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would attend our progress, and how slow it would prove! Having never possessed sight, it would be many years before the most acute and active person could form an idea of a mountain or even of a large edifice. But by the additional help of the sense of seeing, he not only observes the figure of large buildings, but is in a moment possessed of all the beauties of a wide and variegated landscape. The
organ of this sense is the eye. On a slight examination, the eye is found to be a sort of telescope, having its distinct parts, and discovering throughout the most exquisite construction. The medium on which this organ acts are rays of light, everywhere diffused, and always advancing, if they meet with no opposition, in direct lines. The eye, like all the other senses, not only receives externally the medium on which it acts, but carries the rays of light into itself; and on principles purely scientific, refracts and combines them anew.
It does not, however, fall within our plan to give a minute description of the eye, which belongs rather to the physiologist; but such a description, with the statement of the uses of the different parts of the organ, must be, to a candid and reflecting mind, a most powerful argument in proof of the existence and goodness of the Supreme Being. How wonderful, among other things, is the adaptation of the rays of light to the eye! If these rays were not of a texture extremely small, they would cause much pain to the organ of vision, into which they so rapidly pass. If they were not capable of exciting within us the sensations of colour, we should be deprived of much of that high satisfaction which we now take in beholding surrounding objects; showing forth, wherever they are to be found, the greatest variety and the utmost richness of tints.
$ 34. Statement of the mode or process in visual perception. In the process of vision, the rays of light, coming from various objects and in various directions,
strike in the first place on the pellucid or transparent part of the ball of
If they were to continue passing on precisely in the
same direction, they would produce merely one mingled and indistinct expanse of colour. In their progress, however, through the crystalline humour, they are refracted or bent from their former direction, and are distributed to certain focal points on the retina, which is a white, fibrous expansion of the optic nerve.
The rays of light, coming from objects in the field of vision, whether it be more or less extensive, as soon as they have been distributed on their distinct portions of the retina, and have formed an image there, are immediately followed by the sensation or perception which is termed sight. The image which is thus pictured on the retina is the last step which we are able to designate in the material part of the process in visual perception; the mental state follows; but it is not in our power to trace, even in the smallest degree, any physical connexion between the optical image and the corresponding state of the mind. All that we can say in this case is, that we suppose them to hold to each other the relation of antecedent and consequent by an ultimate law of our constitution.
§ 35. Of the original and acquired perceptions of sight. In speaking of those sensations and perceptions, the origin of which is generally attributed to the sense of sight, it is necessary to make a distinction between those which are ORIGINAL and those which are ACQUIRED. Nothing is properly original with the sense of sight but the sensations of colour, such as red, blue, yellow. These sensations (or perceptions, as they are otherwise called, when the internal feeling is combined with a reference to the external cause) are exceedingly numerous. In this respect, the intimations of the sense of sight stand on the same footing with those of taste and hearing; although distinctive names, in consequence of the difficulty of accurately separating and drawing the line between each, are given only in a few cases. All the sensations of colour are original with the sight, and are not to be as
other sense. A part, however, of that knowledge, which we attribute to the sight, and which has the appearance of being immediate and original in that sense, is not so. Some of its alleged perceptions are properly the results of sensations, combined not only with the usual reference to an external cause, but with various other acts of the judgment. In some cases the combination of the acts of the judgment with the visual sensation is carried so far, that there is a sort of transfer to the sight of the knowledge which has been obtained from some other source. And not unfrequently, in consequence of a long and tenacious association, we are apt to look upon the knowledge thus acquired as truly original in the seeing power. This will suffice, perhaps, as a statement of the general fact, while the brief examination of a few instances will help to the more thorough understanding of those acquired perceptions of the sight which are here referred to.
cribed to any
0 36. The idea of extension not originally from sight. It is well known that there is nothing more common than for a person
say, that he sees the length or breadth of any external object; that he sees its extent, &c. These expressions appear to imply (and undoubtedly are so understood) that extension is a direct object of sight. There is no question that such is the common sentiment, viz., that the outlines and surface which bodies permanently expand and present to the view, are truly seen. An opinion different from this might even incur the charge of great absurdity.
But, properly, the notion of extension, as we have already seen, has its origin in the sense of touch. Being a simple and elementary thought, it is not susceptible of definition; nor, when we consider extension as existing outwardly and materially, can we make it a matter of description without running into the confusion of using synonymous words. But, whatever it is, (and certainly there can be neither ignorance nor disagreement on that point, however much language may fail of conveying our ideas,) the knowledge of it is not to be ascribed originally to the sight.
The notion of extension is closely connected with externality. It is not possible to form the idea of extension from mere consciousness, or a reflection on what takes place within us. But making a muscular effort, and thus
applying the touch to some resisting body, we first have the notion of outness; and either from the same application of that sense, or when we have repeated it continuously on the same surface, we have the additional notion of its being extended or spread out. If a man were fixed immoveably in one place, capable of smelling, tasting, hearing, and seeing, but without tactual impressions originating from a resisting body, he would never possess a knowledge of either. Having first gained that knowledge from the touch in the way just mentioned, he learns in time what appearance extended bodies (which are, of course, coloured bodies) make to the eye. At a very early period, having ascertained that all coloured bodies are spread out or extended, he invariably associates the idea of extension with that coloured appearance. Hence he virtually and practically transfers the knowledge obtained by one sense to another; and even after a time imagines extension to be a direct object of sight, when, in fact, what is seen is only a sign of it, and merely suggests it. An affection of the sense of touch is the true and original occasion of the origin of this notion; and it becomes an idea of sight only by acquisition or transfer
Ø 37. Of the knowledge of the figure of bodies by the sight. Views similar to those which have been already advanced will evidently apply to the figure of bodies. We acquire a knowledge of the figure or form of bodies originally by the sense of touch. But it cannot be doubted that this knowledge is often confidently attributed to the sense of sight as well as the touch. Although there is reason to believe that men labour under a mistake in this, it is not strange, when we trace back our mental history to its earlier periods, that such a misapprehension should exist.
A solid body presents to the eye nothing but a certain disposition of colours and light. We may imagine ourselves to see the prominences or cavities in such bodies, when in truth we only see the light or the shade occasioned by them. This light and shade, however, we learn by experience to consider as the sign of a certain
solid figure.--A proof of the truth of this statement is, that a painter, by carefully imitating the distribution of light and shade which he sees in objects, will make his work very naturally and exactly represent, not only the general outline of a body, but its prominences, depressions, and other irregularities. And yet his delineation, which, by the distribution of light and shade, gives such various representations, is on a smooth and plain surface.
9 38. Illustration of the subject from the blind. It was a problem submitted by Mr. Molyneux to Mr. Locke, whether a blind man, who has learned the difference between a cube and a sphere by the touch, can, on being suddenly restored to sight, distinguish between them, and tell which is the sphere and which is the cube, by the aid of what may be called his new sense merely? And the answer of Mr. Locke was, in agreement with the opinion of Molyneux himself, that he cannot. The blind man knows what impressions the cube and sphere make on the organ of touch, and by that sense is able to distinguish between them; but, as he is ignorant what impression they will make on the organ of sight, he is not able, by the latter sense alone, to tell which is the round body and which is the cubic.
It was remarked that solid bodies present to the eye nothing but a certain disposition of light and colours.— It seems to follow from this, that the first idea which will be conveyed to the mind on seeing a globe, will be that of a circle variously shadowed with different degrees of light. This imperfect idea is corrected in this way. Combining the suggestions of the sense of touch with those of sight, we learn by greater experience what kind of appearance solid, convex bodies will make to us. That appearance becomes to the mind the sign of the presence of a globe; so that we have an idea of a round body by a very rapid mental correction, whereas the notion first conveyed to the mind is truly that of a plane, circular surface, on which there is a variety in the dispositions of light and shade. It is an evidence of the correctness of this statement, that in paintings, plane surfaces, variously shaded, represent convex bodies, and with great truth and exactness.