SEEING

, the act of perceiving objects by the organ of sight; or the sense we have of external objects by means of the eye. |

For the apparatus, or disposition of the parts necessary to Seeing, see Eye. And for the manner in which Seeing is performed, and the laws of it, see VISION.

Our best anatomists differ greatly as to the cause why we do not see double with the two eyes? Galen, and others after him, ascribe it to a coalition, or decussation, of the optic nerve, behind the os sphenoides. But whether they decussate or coalesce, or only barely touch one another, is not well agreed upon.

The Bartholines and Vesalius say expressly, they are united by a perfect confusion of their substance; Dr. Gibson allows them to be united by the closest conjunction, but not by a confusion of their fibres.

Alhazen, an Arabian philosopher of the 12th century, accounts for single vision by two eyes, by supposing that when two corresponding parts of the retina are affected, the mind perceives but one image.

Des Cartes and others account for the effect another way; viz, by supposing that the fibrillæ constituting the medullary part of those nerves, being spread in the retina of each eye, have each of them corresponding parts in the brain, so that when any of those fibrillæ are struck by any part of an image, the corresponding parts of the brain are affected by it. Somewhat like which is the opinion of Dr. Briggs, who takes the optic nerves of each eye to consist of homologous sibres, having their rise in the thalamus nervorum opticorum, and being thence continued to both the retinæ, which are composed of them; and farther, that those fibrillæ have the same parallelism, tension, &c, in both eyes; consequently when an image is painted on the same corresponding sympathizing parts of each retina, the same effects are produced, the same notice carried to the thalamus, and so imparted to the soul. Hence it is, that double vision ensues upon an interruption of the parallelism of the eyes; as when one eye is depressed by the finger, or their symphony is interrupted by disease: but Dr. Briggs maintains, that it is but in few subjects there is any decussation; and in none any conjunction more than mere contact; though his notion is by no means consonant to facts, fnd it is attended with many improbable circumstances.

It was the opinion of Sir Isaac Newton, and of many others, that objects appear single, because the two optic nerves unite before they reach the brain. But Dr. Porterfield shews, from the observation of several anatomists, that the optic nerves do not mix or confound their substance, being only united by a close cohesion; and objects have appeared single, where the optic nerves were found to be disjoined. To account for this phenomenon, this ingenious writer supposes, that, by an original law in our natures, we imagine an object to be situated somewhere in a right line drawn from the picture of it upon the retina, through the centre of the pupil; consequently the same object appearing to both eyes to be in the same place, we cannot distinguish it into two. In answer to an objection to this hypothesis, from objects appearing double when one eye is distorted, he says, the mind mistakes the position of the eye, imagining, that it had moved in a manner corresponding to the other, in which case the conclusion would have been just: in this he seems to have re- course to the power of habit, though he disclaims that hypothesis. This principle however has been thought sufficient to account for this appearance.

Originally, every object making two pictures, one in each eye, is imagined to be double; but, by degrees, we find that when two corresponding parts of the retina are impressed, the object is but one; but if those corresponding parts be changed by the distortion of one of the eyes, the object must again appear double as at the first. This seems to be verified by Mr. Cheselden, who informs us, that a gentleman, who, from a blow on his head, had one eye distorted, found every object to appear double, but by degrees the most familiar ones came to appear single again, and in time all objects did so without amendment of the distortion. A similar case is mentioned by Dr. Smith.

On the other hand, Dr. Reid is of opinion, that the correspondence of the centres of two eyes, on which single vision depends, does not arise from custom, but from some natural constitution of the eye, and of the mind.

M. du Tour adopts an opinion, long before suggested by Gassendi, that the mind attends to no more than the image made in one eye at a time; in support of which, he produces several curious experiments; but as M. Buffon observes, it is a sufficient answer to this hypothesis, that we see more distinctly with two eyes than with one; and that when a round object is near us, we plainly see more of the surface in one case than in the other.

With respect to single vision with two eyes, Dr. Hartley observes, that it deserves particular attention, that the optic nerves of man, and such other animals as look the same way with both eyes, unite in the sella turrica in a ganglion, or little brain, as it may be called, peculiar to themselves, and that the associations between synchronous impressions on the two retinas, must be made sooner and cemented stronger on this account; also that they ought to have a much greater power over one another's image, than in any other part of the body. And thus an impression made on the right eye alone by a single object, propagates itself into the left, and there raises up an image almost equal in vividness to itself; and, consequently, when we see with one eye only, we may however have pictures in both eyes.

It is a common observation, says Dr. Smith, that objects seen with both eyes appear more vivid and stronger than they do to a single eye, especially when both of them are equally good. Porterfield on the Eye, vol. ii, pa. 285, 315. Smith's Optics, Remarks pa. 31. Reid's Inquiry, pa. 267. Mem. Présentes, pa. 514. Acad. Par. 1747. Mem. Pr. 334. Hartley on Man, vol. i, pa. 207. Priestley's Hist. of Light and Colours, pa. 663, .&c.

Whence it is that we see objects erect, when it is certain, that the images thereof are painted invertedly on the retina, is another difficulty in the theory of Seeing. Des Cartes accounts for it hence, that the notice which the soul takes of the object, does not depend on any image, nor any action coming from the object, but merely on the situation of the minute parts of the brain, whence the nerves arise. Ex. gr. the situation of a capillament brain, which occasions the | soul to see all those places lying in a right line with it.

But Mr. Molyneux gives another account of this matter. The eye, he observes, is only the organ, or instrument; it is the soul that sees. To enquire then, how the soul perceives the object erect by an inverted image, is to enquire into the soul's faculties. Again, imagine that the eye receives an impulse on its lower part, by a ray from the upper part of an object; must not the visive faculty be hereby directed to consider this stroke as coming from the top, rather than the bottom of the object, and consequently be determined to conclude it the representation of the top?

Upon these principles, we are to consider, that inverted is only a relative term, and that there is a very great difference between the real object, and the means or image by which we perceive it. When all the parts of a distant prospect are painted upon the retina (supposing that to be the seat of vision), they are all right with respect to one another, as well as the parts of the prospect itself; and we can only judge of an object being inverted, when it is turned reverse to its natural position with respect to other objects which we see and compare it with.

The eye or visive faculty (says Molyneux) takes no notice of the internal surface of its own parts, but uses them as an instrument only, contrived by nature for the exercise of such a faculty. If we lay hold of an upright stick in the dark, we can tell which is the upper or lower part of it, by moving our hand upward or downward; and very well know that we cannot feel the upper end by moving our hand downward. Just so, we find by experience and habit, that by directing our eyes towards a tall object, we cannot see its top by turning our eyes downward, nor its foot by turning our eyes upward; but must trace the object the same way by the eye to see it from head to foot, as we do by the hand to feel it; and as the judgement is informed by the motion of the hand in one case, so it is also by the motion of the eye in the other.

Molyneux's Dioptr. pa. 105, &c. Musschenbroek's Int. ad Phil. Nat. vol. ii, pa. 762. Ferguson's Lectures, pa. 132. See Sight, Visible, &c.