ART AND EYESIGHT. 459 



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 ma- 

 jority of people have become somewhat familiar with the term 

 astigmatism and its meaning, but, as it involves a rather com- 

 plicated 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 con- 

 sidered 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 investiga- 

 tion in the same direction would fully corroborate this. If, there- 

 fore, 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 



