Page:Popular Science Monthly Volume 9.djvu/713

 Rh 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 colored 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.

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. If 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 colored fringe. This is caused partly by a cloudiness in the vitreous humor, 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 allow 7 ed to fall upon the eye; for, in that case, the shadows produced are distinct, while they are generally obliterated in ordinary vision, because the light penetrates the eye from all sides.