Page:Popular Science Monthly Volume 1.djvu/485

Rh particular spot, situated somewhat beyond the optic nerve and exactly opposite the centre of the cornea. The light which falls along the main axis of the eye converges at this point. This spot in the retina is marked by a small hollow. For the rest, it is filled up with a structure of its own; and we have reason for assuming that it furnishes the most exact perceptions, not only on account of the greater optical sharpness of the image, but also on account of the higher energy or activity with which it is endowed. It is this spot we make use of when we desire to go into details; for, if we wish to examine closely into the nature of an object, either we approach it to the eye, or bring the eye to bear on the object; but, in both cases, in such a manner as to cause the image to fall exactly on the hollow of the retina, or on the spot of direct vision. This arranging of a position for an object is what we mean when we speak of adjusting the eye.

The images which are not projected on the spot of direct vision are not sharp; for the necessary light falls on the refracting media more or less obliquely. This, and the decrease in the activity of the sight from the hollow to the sides, explain how the objects, the farther they are removed from the fixed point, appear with so much less clearness and sharpness of outline. Indirect or eccentric vision, as it is termed, makes us aware simply of the presence of objects, by giving us some notion of their shapes; but we are unable to distinguish even the biggest letters, if the image of them should fall only one hair's breadth off that one spot in the retina. In reading, the eye must constantly move onward to the end of the line, the single letters thus gradually imprinting themselves on the direct point of vision. On the other hand, indirect vision offers hints for fixing the object in our eye; it warns us of, and prepares us for, the object previous to our devoting our whole attention to it, and it is further of use in procuring a wide survey, by enabling us to see and examine what lies before us. There are some who only possess direct vision. Any one can put himself in the place of an individual so afflicted, by holding a long tube of small calibre to his eye. You naturally distinguish the most minute objects enclosed within the restricted range of vision; but, deprived of the lateral impressions, you could not guide your steps in the street. In short, you must fancy the image of the external world that is imprinted on the retina, like a picture highly finished in the middle, and only roughly sketched out at the sides.

The distance from the spot of direct vision, at which the objects may be perceived by eccentric vision, has its limits. When looking straight before you, you can just perceive a hand which stretches down the whole length of the face, in the direction of the temples. This is the extremest point from which it is possible for light to fall on the retina; but, if you attempt to go beyond it, the hand disappears, its image not being projected on the retina. The combination of all the extreme points from which, with a set eye, impressions may be received,