Imatges de pàgina
PDF
EPUB

PART. I.

CHAP. V.

Of Sight.

CHAPTER V.

OF SIGHT.

1. SIGHT, as well as hearing, is produced by immediate contact of the exciting cause with the organ; which exciting cause is the light reflected, from the objects seen, upon the retinal of the eye; the pictures upon which, by some impressions or irritations upon the optic nerves, the modes of which must be for ever unknown to us, are conveyed to the mind, and produce the sense of vision, the most valuable of ali our

senses.

2. The sensation, therefore, felt upon opening the eyes for the first time, must necessarily be that of the objects seen touching them; as it proved to be in the case of the boy, who, at the age of fourteen or thereabouts, obtained his sight, after having been blind from his birth, by an operation performed upon his eyes by Chiselden. For a considerable time, and till the sense of seeing had been aided and corrected by that of touch, all the objects seen appeared only as variations of light acting upon the eye: for the colours of objects are only different rays of

CHAP. V.

light variously reflected from their surfaces; PART I. and their visible projection is merely gradation and opposition of light and shadow; which, in of Sight. round and undulating bodies, are intermixed gradually; and, in those of angular forms, abruptly. It is, therefore, only by habit and experience that we form analogies between the perceptions of vision and those of touch, and thus learn to discover projection by the eye: for, naturally, the eye sees only superficial dimension; as clearly appears in painting and all other optical deceptions, which produce the appearance of projection or thickness upon a flat surface. The faculty, however, when acquired, as it is in all adult persons who have seen from their birth, is exercised as readily and instantaneously as any natural faculty whatsoever †.

3. The perception of visible projection being thus artificial, that of visible distance must necessarily be so likewise: for distance is only projection extended. Accordingly we find that our improved perception of visible distance extends no further than that experience, by which it has been formed and improved: for of the immense distances of the heavenly bodies

* See Newton's Theory of Light and Colours. + See Dr. Reid's Essay on the Mind, where a very and full explanation of the theory of vision is given.

clear

PART I. from each other, and from the earth, we discover nothing by looking at them; they all appearing Of Sight. to occupy the surface of one blue vault, whose

CHAP. V.

diameter is that of the visible horizon; which the sun, moon, and stars seem equally to touch at their rising and setting. Hence the notion of these luminaries setting in, and rising from the ocean has universally prevailed through all nations: and it has not been by the evidence of improved sense; but by the calculations and discoveries of improved intellect, that the error has been removed.

4. The visible magnitude of bodies depending entirely upon their distance from the eye, we have, of course, as imperfect and inadequate perceptions of it from the unaided sense of vision, as we have of distance. The pen, which I hold between my fingers, occupies a greater space in the retina, when only a foot from the eye, than the spire of Salisbury does, when seen at the distance of a mile; and, consequently, as far as concerns the mere organ of sense, is bigger: for though the real magnitude of an object, which is perceived by a computation of its distance, rendered instantaneous by habit, may affect the imagination, the visible dimensions of it alone are impressed upon the eye; and, consequently, can alone affect the sensation excited.

CHAP. V.

5. Hence we may learn how to estimate the PART I. theory of an eminent writer, who supposes that objects of large dimensions are sublime, because of Sight. the great number of rays, which they emit, crowd into the eye together, or in quick succession, and produce a degree of tension in the membrane of the retina, which, approaching nearly to the nature of what causes pain, must (in his own words) produce an idea of the sublime *. But, to say nothing of this assumed connection between the causes of pain and the ideas of the sublime, the slightest knowledge of optics would have informed him that the sheet of paper, upon which he was writing, being seen thus close to the eye, reflected a greater, and more forcible mass of light; and, consequently, produced more irritation and tension, than the Peak of Teneriffe or Mount St. Elias would, if seen at the distance of a few miles:-yet, surely he would not say that the sheet of paper excited more grand and perfect ideas of the sublime.

6. That the irritation, produced in the membranes of the eye by vision, is proportioned to the quantity of light poured into it, we may perceive by the dilation and contraction of that membrane called the iris; which always expands

* Sublime and Beautiful, Part IV. s. ix.

CHAP. V.

PART I. its circle, as the quantity of light, to which it is exposed, is diminished, and contracts it, as it is Of Sight. increased. In the eyes of animals formed to see with a very small quantity of light; such as cats, owls, &c., this power is very great; and the membrane affected seems to consist of valves, which open and shut, instead of a sphincther, that dilates and contracts. Hence, in the night, when these valves are entirely open, the eyes of these animals present a very singular appearance of large luminous circles; which, in the day, are reduced to small horizontal slits; through which the few rays, that they then want, are suffered to pass: for, to organs of such nice sensibility, any great quantity would be painful; and it is probable that the degree of irritation alone regulates the opening and shutting of the membranes, which admit and exclude it, in the same manner as it does the dilation and contraction of the corresponding membranes in our eyes, without the intervention of the will.

7. The pains and pleasures of vision, however, like those of the other senses, depend upon the modes as well as degrees of irritation: for all the different colours may be properly considered as different modes, in which light acts upon the eyes; colours being only collections of rays variously modified, separated, and combined, ac

« AnteriorContinua »