Page:Popular Science Monthly Volume 1.djvu/477

Rh place can the irritation of each separate spot of the retina produce a peculiar impression corresponding to the presence of the object-point; in short, according to the optical expression for such a relation, a picture of the outside world must be painted on the retina.

And this is indeed what takes place. As, on the one hand, the retina stands as the terminal apparatus of the optic nerve; on the other hand, it acts as a shade subservient to optical purposes; a screen, on which a perspective picture of the outside world is projected. If you compare it with the dull glass on which the picture in the camera-obscura falls, or the prepared plate in the photographer's camera, you have a correct notion of what I mean. As, in the photographer's camera, the picture falls on the sensitive plate, and is impressed on it by means of chemical changes produced by light, so in the eye it falls on the sensitive plate of the retina, whose irritations are telegraphed to the brain in due form.

We henceforth have to consider this image painted on the retina as the real object of the operations of the senses.

But how does the picture imprint itself on the retina? This is done by an optical apparatus close behind the retina and in connection with it; and, in short, by means of that mechanism known to us as the eye.

If we compare the retina with the sensitive plate in a camera-obscura, we shall perceive that the eye has indeed an undeniable resemblance with this well-known optical instrument, the camera-obscura.



This is essentially a box painted black in the inside, with an opening fitted with a collective lens, turned on the objects of the outside world, and which receives the images produced by this lens on the wall behind. In order to show us the image, the one side of the box