trace in his efforts the progress which he is gradually
making;--and, in those striking cases, which are sometimes presented to
us, of the acquisition of sight, in mature life, in consequence of a
surgical operation,--after vision had been obstructed from infancy,--it
has been found, that the actual magnitude and figure, and position,
of bodies, were to be learned like a new language,--that all objects
seemed equally close to the eye,--and that a sphere and a cube, of
each of which the tangible figure was previously known, were not so
distinguishable in the mere sensation of vision, that the one could
be said, with certainty, to be the cube, and the other the sphere. In
short, what had been _supposed_, with every appearance of probability,
was _demonstrated_ by experiment,--that we _learn_ to _see_,--and that
vision is truly, what Swift has paradoxically defined it to be, _the
art of seeing things that are invisible_.
FOOTNOTES:
[113] Essays--Inquiry concerning Human Understanding, Sect. xii. Part 2.
[114] Inquiry into the Human Mind, &c. Chap. v. Sect. 7.
[115] Darwin's Botanic Garden, Canto II. v. 203-6.
[116] Gray, de Princip. Cogit. lib. i. v. 85-96.
[117] Paradise Lost, Book III. v. 1-12.
[118] Ib. v. 38, 39.
[119] Ib. v. 40-41.
[120] Samson Agonistes, v. 93-97.
[121] Young's Paraphrase on a part of the Book of Job, v. 235-240.
LECTURE XXIX.
ANALYSIS OF THE FEELINGS ASCRIBED TO VISION, CONTINUED.
The chief part of my last Lecture was employed in considering
the Phenomena of _Vision_, and particularly in proving, that
vision,--simple and immediate, as it now seems to us, even in its most
magnificent results,--is truly the application of an art, of long
and tedious acquirement,--of that art with which we learn to measure
forms and distances, with a single glance, by availing ourselves of
the information, previously received from other sources;--the mixed
product of innumerable observations, and calculations, and detections
of former mistakes--which were the philosophy of our infancy, and each
of which, separately, has been long forgotten,--recurring to the mind,
in after-life, with the rapidity of an instinct.
Of all the arts, which man can acquire, this is, without question, the
richest, both in wonder and in value--so rich in value, that if the
race of man had been incapable of acquiring it, the very possibility of
their continued existence seems scarcely conceivable; and so rich in
subjects of wonder, that to be most familiar with these, and to study
them with most attention, is to find at every moment new miracles of
nature, worthy of still increasing admiration.
“Per te quicquid habet mundus, mirabile nobis,
Panditur; acceptumque tibi decus omne refertur
Terrarum. Gentes nequicquam interluit æstu
Vicinas pelagus; tu das superare viarum
Ardua, et obtutu Seston conjungis Abydo.
Necmaris angusti tantum discrimina solers
Decipis, oceanique moras; Tu sidera Cœli
Subjicis humanis oculis, et dissita longe
Das spectare loca, et Dias invisere sedes.
Nativa hinc quamvis ferimur gravitate deorsum
Ad Stygias sedes, Ditisque inamabile regnum,--
Mente tamen sursum rapti ad sublimia; molem
Exuimus terrenam, animosque æquamus Olympo.”[122]
On this subject the remarks of Dr Reid, which I am about to quote,
are not less just than they are strikingly expressed. “If we shall
suppose an order of beings, endued with every human faculty but that
of sight, how incredible would it appear to such beings, accustomed
only to the slow informations of touch, that, by the addition of an
organ, consisting of a ball and socket of an inch diameter, they might
be enabled in an instant of time, without changing their place, to
perceive the disposition of a whole army, or the order of a battle, the
figure of a magnificent palace, or all the variety of a landscape? If a
man were by feeling to find out the figure of the peak of Teneriffe, or
of even St Peter's Church at Rome, it would be the work of a lifetime.
“It would appear still more incredible to such beings as we have
supposed, if they were informed of the discoveries which may be made
by this little organ in things far beyond the reach of any other
sense: That by means of it we can find our way in the pathless ocean;
that we can traverse the globe of the earth, determine its figure and
dimensions, and delineate every region of it. Yea, that we can measure
the planetary orbs, and make discoveries in the sphere of the fixed
stars.
“Would it not appear still more astonishing to such beings, if they
should be further informed, That, by means of this same organ, we can
perceive the tempers and dispositions, the passions and affections of
our fellow-creatures, even when they want most to conceal them? That
when the tongue is taught most artfully to lie and dissemble, the
hypocrisy should appear in the countenance to a discerning eye; And
that by this organ, we can often perceive what is straight and what is
crooked in the mind as well as in the body?--How many mysterious things
must a blind man believe, if he will give credit to the relations of
those that see! Surely he needs as strong a faith as is required of a
good Christian.”[123]
The same observation has been put in a strong light, by the
supposition, that it had been as uncommon, to be born with the power
of sight, as it is now to be born incapable of it;--in which case it
has been truly said, that “the few who had this rare gift would appear
as prophets or inspired teachers to the many.”[124] The very easy
predictions thus made, would be found, constantly, or almost constantly
fulfilled, by those who could form no conception of the means by which
the effects predicted were foreseen; and wonderful as the dreams and
visions of prophetic inspiration may appear, they surely could not seem
more wonderful, as a medium of communication, than that by which the
very secrets of the mind, and events apparently the most distant, were
made known, through the intervention of a small ball like the eye.
In shewing the manner by which we learn to combine, with our visual
sensations, the knowledge obtained by touch; or, as I am rather
inclined to think, for reasons formerly stated, the knowledge falsely
ascribed to mere touch; it will not be necessary to go over the
different varieties of figure, magnitude, distance. The most striking
of these is _distance_,--which, indeed, may be truly said to involve
the other two; since the distance of an object is merely the extension
of the long line that intervenes between the object and our eye, and
the consequent magnitude of the intervening objects, and that which
we consider, regarded as one extended whole. Of this one great whole,
what we term the distant object, is nothing more than the boundary. The
cottage, at the end of a field, is a part of that compound magnitude of
which the field and the cottage are separately parts, exactly in the
same manner as the wing of a house, is a part of the compound magnitude
of the whole building. The line of field which connects our eye with
the cottage, may, indeed, be a longer line, but it is a line of
precisely the same sort as that which connects the wings of the house
with our organ of sight, or with each other.
It is vain to think of ascribing the perception of distance to the
measurement of the different angles subtended by objects, at different
distances, or to an equally nice measurement of the different degrees
of inclination of the axis of the eyes, necessary for distinct vision,
in particular cases,--as if all men were instinctively geometers, and
the peasant and the very idiot were incessantly occupied in measuring
angles; for if this measurement were truly _instinctive_, it would
occur, in infancy, as in maturity, and be _immediate_, in those who
have acquired the power of vision, by that surgical operation to
which I alluded in my last Lecture. But the most decisive of all
considerations, with respect to this supposed _geometry_, is, that the
angles, subtended by the object at its different distances, and the
inclination of the optic axis, in the spontaneous accommodation of
the eyes to the distinct vision of the object at different distances,
though truly existing, to the mere optical examiner of the object,
and the light, and the eye, as one compound phenomenon, have no real
existence, as feelings of the mind, of the individual who sees, and
are known but to very few of the immense multitudes, who without the
slightest acquaintance with geometry, or the slightest knowledge of
the very lines, whose angles they are supposed to measure, are yet
able to distinguish the distances of objects as accurately as the most
expert mathematician. How is it possible that the angles, which remote
objects make relatively to the eye, should be known originally, when
the remote objects themselves are not known, but merely the points of
light on the retina? In relation to the eye, as the organ, and to the
mind, as originally sentient in vision, these points of light were
truly all that existed. The light, indeed, traversed a certain space,
in passing from the object to the eye, and the lines of direction of
the different rays, in arriving at one focal point at the retina,
formed truly different angles. But the angles could not be known,
unless the radiant lines themselves were known; and of these, the mind
could have no knowledge. During the whole time of their convergence,
till they reached the expansion of the optic nerve, the rays of light
were as little capable of producing vision, as darkness itself; and,
when they reached the retina, the lines, and consequently the angles,
existed no more. Of whatever use, therefore, such angles may be to
the optician, in laying down, and illustrating the principles of his
science, they are of no use in the actual living measurements of sight.
Man may reason, indeed,--but he must reason from what he _knows_; and,
therefore, if the determination of distance be the result of any
_judgment_, it must be of a judgment formed from feelings which truly
have, or have had existence.
Such feelings, the elements of our visual judgments, it is not very
difficult to discover.
The great principle, in this case, is the principle of association,
by which the notions derived from touch,--or, at least, the notions
which are commonly supposed to be derived from that sense, are
suggested immediately by the visual feelings which coexisted with the
sensations of touch; in the same manner, as the words of a language,
when a language has been fully learned, suggest whatever the words may
have been used to denote. A child, whose eye has already learned to
distinguish objects, hears the word _cup_ frequently repeated, when a
cup is held before him; and the _word_ afterwards suggests the _thing_.
This process every one understands. But we are not equally aware, that,
in the prior stage of learning to distinguish the cup by the eye, the
child went through a process exactly similar,--that the visual feeling,
which the rays of light from the cup excited, coexisted with the
tactual and muscular feeling, when he handled the cup; and that the one
feeling was thus associated, forever after with the other.
The means by which we acquire our knowledge of the distance of objects,
may be reduced to three,--the difference of the affections of the
_optic nerve_,--the different affections of the _muscles_, employed in
varying the refracting power of each eye, according to the distance
of objects, and in producing that particular inclination of the axis
of the two eyes, which directs them both equally on the particular
object,--and, _thirdly_, the previous knowledge of the distance of
other objects, which form, with that which we are considering, a part
of one compound perception.
To begin, then, with the affections of the retina. These become signs
of distance, in two ways, by the extent of the part of the retina
affected, and by the more or less vivid affection of the part.
It is evident, from the laws of optics, that, according to the distance
of the object from the eye, there must, when all other circumstances
are the same, be a difference of the extent of the retina, on which
the light falls. This illuminated portion of the nervous expanse, as
supposed to be instantly perceived, is what is termed the visible
figure of an object; and, though I am disposed to question the
knowledge, which the mind is believed to acquire of this figure, from
the mere sensation of colour, to which the affection of the retina
gives rise,--I am far from denying, that the sensation itself, whatever
it may originally be, will be different according to the extent of the
retina affected, as the sensation of _heat_ is different, according
to the extent of the surface, which has grown warmer or colder,--or
of _fragrance_, according as a small number of odorous particles have
acted on a portion of the surface of the organ of smell, or a greater
number of these on a greater portion of that surface. The different
feelings, then, when more or less of the retina has been affected, are
capable of being associated with other feelings, which may coexist
with them. An object, held at the distance of a foot from the eye,
affects one part of the retina,--held at arm's length, it affects less
of the retina; and this difference, not indeed as perceived in figure,
but as perceived in the variety, whatever that may originally be, of
the resulting sensation, being found constant and uniform, becomes of
itself significant of the distance.
Another mode, in which the affection of the retina becomes significant
of distance, is by the _brightness_ or dimness of the visible figure,
and its distinctness or indistinctness of outline; or, as I would
rather say, by the peculiar sensations, without regard to figure,
which accompany those varieties of light. Since, at a distance, less
light falls from objects on the eye, and their outline becomes less
definite, a new measure is thus obtained, in addition to that which
is derived from the mere difference in extent of the retina affected.
In the illusion of this spontaneous measurement, consists the chief
magic of the painters art. By different shades of colour, he produces
corresponding perceptions of distance; and thus, making one part of a
plane surface seem more remote than another, converts it, as far as
the mere eye can judge, into a cube or sphere, or any other solid,
which he chuses to present to us. By the indistinct outline which he
gives to his small figures, in the back ground of a landscape, he
leads us to consider them, not as diminutive in themselves, which we
should conceive them to be, if, with equal smallness, their outline
were clearer, but merely as less or more remote. He is thus able to
vary his figures in three ways, to make them larger or smaller, more or
less bright, and more or less precisely defined; and, by uniting these
varieties, in various proportions, to distinguish not merely what is
large from what is small, but the diminutive from the distant, and the
gigantic from the near.
Accordingly we find, that, in circumstances, in which the medium of
transmission of light from objects is much altered, our perception of
distance and magnitude becomes less accurate. In a fog, objects appear
to us greatly magnified; because, the effect produced on the retina,
in the extent of the visible figure and its dimness and indefinite
outline, is truly the same, as when a larger object, in the common
state of the atmosphere, is seen by us at a distance. From the same
principle, objects seen under a brighter sky, and in purer air, seem
nearer than they really are, to those, whose notions of distance have
been acquired in a less happy climate. This has been remarked, by
travellers in Italy, and particularly by one of the most illustrious
of those who have visited that beautiful country,--a traveller, whose
attention had been particularly turned to observations of this sort.
The very acute observer, of whom I speak, is Berkeley, in whose Theory
of Vision there is to be found a very interesting Section, in which he
at once describes this impression, and accounts for it.
Our affections of the retina, then, both in the extent of the nervous
expansion affected, and in the species of affection, afford one set
of feelings, with which the notion of distance may be associated, in
the same manner as the sounds or visual characters of a language may
be associated with the conceptions which they denote, or any other
feelings with any other feelings.
The next set of feelings which we have to consider, in relation to our
perception of distance, belong to a class, of the importance of which
I have had frequent occasion to speak, _the muscular feelings_, in the
contraction of those muscles, which adapt the nice refracting apparatus
in each eye, to the degree of refraction, necessary for distinct vision
in the particular case, and produce that inclination of the axis of
vision to each other, which is necessary for directing both eyes
equally on the object. The muscular feeling may be slight indeed, but
still it is sufficient to modify, in some degree, the whole compound
sensation of the moment. One degree of contraction is attended with a
particular feeling; another degree with a different feeling; and, as
there are various muscles, subservient to the motions of the eyes, some
of which are exerted, while others are quiescent,--the feeling, it is
evident, must vary, not with the degree of contraction merely, but also
with the muscles contracted. A certain muscular feeling, however simple
or complex, accompanies the mere visual sensation, and blends with it;
and it is with this compound feeling, _muscular_ and _visual_, that the
notion of distance is associated.
The muscular adaptation, however, it may be remarked, seems, in a great
measure, to imply the very knowledge which it is supposed to give;
since we cannot, instantly and voluntarily, adapt our eyes to the state
necessary for distinct vision, at a particular distance, unless we have
previously known that particular distance. The necessary adaptation,
however, if it be not the result of a rapid change of various degrees
of contraction in each particular case may depend, not on our knowledge
and will, but on an instinctive connexion of certain motions with
certain feelings, in which there is as little consciousness of design,
as in that very analogous instinct, or connexion of motions with
feelings, which increases or diminishes the diameter of the pupil,
according to the quantity of light which is poured upon the eye, when
the individual, far from willing the contraction, does not know even
that such a contraction has taken place.
A third element, in the calculation of the distance of an object, is
the previous knowledge of the distance of other objects, which form
together with it one compound perception. Thus, when we look along a
road, and observe a man on horseback, who has nearly approached a house
which we know, we have of course little difficulty in determining the
distance of the rider. Every one must have felt how much easier his
judgments of the distance of moving objects are, in scenes with which
he is in some degree acquainted, than in a country which is new to him;
and what aid the interposition of a variety of objects gives, even
though we may not be well acquainted with the exact extent and distance
of each. To an inexperienced eye, therefore, in a first voyage, a ship
at a distance seems far nearer than it truly is, from the absence of
varied intervening objects in the line between. Even in the case of
a river, which is not so broad as to prevent us from distinguishing
objects on the opposite side, it is with great difficulty that we
attempt to guess the distance, with any approach to exactness. There is
a constant tendency to suppose the breadth of the river less than it
is, and consequently the objects on the opposite bank nearer than they
are. For the same reason, the horizontal line, in which innumerable
objects intervene between the eye and the horizon, appears so much
longer than the line of altitude of the meridian, that the vault of the
sky does not seem a hemisphere, but a far smaller segment of a great
sphere. On this subject, however, rich as it is in illustration, my
time will not allow me to dwell longer. But I regret this the less, as
the subject is one of those, which in the department of optics, come
under the consideration of one of my Colleagues, whose happy Genius has
the art of describing fully what the narrow compass of his lectures may
have obliged him to state briefly; and who leaves little for others
to add, even on subjects to which he alludes only for incidental
illustration.
These few very slight remarks, however, will be sufficient to show, in
what manner the notion of distance may be associated with mere _visual
feelings_, that in themselves originally involve no notion of distance,
as the words of a language, which, in themselves, either as sounds or
characters, involve no relation to one object more than to another,
become instantly significant of particular objects, and excite emotions
of love or joy, or hate, or indignation, like the very presence of some
living friend or foe.
It has been very justly remarked, that, if all men had uniformly spoken
the same language, in every part of the world, it would be difficult
for us not to think that there is a natural connexion of our ideas
and the words which we use to denote them; and it is not wonderful,
therefore, that a similar illusion should take place with respect to
what may be termed the universal language of vision; since, in the
case of visual perception, all men may be truly said to have the same
language; the same sensations of sight, being to all significant of
magnitude and distance. And it is well that the judgments which we
form, on these important points, are thus prompt and spontaneous; for,
if we had to wait till we had calculated the distance and magnitude of
every thing around us, by a measurement of angles, we should be cut
off, in our optical career, before we could, with all our geometry,
determine, with precision, whether the things which we needed most,
or the objects of greatest peril to us, were ten or a thousand paces
distant, and whether they were of the bulk of a mole hill or of a
mountain.