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.

CHAPTER III.

Recognition of the Interior Portions of the Eye-The Ophthalmoscope.

THE retina is the point where the physical process of vision passes into the physiological process. Until it impinges upon the retina, the light which penetrates the eye has only undergone physical changes, consisting chiefly in refraction, the last perceptible result of which is the production of the image upon the retina. From this point the process passes from our immediate observation, and the difficulty of discovering its character increases at each step. The image upon the retina is reversed, and yet we see every object in the field of vision upright. This is the result of the experience, which we have acquired from childhood, in the exercise of the organ of sight. The point A (fig. 10), which is on the right, is imprinted upon the left portion of the retina, and we therefore know by experience that a ray coming from the right, must strike the left portion of the retina; and because we always imagine the objects we see to be external to ourselves, we must do so by unconsciously following the line a A, through the optical centre k. In this manner the eye projects a uniform field of vision, which is

obtained by drawing, from every point of the retina outward, straight lines through the optical centre of the eye, which lines will terminate upon a convex surface.

This is really the manner in which the eye interprets, in all cases, its sensations of sight. For luminous appearances may be produced, without our perceiving any external object, but merely a part of the eye or an inward irritation; and yet, in the same manner, we imagine them to be external to ourselves.

If we shut the eye and press the head of a pin upon the outer edge of the eyeball, we shall see in the dark field of vision a white or coloured spot of light, which has the same form as the compressing body: it will be seen upon the left side of the field if the right side is pressed, and upon the upper half if the lower is pressed, and vice versa. The retina, therefore, extends as far as the part which projects beyond the socket of the eye, and can be irritated by pressure. It is well known that when the eye is struck, a cloud of sparks is seen, which is caused by the mechanical concussion of the retina. These luminous images, often perceived involuntarily, take, speaking scientifically, the form of the body producing the pressure; at the same time we observe the relation between the position of the irritation and the position of the sensation of sight. We transpose a point on the left side of the retina to the right, because we imagine that a ray of light has penetrated the eye from the right, which must fall upon the left half of the retina

We are also able to perceive particles within the interior of the eye, which are found in the transparent media There are many persons who always see round particles

or filaments, which seem to float about in the field of vision. They may be more distinctly seen when looking upon a bright surface-a cloudy sky, or through a microscope. They follow every motion of the eye, and have, moreover, a peculiar motion of their own. These particles are produced by filaments and cells, which may be found floating about in the narrow space between the hyaloid membrane and the retina. They cast their shadow directly upon the retina, which then, from experience, refers them to external objects.

If

It has also been discovered by more careful observation that the refracting media of the eye are not absolutely transparent, but that a kind of cloudiness is seen. in places which throws a shadow upon the retina. we look at the sky through a small hole in a sheet of paper, held a short distance from the eye, the hole will appear to be surrounded by a coloured fringe. This is caused partly by a cloudiness in the vitreous humour, and partly by the peculiar radiating formation of the lens, already described. All such phenomena are called entoptic, because they deal with the perceptions of the internal portions of the eye. They are produced by the incident rays of light casting shadows of these particles upon the retina. They are best seen when an isolated pencil of light, like that admitted through a small aperture, is allowed to fall upon the eye; for, in that case, the shadows produced are distinct, whilst they are generally obliterated in ordinary vision, because the light penetrates the eye from all sides.

One of the most interesting entoptic phenomena is the Arborescent Figure, discovered by Purkinje. If, towards evening, we place ourselves opposite a dark wall in