larger field of vision, the margins of which are depicted distinctly and without distortion. If the retina had a plane surface, like the ground glass plate in a camera, it must necessarily be much larger than is really the case if we were to see as much; moreover, the central portion of the field of vision alone would give a good clear picture.

CHAPTER II.

The Adjustment of the Eye-Short Sight and Long Sight.

WE know from experience that we are able to see distinctly objects at different distances from our eye. But on careful observation we shall find that we cannot form simultaneously a perfect picture of objects at different distances from us. Suppose that we are in a room at a little distance from the window, and that we then hold up a finger a few inches before one eye, and close the other; if we now fix the eye steadily upon the finger so as to see it distinctly, the window in the background will be seen indistinctly. If we fix our eyes upon the window frame so as to see it distinctly, then the outline of the finger becomes indistinct, so that we can see distinctly at will either the window frame or the finger. Thus the eye is adjusted for objects at different distances, and this property of the eye is called its adjustment. When our eyes are wandering over objects at different distances from us, this adjustment is constantly at work, although we are quite unconscious of it ourselves.

In fig. 8 the image of the arrow A B is depicted at a b. If, however, the object A B is at a greater distance,

then the image a b will be formed nearer the focus F', at the same time becoming smaller. The position of the image changes with that of the object. When, therefore, a picture is about to be produced in the camera obscura, the inner case b containing the ground glass plate g must be moved forward into the outer case a for distant objects, and drawn out again for near objects. By means of the screw r the lens may be moved a little backwards and forwards, which will still further ensure the clear delineation of the picture. This also applies to the eye. The object A B in fig. 10 may be depicted quite clearly at a b in the eye. If the object approaches or retreats whilst the eye remains in the same position, the picture of the object will become indistinct, because in the first case it would be formed before, and in the second behind the retina.

Let us suppose a luminous point a, fig. II, to be at a certain distance from the eye, then a distinct image of this point will be formed if the retina lies at the point of convergence c; if, however, the retina is at a greater distance at g, g, so that the focus is formed in front of it, then, instead of a point, a blurred circle is produced. A similar blurred image would be formed

upon the retina if it lay at ƒƒ,, in front of the image, and if all the points of an object were not depicted upon the retina at their foci, the whole object would appear indistinct and blurred. The experiment, which we mentioned at the commencement of the chapter, may be easily explained in this manner. When the eyes are fixed upon the finger the image of the window frame behind it does not fall upon the retina at the focus of the rays it emits, and conversely, if the window is fixed by the eye, the finger appears blurred for a similar reason. The image of the body which we see distinctly always falls upon the retina, whilst the image of the body which we see indistinctly would fall in the first case before, and in the second behind the retina. From this the important inference is to be drawn, that the eye possesses a power of adjustment, by means of which objects at various distances are depicted distinctly upon the retina.

We might very well be inclined to think that the method employed for this adjustment of the eye should be similar to that employed in the camera obscura. It was formerly considered as possible that in the process of adjustment the retina moved backwards and forwards. by the elongation of the eye-ball, through the action. of muscles in the socket, so that the background of the eye could with equal ease be moved in either direction. This supposition has, however, been refuted, and it has been shown that the method employed is quite different to that of the camera obscura, and so perfect that it cannot be imitated artificially. Let us suppose the eye to be adjusted for a great distance, such as some point upon the horizon, or in the sky. The rays

from this point will penetrate the eye in almost parallel lines, and will, therefore, unite at the focus of the eye, which, in a normal eye, lies exactly in the central point of the retina. If, however, the object we are looking at is nearer the eye, then, according to the laws of optics, its image will fall beyond the focus of the eye, and, therefore, behind the retina. The eye, however, is enabled by adjustment to bring the point at which the image is formed back to the retina, which can easily be explained by a greater convergence of the rays which penetrate the eye, or in other words, they must suffer greater refraction in order that they may meet sooner than they would if they suffered less refraction, and thus produce a distinct image upon the retina.

The reason why the light-rays suffer a stronger refraction when the eye is fixed upon nearer objects is, that it has been observed, that when the eye, after viewing a distant object, views a nearer one, the crys

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Fig. 12.

talline lens becomes thicker by an increase in the curvature of its anterior surface. Fig. 12 shows the alteration in the curvature of the lens when a near object is looked at, after a drawing by Helmholtz.

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