132 
POPULAK SCIENCE REVIEW. 
if the point A be brought nearer the lens, as to a', the rays 
proceeding from the latter point, being more divergent than 
those from the former, will be less affected by the lens, and will 
be united at a point b', behind B. In' order that the rays pro- 
ceeding from a' may still be united at b, either the lens c must 
be made more powerful or it must be moved further from B, as 
into the position shown by the dotted line c'. In the eye, as in the 
camera, the point B — that is to say the place of the screen — re- 
mains unchanged, and the adjustment required for different dis- 
Fig. 1. 
tances of the object has to be provided for. In the camera the lens 
is moved to and fro by rack- work ; in the eye there exists an ar- 
rangement for increasing the power of the lens by voluntary effort. 
The eye is represented in diagram in fig. 2, and c is its lens. In a 
natural eye, this lens is just sufficiently powerful to unite at the -J 
retina, b, rays of light that are nearly parallel — such as A A — 
and that come from a very distant point. In order to unite at 
B rays that come from the nearer point a', the lens is rendered 
more powerful by a voluntary effort ; and the effect of this effort, 
as shown by the dotted line, is to increase the curvature of one { 
Fig. 2. 
or'^both of its surfaces. The faculty of thus increasing the power 
of tlie len.s, so as to see near objects clearly, is called accommo- ^ 
dation. The accommodation necessarily has a definite limit; ^ 
and hence, if we bring print nearer and nearer, it soon arrives j 
at a place where it can no longer be read. The nearest point ^ 
at wiiich it can be read is called the near point of distinct 
vision — or simply the near point. The stronger the accommo- 
dation, the nearer will this near point be to the eye. As life 
advances, the accommodation diminishes; partly because the 
lens itself becomes less yielding, partly — perhaps, because the 
