136 
POPULAR SCIENCE REVIEW. 
and it causes the rays of light to diverge, or spread out towards 
r, as if they came from b, which also is called the principal 
focus, and the distance cb the focal length. The reader who 
carefully considers these diagrams will see that, if we place 
before a short-sighted eye a concave lens of six inches focal 
length, it will make the rays from distant objects come to the 
eye in the same state or direction as if they came from an object 
exactly six inches distant. They will then be united upon the 
retina, and clear vision of the distant objects will be obtained. 
On the other hand, if we place before a presbyopic eye a convex 
lens of six inches focal length, we cause the rays of light from 
an object only six inches distant to strike the eye as if they 
came from far distance, and the object will be seen distinctly. 
Since people with natural eyes, in order to be presbyopic, 
have only to live to be forty-five or fifty years of age, the 
spectacles that they will then require are matters about which 
it concerns everybody to be informed. There are old prejudices 
against beginning glasses too early,” against using them too 
strong,” and against changing to a higher power too frequently. 
The object of glasses is to enable the eyes to work comfortably, 
without fatigue ; and the glasses should always be strong 
enough to effect this object, and should be used as soon as in- 
convenience is felt. On this point it is difficult or impossible 
to lay down any rule that will not be subject to frequent ex- 
ceptions ; but when the sight has been previously good, the 
first indications of the need for glasses will be a little mistiness 
of fine print by candle light, a difficulty of distinguishing 
between somewhat similar letters, such as m and and a 
tendency to draw the candle or lamp nearer, or even to hold it 
between the eyes and the book. Grlasses having a focal length 
of sixty inches will then usually suffice for a time, and the 
ascent to higher powers should be gradual. The first spectacles 
should at first only be used for reading in the evening; and, 
when no longer sufficient, they may be superseded for evening 
work by others, and the first pair reserved for reading by day- 
light, or for writing, which requires less critical vision, more 
especially if care be taken to use ink that flows black from the 
pen. The objection to the higher powers is, that convex lenses 
contract the range of vision by bringing the far point to their 
own focal distance. If we wear lenses of twelve-inch focal 
distance, we cannot, through them, see clearly at any point 
beyond that distance. And if the powers be too high, the focal 
distances too short, and tlie range of vision too much curtailed, 
the muscles of tlie eyes are strained in order to keep them both 
directed to a point within this range, and the circulation is 
impeded l)y the bent neck and stooping posture that are re- 
quired in order to bring tlie l ead near to the work. Moreover, 
