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fity 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 sphincter, 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, according to the different textures of the surfaces of the bodies, from which they are reflected, or the substances of those through which they are refracted

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s. There are, indeed, scarcely any human eyes of such extreme sensibility, unless in a morbid state, as to feel any absolute pain from colours composed of reflected rays: for unless the reflection be from the surface of a concave mirror, in which the rays are collected and condensed, the effect of light is necessarily weakened by being reflected; whence the refracted colours of a prism or a rainbow are always more vivid and bright than those which are reflected from any opaque substance. There are, however, some kinds of birds and quadrupeds, such as turkeys and oxen, to whom scarlet is evidently painful; painful; as they will run at it, and attack it with the utmost virulence and fury. Green, on the contrary, appears to be grateful to the eyes of all animals; though colours, as well as sounds and flavours, are more pleasing when harmoniously mixed and graduated, than when distinct and uniform. Indeed, they almost always are graduated and broken in nature: for, though an object be of one colour throughout, unless it present one equal superficies to one equal degree of light, that colour will be variously graduated and diversified to the eye by every undulating or angular projection or indenture of its form. In every individual pink or rose, whether its colour be white, yellow, or red,

there are infinite varieties and gradations of tint, produced, not only by the different degrees and modifications of light and shadow, but by the various reflected rays, which one leaf casts upon another, according to their different degrees of opacity and exposure.

9. When many sorts and varieties of these rich and splendid productions of nature are skilfully arranged and combined, as in the flower-pots of Vanhuysum, they form, perhaps, the most perfect spectacle of mere sensual beauty that is any where to be found. The magnificent compositions of landscape are, indeed, spectacles of a higher class; and afford pleasures of a more exalted kind: but only a small part of those pleasures are merely sensual; the venerable ruin, the retired cottage, the spreading oak, the beetling rock, and limpid stream having charms for the imagination, as well as for the sense; and often bringing into the mind pleasing trains of ideas besides those, which their impressions upon the organs of sense immediately excite. As far, however, as they do afford sensual pleasure, it depends upon the same principle as the pleasures of the other senses already treated of; that is, upon a moderate and varied irritation of the organic nerves: for, if the irritation be too strong; that is, if the transitions of colour be too violent and sudden, and the oppositions

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of light and shadow too vigorous and abrupt, the effect will be harsh and dazzling, and the sensation painful, or, at least, unpleasant; while, if they be too monotonous and feeble, the effect will be flat and insipid, and the sensation too languid to be pleasing.

10. In this, however, as in all other pleasures of sense, the scale of the pleasing and displeasing impressions cannot be graduated according to any abstract general rule, but must be adapted to the different degrees of sensibility of different organs; which vary, not only constitutionally, but habitually; the eye, as well as the palate, being liable to be vitiated, and consequently to require such stimulants to give it pleasure, as give pain to those of more refined sensibility. On the contrary, there are persons whose eyes have naturally a sort of morbid irritability, which renders those degrees of light and modifications of colour, which are merely sufficient to be pleasant to others, quite painful to them. In this case, however, as in all others of the kind, the just scale, and criterion of taste, must be taken from the natural feelings of the mass of mankind for we have here no rules of calculation to appeal to; and rules of analogy are true or false accordingly as they are respectively supported or opposed by the greater number of

instances.

11. Smoothness being properly a quality perceivable only by the touch, and applied metaphorically to the objects of the other senses, we often apply it very improperly to those of vision; assigning smoothness, as a cause of visible beauty, to things, which, though smooth to the touch, cast the most sharp, harsh, and angular reflections of light upon the eye; and these reflections are all that the eye feels or naturally perceives; its perception of projecting form or tangible smoothness being, as before observed, entirely artificial and acquired; and, therefore, unconnected with pure sensation. Such are all objects of cut glass or polished metal; as may be seen by the manner in which painters imitate them: for, as the imitations of painting extend only to the visible qualities of bodies, they show those visible qualities fairly and impartially-distinct from all others, which the habitual concurrence of other senses has joined with them in the mind, in our perceptions of them in nature. Yet the imitative representation of such objects in painting is far less harsh and dazzling than the effects of them in reality for there are no materials, that a painter can employ, capable of expressing the sharpness and brilliancy of those angular reflections of the collected and condensed rays, which are emitted from the surfaces of polished metals; so that the only way of nitating them

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