It may be seen from the foregoing figure that if the retina, which normally would lie at 2, were placed nearer the dioptric apparatus, say at 1, or further from it, at 3, it would not meet the exact point of the luminous cone, but would receive the rays either before they came to a point, or after they had diverged from it. Thus indistinct rings of light would be seen instead of one luminous point, and an image would be blurred and indefinite.
From this it follows that the eye, when quite passive, can only get an exact image of bodies which are placed at a certain distance from it, just as, for any given state of a camera, only those bodies in one plane come into focus and give a clear picture on the screen. If the dioptric apparatus of the eye were rigid and unalterable, since the relation of the retina to it is permanently the same, we could only see those objects clearly which are at a given distance from the eye. We know, however, that we see as distinct an image of distant as of near objects, and we can look through the window at a distant tree, or can adjust our eyes so as to see a fly walking on the window pane. We cannot see both distinctly at the same moment. This power of focusing may be demonstrated by what is known as Scheiner's experiment, which is carried out in the following way.
Two pin holes are made in a card at a distance from each other not wider than the diameter of the pupil. The card is then brought close to the eye, so that a small object - such as the head of a bright pin - can be seen through the holes. The dioptric media being-fixed, moving the object nearer to or further from the eye would have the same effect as changing the relation of the retina to m n or p q in Fig. 225, by means of which we may explain the following observations: (1) The eye being fixed upon the object (of which only one image is seen), move the pin rapidly away; two objects now appear, showing that the rays coming through the holes have met before they reach the retina, as at p q. (2) Move the pin near the eye; again two very blurred objects are seen, for the rays have not met when they strike the retina, as at m n. (3) Keeping the object in the same position, alter the gaze, as if to look first at distant and then at near objects; in both extremes two images are seen. (4) When the object is in exact focus, as at c, the closure of one of the holes does not affect the single image. (5) When two images are seen, closing the right-hand hole at g causes the right or left image to disappear, according as the focus c falls short of m?i or is beyond p q, the retina. (6) By moving the pin's head nearer the eye, a point is reached at which the object cannot be brought to a focus as a single image. This limit of near accommodation marks the near point. A little attention teaches us that looking at the near object requires an effort which looking at the distant one does not; in fact, we have to do something to see things near us distinctly. This'act is the voluntary adjustment of the eye which we call its accommodation for near vision. 48
Fig. 225. To illustrate Scheiner's experiment; for explanation, see text.