experiment mentioned above when the needle is held so near the holes that the two pencils have not united upon the retina. The eye then, even with the greatest effort, cannot increase the refracting power of the lens sufficiently to adjust it for the needle. If, however, the point is found at which it is possible to see one image only, this point will clearly be the near-point.

Moreover, it is interesting to observe that when we perceive two images and 9, the image 9, which is

formed by the lower hole f, appears above somewhere near Q, whilst the picture p appears below somewhere near P. For if the hole ƒ is covered Q disappears, and if e is covered P disappears. The reason is simple. The picture formed upon the retina is, we know, an inverted one. Therefore what is depicted upon the lower part of the retina is found in the upper part of the field of vision, and that which is depicted on the right, in the left, and vice versa. Therefore in our imagination the retinal picture q is transposed to Q, and the picture to

P.

For the success of the experiment it is important to take care that the distance between the two holes should not be greater than the diameter of the pupil, else the

two pencils of light will not both be able to penetrate the eye. By an instrument founded on the principle of this experiment, the Optometer, in which one cylinder is moved within another, as in a telescope, the near-point may be determined with perfect accuracy,

Now, in most cases, there is also a limit to distinct vision when the eye is adjusted for distant objects. It is indeed possible for persons with perfectly healthy clear-sighted eyes to see objects with clearly defined outlines at any distance within the horizon, and they are therefore able to adjust their eyes for infinite distances. Many people, however, cannot do this, that is to say, they are more or less short-sighted, and there is a certain distance where their distinct vision ceases. This point is called the far-point of the eye.

The far-point of the normal eye is at an infinite distance. When the parallel rays of an object at an infinite distance, a star for instance, fall upon the eye, they unite in the focus of the eye, which in a normal eye would be exactly at the retina. It is evident, however, that this ideal formation is seldom met with, from the fact that people imagine that a star radiates light, whilst in reality it is only a luminous point. In the shortsighted eye, the focus lies more or less in front of the retina within the vitreous humour; this is caused by the diameter of the eye, and particularly the lens, along the line of its axis being too great in comparison to the refractive power of its media. Now if a luminous point is moved towards the eye, then according to the laws of optics, the image will be removed from the focus towards the retina, which, however, it will not reach till it has been moved to within a certain distance from the eye.

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. The 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.

-B

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