134 
rorULAR SCIENCE REYIEW. 
in two opposite directions. If we imagine this natural eye to 
be nearly spherical, we shall see that, in an eye like A, fig. 3, 
wliich is more or less egg-shaped, and longer than a sphere, the 
rays of light from distant objects, instead of meeting at n, on 
the retina, would meet at b' in front of it. On the other hand, 
in an eye like a', that is flattened, or too short from back to 
front, the rays would reach b before they were united ; and if 
the coats of the eye were transparent, would pass through them, 
and meet at b' behind the retina. In neither case would there 
be distinct vision. If we now turn back to figure 1, we see 
that there, by bringing the point A nearer the eye, to a', the 
focus B is put farther back to b^ This is just what we want to 
do for the eye A in fig. 3. Its focus must be put back until it 
falls upon the retina. To do this, we bring the object nearer 
tlie eye, and as ‘soon as it reaches a certain point, the object 
becomes clearly visible. Such an eye is said to be short-sighted, 
or near-sighted, and nearness of sight is defined by the distance 
Fig. 3. 
of the farthest point of distinct vision, just as presbyopia is 
defined by the nearest point of distinct vision. The other 
defect, the flat eye, is technically called hypermetropia, and 
there is no good English name for it. In it (a', fig. 3) it is 
manifest that the lens requires to be made more powerful ; and 
hence a certain amount of accommodation is called into play, 
even for distant objects, and cannot be wholly relaxed. A 
natural eye has its accommodation wholly relaxed when looking 
at distant objects, and hence wholly in reserve for near ones. 
A flat eye is perhaps using one-half of its accommodation 
always, and has only the other half in reserve for near objects. 
\Vhen looking at near objects, perhaps all of this remaining 
half is employed, and hence the eye soon suffers pain and in- 
convenience from over-fatigue. 
^^e may still further elucidate these several conditions by 
simple diagrams. The natural eye, when placed at the point A, 
N' F-^ 
looking along the line towards its ,‘far point F, has that far 
point at an infinite distance, as at a fixed star, and its distant 
