SPHERICAL AND CHROMATIC ABERRATION. 425 screen be held at F, it will receive, not only the rays which are brought to a focus there, but also those which, having met at f, have crossed and passed-on to G and H. 547. Now this indistinctness is ordinarily got over in practice, by employing only the central portion of the lens; so that only those rays which correspond to R' L', R′ L', shall pass through it. This we observe in ordinary Microscopes and Telescopes;-a stop or perforated partition being interposed behind the lenses, so as to allow the light to pass through only a small aperture in their centre. By this plan a great deal of light is cut off, so that the image is rendered dark. The spherical aberration may be completely corrected, however, by a certain adaptation of two or more lenses whose surfaces have different curvatures; the effect of which is, to bring all the rays that have passed through every part of this compound lens to the same focus. Now this very adjustment is made in the eye, by the arrangement of the curvatures of the cornea and of the two surfaces of the crystalline lens ; and in the well-formed eye it is so perfect as to produce complete distinctness in the image or picture thrown upon the retina. The only case in which this would not occur, is when an object is brought very near the eye; for the rays then diverge from each other at so wide an angle, that those which fall upon different parts of the lens would not be all brought to the same focus. This error is corrected by the contraction of the pupil, which takes place involuntarily when an object is brought very near the eye, and thus cuts-off the rays that would otherwise render the picture indistinct. 548. But there is another imperfection to which ordinary optical instruments are liable, that is completely corrected in the eye. If we look through a common Microscope, especially when a high power is employed, by the light of a lamp or candle, we see that the edges of the image are bordered by coloured fringes, which very much impair its distinctness, and prevent it from being seen in its true aspect. This is the result of what is termed chromatic aberration; and it results from the fact, that the rays of different colours, which are all blended in ordinary colourless light, are refracted by the same lens in different degrees, so as to be brought to a focus at different points. Thus we will suppose that the lens 426 CHROMATIC ABERRATION :—ACHROMATISM OF THE EYE. LL (fig. 211) has been corrected for spherical aberration; and that R L, R L, are violet rays falling upon it, whilst R' L', R' L', are red rays. The former are capable of being refracted in a much higher degree than the latter; so that they are brought to a focus at f, whilst the others do not meet until F. Hence if a screen be placed to receive the image at f, the picture will be formed by the violet rays only, and will be surrounded by red fringes occasioned by the red rays which are passing on to their focus at F; whilst, on the other hand, if the screen be placed at F, the picture will be chiefly formed by the red rays, and will be surrounded by violet fringes produced by the violet rays, which, having met in f, have crossed and passed-on to G and H. Now as from each point of almost every object proceed rays in which the different colours are blended, the refraction of an ordinary lens produces a separation of these, and a consequent indistinctness and false colouring in the picture. This is particularly the case with regard to the rays that pass through the outer portion of the lens; for, as these are subject to greater change in their direction than are those which pass through its centre, the separation of the differently-coloured rays of which they are composed is more considerable. 549. In practice, this error is got over, like the preceding, by very much contracting the aperture of the lens; so that only the central rays, in which the colours are but little separated, are allowed to pass. But it may be perfectly corrected by combining lenses formed out of different materials, which possess a different refracting power; the errors of these being made to counterbalance one another. Such lenses, which are termed achromatic, are now employed in all superior Telescopes and Microscopes; but no artificial combination can surpass that which exists in the Eye, the different density of whose humors is adjusted in such a manner as completely to answer this purpose. The contraction of the pupil which takes place when we look at a very near object, prevents the only imperfection which could occur; and thus the picture on the retina, in a healthy eye, is always rendered free from false colours. It is said that the first idea of uniting glasses of different kinds, so as to produce an achromatic lens, was taken from the Eye; and this is not at all improbable. In this, as in many other instances, Nature ADJUSTMENT OF THE EYE FOR VARYING DISTANCES. 427 has served as a guide to Art; or, in other words, the Divine Artificer has thus condescended to teach the human workman. 550. There is another wonderful arrangement in the healthy Eye, which the optician can only imitate in his instruments in a very bungling manner. It is that by which the eye adapts itself to view objects at different distances from it, with equal distinctness. If we look at a near object with a Telescope, adjusting the instrument so as to see it distinctly, and then turn it towards a remote object, we shall not see the latter with equal clearness until the instrument has been again adjusted. If we then turn it back to the nearer object, we shall find that the change in the adjustment occasions the representation of it to be now indistinct; and in order to bring back the image to its former clearness, it is requisite to re-adjust the instrument to its first condition. This is a necessary consequence of the optical law, that the distance of the image from the lens which forms it, varies with that of the object,-being increased as the object is brought nearer, and diminished as it recedes. If the Eye were constructed in the same manner, we should not be able to see distinctly, without the aid of artificial assistance, at any other distance than that for which it is adjusted. Hence if a perfect picture of an object situated at twelve inches' distance from the eye, were formed upon the retina, we should not be able to see it clearly when brought to the distance of six inches, nor when removed to the distance of six feet; because in the first of these cases the rays would not be brought to a focus upon the retina, but at a point behind it (if they were allowed to pass on unchecked); whilst in the second, they would be brought to a focus at a point nearer than the retina, and would consequently begin to separate again before they reach it. 551. But the healthy eye possesses a power of perfect adjustment to the viewing of objects situated at different distances; and this without any effort or intention on our parts, but, as it were, by an instinctive operation. That such a change really takes place, we may readily convince ourselves, by looking at a near and at a distant object placed in the same line, —a pencil-case, for instance, held up at a few inches from the eye, and a chimney half a mile off. We shall find that no effort of attention will enable us to see them both 428 LONG AND SHORT SIGHT:-SPECTACLES. distinctly at the same time; but that, on whichever of the two objects we fix our eyes, we shall see it clearly, whilst the other will become indistinct. Recent observations have conclusively shown that this adjustment is brought about by an alteration in the curvature of the crystalline lens; its convexity being increased when a near object is looked at, so as to act more powerfully in bringing its diverging rays into convergence; and being diminished when the gaze is turned towards a distant object. 552. In advanced life, however, from the diminution in the convexity of the cornea and in the refracting power of the humors, the eye can no longer accommodate itself to near objects; their rays not being brought to a focus by the time that they reach the retina. But as it is still able to see distant objects clearly, it is said to be long-sighted. By the use of a convex glass, however, adapted to supply that additional amount of refraction which is required, near objects may be distinctly seen. A contrary state of the eye not unfrequently exists, in which the cornea is too convex, and the refracting power of the humors is too high; from which it happens that the rays proceeding from distant objects are brought to a focus too soon, so as to cross each other before they reach the retina. But as such an eye can form a very distinct picture of a near object, it is said to be near-sighted. This imperfection is remedied by interposing a concave lens between the object and the eye, by which its refracting power is diminished to the necessary degree. 553. In the choice of spectacles or eye-glasses for these purposes, particular care should be taken that they are not too powerful; since great mischief is frequently done to the eye, by the employment of lenses of too great a curvature. A person who in youth and middle age has enjoyed good sight, very commonly finds it necessary to employ spectacles for assistance in reading and writing, as his age advances towards fifty years; and he should be very cautious, when first availing himself of their assistance, to employ those of the longest focus. As his age advances, it will be necessary to substitute more powerful lenses for these; but this should be done very gradually; and in no instance should a glass be employed that produces any apparent enlargement in the object, its proper use being only to render the object distinct. The evil SPECTACLES: -SENSITIVE SPOT OF RETINA. 429 influence of using spectacles of too high a power, soon manifests itself in the strained feeling which the eyes experience for some time; but this feeling at last subsides, in consequence of the eye having adapted itself to the glasses, and having thus undergone a change which it might otherwise take years to produce. In this manner the eyes of a person at sixty may be brought to the state which, under more careful management, might have been deferred ten or fifteen years longer.—Similar remarks apply to the use of concave lenses by short-sighted persons. They should never be employed of a higher power than is requisite to see objects with distinctness, when at a moderate distance; and on no account should any glasses be used that diminish their apparent size. As age advances, the eyes of short-sighted persons usually become more flattened, and are then able to adapt themselves to objects at a variety of distances; so that persons who have been short-sighted when young, are not unfrequently able to see distinctly at an advanced age, without the assistance of convex glasses. 554. The power of receiving and transmitting visual impressions is by no means uniform over the whole retina. In the whole field of vision which at any time lies before the eye, we only see with perfect distinctness that part to which its axis (namely, that diameter of the sphere which passes through the centre of the pupil) is directed, and of which the image, therefore, is formed upon "the yellow spot" (§ 535) which lies at the posterior pole of the axis. Nevertheless we have a sufficiently distinct perception of the remainder of the field, to enable us to judge of the general relations of its objects to each other and to those which we distinctly see: thus, whilst reading or writing, we can only recognise letters and words at any one moment within a spot which a sixpence or a shilling would cover, but we may distinguish the lines over the whole area of the page, and can plainly see the position of the book or paper upon the desk or table, together with the position of this in the apartment. In the act of reading or writing, as in surveying the different parts of a landscape or a picture, or in examining any solid object that is brought under our notice, we direct the axis of the eye successively to one point after another, until we have satisfied ourselves that we have gained a distinct view of every part, |