ON THE USE AND CHOICE OF SPECTACLES. 133 
strength of the muscle of accommodation declines; and the 
result is that, from youth onwards, the near point constantly 
recedes farther and farther from the eye. When the sight is 
perfect, it is usual to test the place of the near point with 
El illiant typ®. At eleven years of age, this can usually be read 
as close as at three inches from the eye. At fifty years of 
age, it would scarcely be read nearer than at eighteen inches ; 
and at sixty, scarcely nearer than at two feet. And, as the light 
falling upon the eye from any surface diminishes as the square 
of the distance increases, it follows that a pupil of the same 
dimensions would receive from a printed character, at eighteen 
inches, only one thirty-sixth as much light as from the same 
character at three inches. At two feet, it would receive only 
one sixty-fourth as much as at three inches. And this explains 
the practical inconvenience arising from the diminished accom- 
modation of advancing life. In time, the near point comes to be 
so far off, that we do not receive from the object light enough 
to see it distinctly. Dr. Kitchener, writing in days before 
people had gas in their houses and many kinds of brilliant light 
at command, said that the first sign of the need of spectacles 
was a tendency to bless the man who invented snuffers. The 
same rule still applies ; and men between forty and fifty will 
still be found to seek the best artificial light, before their eyes 
have changed so much that it becomes inconvenient to hold the 
object at the required distance. The change, as has been said, 
is gradual and steadily progressive ; and it is therefore im- 
possible to fix upon any natural beginning of what is called 
‘‘aged sight,” or, technically, “presbyopia.” For the sake of 
convenience, it is necessary to have some definition of the 
meaning of the term ; and Professor Donders suggests that a 
person may be called presbyopic when, in consequence of 
gradual failure of the accommodation, the near point is farther 
than eight inches from the eye By that time, inconvenience 
will usually be felt in attempting to read, or to do fine work by 
artificial light ; and then the time for spectacles has arrived. 
We have seen already that the aim of the function of accommo- 
dation is to increase the power of the natural lens of the eye. 
When this can no longer be sufficiently increased, it may be 
added to. To place an artificial lens outside the eye adds to 
the power of the whole optical apparatus, and removes the 
inconveniences incidental to failure of accommodation. This is 
the ordinary purpose of spectacles when they are first required 
in advancing life, or from the forty-fifth to the fiftieth year. 
The natural eye, however, in which the nearly parallel rays 
from distant objects are united upon the retina without an 
effort, and in which the accommodation is only called into play 
for near objects, forms a standard that may be departed from 
