Page:Popular Science Monthly Volume 47.djvu/473

Rh The chief imperfection in the vision to which I refer is astigmatism, although either with that, or independently of it, there is usually with artists excessive contraction of the muscle used in focusing the eye—the so-called ciliary muscle. The majority of people have become somewhat familiar with the term astigmatism and its meaning, but, as it involves a rather complicated principle in optics, it may be well to define it here. Technically it might be described as an asymmetry of the eye in which the radius of curvature in one meridian is greater or less than the radius of curvature in another. This definition may be easily understood by a simple illustration. If the transparent portion in the front part of the eye, known as the cornea, were perfectly regular, like the surface of the ordinary sunglass, the rays of light would all tend to converge to a single point; but if the globe were compressed in any one direction—for example, from above downward—then this transparent portion of the eye would not have a regular curvature, but would be somewhat like the top of a Derby hat, held with the long diameter horizontally and the convex surface forward. Practically this is what usually exists in the human eye. As the globe is compressed above and below by the upper and lower lids, it is to a certain extent flattened. This is the usual form of astigmatism, or astigmatism with the rule, as it is called. Other causes tend to make the axes of these two curvatures oblique to each other, or may change their position in various ways, which need not be considered here.

According to the popular idea, the human eye is a perfect instrument, but this, in the vast majority of cases, is not the truth. Nearly every one is astigmatic. Many a person whose eyes are quite imperfect would laugh at the idea if this were told him. He has perhaps always prided himself upon having the best of vision. But the fact is that only a very small per cent of eyes are really free from this unequal curvature which we call astigmatism.

One series of observations made by Dr. Roosa of two hundred eyes, whose owners supposed them to be perfect, and which were apparently perfectly normal, showed that only about one per cent were, beyond question, absolutely perfect, and my own investigation in the same direction would fully corroborate this. If, therefore, a variation from the normal type is so frequent, it is but natural to suppose that artists should have at least their share of astigmatism. But the fact is that among artists astigmatism is not only more frequent, but also that it exists, on the average, in a higher degree, probably, than among any other class of persons.

We can understand the reason for this if for a moment we