Page:Popular Science Monthly Volume 1.djvu/189

Rh greatest tension if we adapt our eye to the nearest point. It is, on the contrary, in complete repose if we adapt it to the farthest point. The optical state of the eye during its adaptation for the farthest point, when every effort of accommodation is completely suspended, is called its refraction.

There are three different kinds of refraction: firstly, that of the normal eye; secondly, of the short-sighted eye; thirdly, of the over-sighted eye:

1. The normal eye, when the activity of its accommodation is perfectly suspended, is adjusted for the infinite distance; that is to say, it unites upon the retina parallel rays of light.

2. The short-sighted eye has, in consequence of an extension of its axis, a stronger refraction, and unites, therefore, in front of the retina the rays of light which proceed from infinite distance. In order to be united upon the retina itself, the rays of light must be divergent; that is to say, they must come from a nearer point. The more short-sighted the eye is, the stronger must be the divergence; such an eye, in order to see distinctly distant objects, must make the rays from a distant object more divergent, by aid of a concave glass. We determine the degree of short-sightedness by the power of the weakest concave glass that enables the eye to see distinctly at a great distance.

3. The over-sighted, or hypermetropic eye, on the contrary, has too weak a refraction: it unites convergent rays of light upon the retina; parallel or divergent rays of light it unites behind the retina, unless an effort of accommodation is made. The degree of hypermetropy, or over-sightedness, is determined by the focal distance of the strongest convex glass with which objects can still be distinctly seen at a great distance.

Hypermetropy has no essential influence upon painting; it only reduces the power of application, and must therefore be corrected by wearing convex glasses. This can never be avoided if the hypermetropy is so great as to diminish the distinctness of vision. Short-sightedness, on the contrary, generally influences the choice of the subject of the artist and also the manner of its execution. As a very small handwriting is an indication of short-sightedness, so we find that artists who paint small pictures, and finish the details with great minuteness, and with fine touches of the brush, are mostly short-sighted.

Sometimes the shape of the eye diverges from its normal spherical form, and this is called astigmatism. This has only been closely investigated since Airy discovered it in his own eye. Figure to yourself meridians drawn on the eye as on a globe, so that one pole is placed in front: then you can define astigmatism as a difference in the curvature of two meridians, which may, for instance, stand perpendicularly upon each other; the consequence of which is a difference in the power of refraction of the eye in the direction of the two meridians. An eye may,