SIGHT. 499 



is placed at less than six inches distance from the eye. But from 

 that point outward, the eye can adapt itself to any distance at which 

 light is perceptible, even to the immeasurable distances of the fixed 

 stars. A much greater accommodating power, however, is re- 

 quired for near distances than for remote, since the difference in 

 divergence between rays, entering the pupil from a distance of one 

 inch and from that of six inches, is greater than the difference be- 

 tween six inches and a yard, or even distances which are immea- 

 surably remote. Accordingly, near-sighted persons can see objects 

 distinctly when placed very near the eye ; since, as their lens con- 

 verges the rays of light more powerfully than usual, they can be 

 brought to a focus upon the retina, even when excessively diverg- 

 ent at the time they enter the eye. But distant objects become, 

 indistinct, since, however far backward the lens is moved, the rays 

 are still brought to a focus and cross each other, before reaching 

 the retina, as in Fig. 161. Near-sighted persons, therefore, have a 

 limited range of accommodation, like all others, only it is confined 

 within short distances, owing to the excessive refracting power of 

 the lens. 



On the other hand, long-sighted persons can see remote objects 

 without trouble, since a very little movement of the lens will be 

 sufficient to adapt it for long distances ; but within short distances, 

 the divergence of the rays becomes too great, and they cannot be 

 brought to a focus. 



Circle of Vision. Since the opening of the pupil will admit rays 

 of light coming from various directions, there is in front of the eye 

 a circle, or space, within which luminous objects are perceived, and 

 beyond which nothing can be seen, because the rays, coming from 

 the side or from behind, cannot enter the pupil. This space, within 

 which external objects can be perceived, is called the " circle of 

 vision." But, for short distances, there is only a single point, in the 

 centre of the circle of vision, at which objects can be seen distinctly. 

 Thus, if we place ourselves in front of a row of vertical stakes or 

 palisades, we can see those directly in front of the eye with perfect 

 distinctness, but those at a little distance on each side are only per- 

 ceived in a confused and uncertain manner. On looking at the 

 middle of a printed page, in the direct range of vision, we see the 

 distinct outlines of the letters ; while at successive distances from 

 this point, the eye remaining fixed, we can distinguish first only 

 the separate letters with confused outlines, then only the words, and 

 lastly only the lines and spaces. 



