The

For very short-sighted persons it is well known that this distance is exceedingly small, so that they are not even able to form distinct images of objects in a room. far-point for the eye of a short-sighted person will therefore be the point where the image of objects is imprinted exactly upon the retina, whilst every point at a greater distance will only produce a hazy image.

Short-sighted persons make use of spectacles consisting of concave glasses to enable them to see distinctly at a distance. These glasses have the peculiarity of increasing the divergence of the rays of light, so that the convergent rays which fall upon them will intersect each other so as to form an image at a greater distance. Fig. 16 shows that rays which converge at the point B will, by a concave glass, be made to converge at the point A. Let us suppose that in a short-sighted eye the image of a luminous point has been formed at B, and that the retina lies behind it at A; now by making use of a pair of concave spectacles, the image will be formed upon the retina. The more short-sighted a person is, the more concave must his spectacles be, to increase the divergence of the incident rays to such an extent that the image may be formed upon the retina.

Another very frequent defect in the eye is longsightedness. The structure here is the opposite of that just described, and consists in the focus of the eye lying behind the retina. When, therefore, parallel rays, as for instance those from a star, fall upon the eye, they do not unite until they have passed beyond the retina. Persons so affected, however, are generally able, by adjusting the eye, to move the image forward, and thus to see distant objects distinctly. Near objects however they see in

distinctly because they cannot sufficiently adjust the eye so as to form the image upon the retina, therefore the near-point for them must be at some distance from the eye. Long-sighted persons consequently use convex glasses to enable them to see near objects distinctly, for a convex lens converges the rays, and brings them sooner to a focus, and therefore moves the image forward. When a long-sighted person wishes to read, he must either put on his spectacles or hold the book at some distance till it reaches his near-point, whilst on the other hand a shortsighted person can read quite well without spectacles, if he only holds the book near enough to his eyes.

Fig. 16.

Short-sighted persons can see objects when placed close to the eye even better than those who enjoy normal sight, because their near-point lies nearer to the eye, and all objects apparently increase in size as they approach the eye.

Another less striking defect, which is seldom met with in a normal eye, but is often strongly developed in cases of illness, is caused by the unsymmetrical formation of the interior of the eye. In looking at the concentric circle (fig. 17) with one eye, we shall observe that the lines are never all distinct at the same time, but, as we adjust the eye, two opposite sections will alternately appear clear and distinct as their positions

change. From this it follows that the curvature of the eye is not exactly the same in a horizontal and vertical direction. Consequently, rays from vertical lines have a different focus, or point of convergence, to rays from horizontal lines. Thus, for instance, in the adjustment for

horizontal lines, vertical lines at the same distance will appear indistinct, and vice versa. This is the case in the figure of the concentric circle, since the horizontal and vertical directions of the lines merge into each other.

One other circumstance may be mentioned which

takes place in the refraction of light in the eye. It is well known that inferior opera-glasses and telescopes represent objects with coloured edges, and, in order to avoid this defect, so-called achromatic lenses are made use of, which are constructed so as to prevent the resolution of light into coloured rays. Every common lens gives images with coloured edges, because the points of convergence of the coloured rays, of which white light is composed, do not coincide, the refrangibility of each colour of the spectrum increasing from red to blue.

Thus we cannot at the same time see a red line and a contiguous blue line distinctly; for when the eye is adjusted for the red line the image of the blue one lies in front of the retina, and when the eye is adjusted for the blue line the image of the red one lies behind the retina. Now the eye is not a perfect achromatic instrument, although objects do not appear with coloured edges in ordinary vision. If we look at the sharp edge of a dark object against a white surface, as, for instance, the horizontal bars of a window frame against a cloudy sky, the upper edge will appear of a yellow tinge and

the lower one of a blue tinge, particularly if we do not carefully adjust the eye for it. The coloured rays are seen here, as in optical instruments, because the resolved rays are not hidden by the edge of the window frame, whilst within a white surface the rays overlap and reform white light.

From an insufficient power of adjustment bright objects, if seen against a dark ground, at a certain distance from the eye, will appear to be surrounded by a coloured fringe, which causes the light surfaces to appear larger than they really are. This phenemenon is called Irradiation. If, for instance, we look at the two equal squares in fig. 18, the black one on a light, and the light one on a dark ground, at a little distance from the eye the latter will look larger than the former. In fig. 19 the two white squares, when seen from a little distance, will appear to run into each other, and to be joined together by a white bridge, since the resolved rays overlap. It is a well-known fact, that people look larger in light clothes than in dark, which may also be explained as the effect of irradiation. This circumstance is not unknown in the art of dress, but is carefully studied. That a black dress contributes to an elegant figure, ladies know very well.

Fig. 19.