the ancient philosophers; some of them imagining that vision was
caused by the reception of rays into the eye; while a great many
others thought it more agreeable to nature, that certain emanations,
which they called visual rays, should flow from the eye to the
object.
We shall now inquire more particularly how each part of the eye is
peculiarly fitted to produce distinct vision. Though the eye is
composed of different humours, yet one might have been sufficient to
collect the rays into a focus, and form the picture of an object upon
the retina. By the experiments of the accurate Dr. Robertson, it
appears that there is less difference in the density, as well as in
the refracting power of the humours, than has been generally thought:
by weighing them in a hydrostatic balance, he found that the specific
gravities of the aqueous and vitreous humours were very nearly equal,
each being nearly equal to that of water: and that the specific
gravity of the crystalline did not exceed the specific gravity of the
other humours in a greater proportion than that of about 11 to 10.
Hence it would seem to follow, that the crystalline is not of such
great use in bringing the rays together, and thereby forming the
pictures of objects on the retina, as has been commonly thought by
optical writers; for though in shape it resembles a double convex
lens, and is, on that account, fitted to make the rays converge; yet,
be cause it is situated between two humours nearly of the same
refractive power with itself, it will alter the direction of the
light but a little. From this, the reason is evident why the sight
continues after the operation for the cataract, in which the
crystalline is depressed, or extracted, and why a glass of small
convexity is sufficient to supply the little refraction wanting,
occasioned by the loss of this humour. But without doubt, several
important purposes are effected by this construction of the eye;
which could not have been attained if it had been composed of one
humour only. Some of those purposes seem sufficiently evident to us;
for instance, by placing the aqueous humor before the crystalline,
and partly before the pupil, and making the cornea convex, a greater
quantity of light is made to enter the eye than could otherwise have
done without enlarging the size of the pupil; the light will also
enter in a less diverging state than it could have done if the pupil
had been enlarged, and consequently be more accurately collected to a
focus on the retina; for a perfect eye can only collect such rays to
a focus on that membrane, as pass through the pupil nearly in a state
of parallelism.
Another, and perhaps a principal advantage derived from the different
humours in the eye, is, probably, to prevent that confusion arising
from colour, which is the consequence of the different degrees of
refrangibility of the rays of light. From the experiments of Mr.
Dollond, it appears, though contrary to the opinion of Newton, and
most other optical writers, that different kinds of matter differ
extremely with respect to the divergency of colour produced by equal
refractions; so that a lens may be contrived, composed of media of
different dispersing powers, which will form the image of any object
free of colour; this discovery Mr. Dollond has applied to the
improvement of telescopes, with great success. It is by no means
improbable, that nature has, for the same purpose, placed the
crystalline lens betwen two media of different densities, and,
probably, different dispersing powers, so that an achromatic image,
free from the prismatic colours, will be formed on the retina. Indeed
we find a conjecture of this kind, so long since as Dr. David
Gregory's time, he says, in speaking of the imperfection of
telescopes, "Quod si ob difficultates physicas, in speculis idoneis
torno elaborandis, et poliendis, etiamnum lentibus uti oporteat,
fortassis media diversae densitatis ad lentem objectivam componendam
adhibere utile foret, ut a natura factum observamus in oculo, ubi
crystallinus humor (fere ejusdem cum vitro virtutis ad radios lucis
refringendos) aqueo et vitreo (aquae quoad refractionem hand
absimilibus) conjungitur, ad imaginem quam distincte fieri poterit, a
natura nihil frustra moliente, in oculi fundo depingendam."
In describing the eye, I observed, that the crystalline humour was
not every where of the same consistence, being much more hard and
dense towards its centre, than externally: in the human eye, it is
soft on the edges, and gradually increases in density as it
approaches the centre: the reason of this construction is evident, at
least we know of one use which it will serve; for, from the
principles of optics, it is plain that the rays which fall at a
distance from the axis of the crystalline, by reason of their greater
obliquity, if the humour were of the same density in all its parts,
would be more refracted than those which fall near its axis, so that
they would meet at different distances behind the crystalline humour;
those which pass towards its extremity, nearer, and those near its
axis, at a greater distance, and could not be united at the same
point on the retina, which would render vision indistinct; though the
indistinctness arising from this cause, is only about the 1/5449 part
of that which arises from the different refrangibility of the rays of
light, as Sir Isaac Newton has demonstrated. Nature has, however,
contrived a remedy for this also, by making the crystalline humour
more dense and solid near its centre, that the rays of light which
fall near its axis, may have their refraction increased, so as to
meet at the same point with those which fall at a distance from its
axis.
_Of the manner in which the Eye conforms itself in order to see
distinctly at different Distances._
It has been much disputed in what manner the eye conforms itself to
see distinctly at different distances; for it is evident, that,
without some change, the rays which flow from objects at different
distances, could not be collected into a focus at the same point,
and, consequently, though the eye might see distinctly at one
distance, it could not at another.
This subject has given rise to a variety of opinions, but few of them
are satisfactory; and though several of them might explain the
phenomena of vision, at different distances, yet it is by no means
proved that those supposed changes do take place in the eye. I shall
content myself with just mentioning the principal opinions on this
subject, without engaging in a controversy, which has for a long time
employed the ingenuity of philosophers to little purpose.
Some are of opinion, that the whole globe of the eye changes its
figure; becoming more oblong when objects are near, and more flat
when they are removed to a greater distance; and this change in the
figure of the eye is differently explained by different authors; some
maintain that it is rendered oblong by the joint contraction of the
two oblique muscles: others think that the four straight muscles
acting together, compress the sides of the globe, and by this
compression, reduce it to an oblong figure, when objects are near;
and that, by its natural elasticity, it recovers its former figure
when these muscles cease to act. Others again think that when these
four straight muscles act together, they render the eye flat by
pulling it inwards, and pressing the bottom of it against the fat;
and that it is reduced to its former figure, either by the joint
contraction of the two oblique muscles, or by the inherent elasticity
of its parts, which exerts itself when the muscles cease to act.
That, if such a change should take place in the eye, it would produce
distinct vision, will be readily granted; but that such a one does
not take place, at least in any of these ways, is, in my opinion,
very certain. Dr. Porterfield thinks that the crystalline lens has a
motion by means of the ligamentum ciliare, by which the distance
between it and the retina is increased or diminished, according to
the different distances of objects. The ligamentum ciliare, he says,
is an organ, the structure and disposition of which excellently
qualify it for changing the situation of the crystalline, and
removing it to a greater distance from the retina, when objects are
too near for us; for that, when it contracts, it will not only draw
the crystalline forwards, but will also compress the vitreous humour,
lying behind it, so that it must press upon the crystalline, and push
it from the retina. Although this hypothesis will, in a great
measure, account for distinct vision at different distances, yet it
could only be of use where the rays enter the eye with a certain
degree of divergency, while, however we are sure, that in looking at
very distant objects which are at different distances from us, the
eye undergoes a change. But a sufficient objection to Dr.
Porterfield's hypothesis is, that it is by no means proved that the
crystalline lens can be moved in the manner he supposes, or that the
ligamentum ciliare is possessed of muscular fibres; on the contrary
some eminent anatomists deny that they are.
We shall now take a view of the opinion of M. de la Hire, who
considered this subject, as well as almost every other relating to
vision, with the closest attention; he maintains, that, in order to
view objects distinctly at different distances, there is no
alteration but in the size of the pupil, which is well known to
contract and dilate itself according to the quantity of light flowing
from the object we look at, being most contracted in the strongest
light, and most dilated when the light is weakest; and consequently
will contract when an object is held near the eye, and dilate as it
is removed, because in the first case the quantity of light entering
the eye is much greater than in the last. That this contraction of
the pupil will have the effect of rendering vision distinct,
especially when objects are within the furthest limits of distinct
vision, will plainly appear, if we consider the cause of indistinct
vision. Dr. Jurin has shown, that objects may be seen with sufficient
distinctness, though the pencils of rays issuing from the points of
them do not unite precisely in another point on the retina, but
instead thereof, if they form a circle which does not exceed a
certain magnitude, distinct vision will be produced; the circle
formed by these rays on the retina he calls the circle of
dissipation. The pupil will, by contracting, not only diminish the
circles of dissipation, and thereby help to produce distinct vision,
but will also prevent so great a quantity of light from falling near
the circumferences of those circles; and Dr. Jurin has shown, that,
if the light on the outer side of the circles of dissipation is
diminished, the remainder will scarce affect the sense. In both these
ways, the contraction of the pupil has a tendency to diminish the
circles of dissipation, and, consequently, to produce distinct
vision. This is likewise confirmed by experiment, for when an object
is placed so near, that the pupil cannot be so much contracted as is
necessary for distinct vision, the same end may be obtained by means
of an artificial pupil: for, if a small hole is made in a card, a
very near object may be viewed through it with the greatest ease and
distinctness. Also, if a person have his back turned towards a
window, and hold a book so near his eyes as not to be able to read,
if he turn his face to the light, he will find, that he will be able
to read it very distinctly; which is owing to the contraction of the
pupil by means of the light.
M. Le Roi, a member of the Royal Academy of Montpelier, has attempted
to defend the opinion of M. de la Hire, and, indeed, it seems, of all
others, the best supported by facts; but perhaps it may not account
so well for vision at great distances. It is likewise rendered more
probable by viewing the pictures of external objects, formed in a
dark chamber, by rays coming through a hole in the window shutter;
those pictures will be rendered distinct, by dilating, or contracting
the aperture, without the assistance of a lens, accordingly as the
object is more or less distant; those who have had the crystalline
lens depressed, or extracted, by means of one glass can see objects
pretty distinctly at different distances. These, and several other
arguments that might be brought, tend to prove that the eye
accommodates itself to view objects distinctly at different
distances, chiefly by means of the motion of the pupil; and though
this does not explain the phenomenon so satisfactorily as we could
wish, yet it is certain, that it has a share in it; we are however
certain, that, in whatever manner it may be produced, the eye has a
power of accommodating itself to view objects distinctly enough at
several different distances.
_Concerning the Seat of Vision._
No subject has been more canvassed than that concerning what is
improperly called the seat of vision. In early times, the crystalline
lens was thought to be best qualified for this office; but this
substance, though situated in the middle of the eye, which Baptista
Porta thought to be the proper centre of observation, had universally
given place to the better founded pretensions of the retina: and,
from the time of Kepler, few ventured to dispute its claim to that
office, till M. Mariotte was led, from some curious circumstances, to
think that vision was not performed by the retina, but by the choroid
coat. Having often observed in the dissections of men, as well as of
brutes, that the optic nerve is not inserted exactly opposite to the
pupil, that is, in the place where the picture of the objects upon
which we look directly, is made: and that in man it is somewhat
higher, and on the side towards the nose, he had the curiosity to
examine the reason of this structure, by throwing the image of an
object on this part of the eye. In order to do this, he fastened on a
dark wall, about the height of his eyes, a small round paper, to
serve for a fixed point of sight; and he fastened such another paper
on the right hand, at the distance of about two feet, but rather
lower than the former, so that light issuing from it, might strike
the optic nerve of his right eye, while the left was kept shut. He
then placed himself over against the former paper, and drew back by
degrees, keeping his right eye fixed, and very steady upon it, and
when he had retired about ten feet, he found that the second paper
entirely disappeared. This, he says, could not be imputed to the
oblique position of the second paper, with respect to his eye,
because he could see more remote objects on the same side. This
experiment he repeated by varying the distances of the paper and his
eye. He also made it with his left eye, while the right eye was kept
shut, the second paper being fastened on the left side of the point
of sight; so that by the situation of the parts of the eye, it could
not be doubted that this defect of vision is in the place where the
optic nerve enters, where only the choroides isdeficient.
From this he concludes, that the defect of vision is owing to the
want of the choroid coat, and, consequently, that this coat is the
proper organ of vision. A variety of other arguments in favour of the
choroides occurred to him, particularly he observed that the retina
is transparent, which he thought could only enable it to transmit the
rays further, and he could not persuade himself that any substance
could be considered as being the termination of the pencils, and the
proper seat of vision, at which the rays are not stopped in their
progress.
Mr. Pequet, in answer to Mariotte's observation, says, that the
retina is very imperfectly transparent, resembling oiled paper, or
horn: and, besides, that its whiteness demonstrates that it is
sufficiently opaque for stopping the rays of light as much as is
necessary for vision: whereas, if vision be performed by means of
those rays which are transmitted through such a substance as the
retina, it must be very indistinct.
Notwithstanding the plausibility of this opinion of M. Mariotte, and
the number of celebrated men who joined him in it, I must confess,
that none of their arguments, though very ingenious, have been able
to make me a convert to that opinion.
If we argue from the analogy of the other senses, in all of which the
nerves form the proper seat of sensation, we shall be induced to give
judgment in favour of the retina. And this argument from analogy is
much strengthened, by considering that the retina is a large nervous
apparatus, immediately exposed to the impressions of light; whereas
the choroides receives but a slender supply of nerves, and seems no
more fitted for the organ of vision than any other part of the body.
But facts are not wanting which make still more in favour of the
retina. It appears from observations made upon the sea calf and
porcupine, that these animals have their optic nerves inserted in the
axis of the eye, directly opposite the pupil, which renders it very
improbable that the defect in sight, where the optic nerves enter,
can be owing to the want of the choroides in that place; for were
this true, then in those creatures which have the optic nerves
inserted in the axis of the eye, and which by consequence do directly
receive on the extremity of the nerve the pictures of objects, all
objects would become invisible to which their eyes are turned,
because the choroides is wanting in that place where the image falls;
but this is contrary to experience.
M. Le Cat, though he strenuously supports Mariotte's opinion, takes
notice of a circumstance, which, if he had properly considered it,
might have led him to a contrary conclusion: from a beautiful
experiment he obtains data, which enable him with considerable
accuracy to determine the size of the insensible spot in his eye,
which he finds to be about 1/30 or 1/40 of an inch in diameter, and
consequently only about 1/5 or 1/6 of the diameter of the optic
nerve, that nerve being about 1/6 of an inch in diameter. I find that
in my eye likewise, the diameter of the insensible spot is about 1/40
of an inch, or something less. Whence it is evident that vision
exists where the choroid coat is not present, and consequently that
the choroid coat is not the organ of vision.
It is probably owing to the hardness and callosity of the retina
where the nerve enters, that we have this defect of sight, as it has
not yet acquired that softness and delicacy which is necessary for
receiving such slight impressions as those of the rays of light, and
this conjecture is rendered still more probable by an observation of
M. Pequet, who tells us, that a bright and luminous object, such as a
candle, does not absolutely disappear, but one may see its light,
though faint. This not only shows that the defect of sight is not
owing to a want of the choroides, but also that the retina is not
altogether insensible where the nerve enters. These circumstances, in
my opinion, render it certain, that the retina, and not the choroid
coat, is the organ of vision.
_Of our seeing Objects erect by inverted Images._
Another question concerning vision, which has very much perplexed
philosophers, is this; how comes it that we see objects erect, when
it is well known that their images or pictures on the retina are
inverted? The sagacious Kepler, who first made this discovery, was
the first that endeavoured to explain the cause of it.
The reason he gives for our seeing objects erect, is this, that as
the rays from different points of an object cross each other before
they fall on the retina, we conclude that the impulse we feel upon
the lower part of the retina comes from above; and that the impulse
we feel from the higher part, comes from below. Des Cartes afterwards
gave the same solution of this phenomenon, and illustrated it by the
judgment we form of the position of objects which we feel with our
arms crossed, or with two sticks that cross each other. But this
solution is by no means satisfactory: first, because it supposes our
seeing objects erect to be a deduction from reason, drawn from
certain premises, whereas it seems to be an immediate perception; and
secondly, because all the premises from which this conclusion is
supposed to be drawn, are absolutely unknown to far the greater part
of mankind, and yet they all see objects erect.
Bishop Berkeley, who justly rejects this solution, gives another,
founded on his own principles, in which he is followed by Dr. Smith.
This ingenious writer thinks that the ideas of sight are altogether
unlike those of touch; and since the notions we have of an object by
these different senses, have no similitude, we can learn only by
experience how one sense will be affected, by what, in a certain
manner, affects the other. Thus, finding from experience, that an
object in an erect position, affects the eye in one manner, and that
the same object in an inverted position, affects it in another, we
learn to judge, by the manner in which the eye is affected, whether
the object is erect or inverted. But it is evident that Bishop
Berkeley proceeds upon a capital mistake, in supposing that there is
no resemblance between the extension, figure, and position, which we
see, and that which we perceive by touch. It may be further observed,
that Bishop Berkeley's system, with regard to material things, must
have made him see this question, in a very different light from that
in which it appears to those who do not adopt his system.
In order to give a satisfactory answer to this question, we must
first examine some of the laws of nature, which take place in vision;
for by these the phenomena of vision must be regulated.
It is now, I believe, pretty well established, as a law of nature,
that we see every point of an object in the direction of a right
line, which passes from the picture of that point on the retina,
through the centre of the eye. This beautiful law is proved by a very
copious induction of facts; the facts upon which it is founded are
taken from some curious experiments of Scheiner, in his Fundamenta
Optices. They are confirmed by Dr. Porterfield, and well illustrated
by Dr. Reid. The seeing objects erect by inverted images is a
necessary consequence of this law of nature: for from thence it is
evident that the point of the object whose picture is lowest on the
retina, must be seen in the highest direction from the eye; and that
the picture which is on the right side of the retina, must be seen on
the left.
_Of seeing Objects single with two Eyes._
That we should have two pictures of an object, and yet see it single,
has long been looked upon as a curious circumstance by philosophers:
and of consequence, many attempts have been made to account for it,
few of which, however, are satisfactory.
As it would take up too much time to give a view of all the opinions
on this subject, I shall pass over the opinions of Galen, Gassendus,
Baptista Porta, Rohault, and others, which do not deserve a serious
refutation; and shall content myself with making a few observations
on the hypothesis of Bishop Berkeley.
But it seems the most proper way of proceeding, first of all to
consider the phenomena of single and double vision, in order, if
possible, to discover some general principle to which they lead, and
of which they are necessary consequences; and, for the sake of
perspicuity, we shall premise the following definition.
When a small object is seen single with both eyes, those points on
the two retinas on which the pictures of the object fall, may be
called corresponding points: and when the object is seen double, we
shall call such points, non-corresponding points.
Now we find that in sound and perfect eyes, when the axes of both are
directed to one point, an object placed in that point is seen single;
and in this case, the two pictures which show the object single, are
painted on the centres of the retinas. Hence, the centres of the two
retinas correspond.