An Invariable Symptom of Imperfect Sight 115
and are interspersed with rods, the number of which in creases toward the margin of the retina. The precise function of these rods and cones is not clear; but it is a fact that the center of the fovea, where all elements ex cept the cones and their associated cells practically dis appear, is the seat of the most acute vision. As we withdraw from this spot, the acuteness of the visual perceptions rapidly decreases. The eye with normal vision, therefore, sees one part of everything it looks at best, and everything else worse, in proportion as it is removed from the point of maximum vision; and it is an invariable symptom of all abnormal conditions of the eyes, both functional and organic, that this central fixa tion is lost.
These conditions are due to the fact that when the sight is normal the sensitiveness of the fovea is normal, but when the sight is imperfect, from whatever cause, the sensitiveness of the fovea is lowered, so that the eye sees equally well, or even better, with other parts of the retina. Contrary to what is generally believed, the part seen best when the sight is normal is extremely small. .The text-books say that at twenty feet an area having a diameter of half an inch can be seen with maxi mum vision, but anyone who tries at this distance to see every part of even the smallest letters of the Snellen test card-the diameter of which may be less than a quarter of an inch-equally well at one time will imme diately become myopic. The fact is that the nearer the point of maximum vision approaches a mathematical point, which has no area, the better the sight.
The cause of this loss of function in the center of sight is mental strain; and as all abnormal conditions of the eyes, organic as well as functional, are accompanied by