ON THE USE AND CHOICE 'OF SPECTACLES. 
137 
any rapidly recurring need to change the spectacles does not 
speak only of presbyopia, but gives warning of the approach of 
disease, and of the need to consult a surgeon. As an approxi- 
mative standard, Professor Bonders gives the following table of 
the glasses that would be required on account of presbyopia, by 
eyes originally natural, at different periods of life. If this 
scale were much departed from, it would be fair to presume 
that some defect besides presbyopia was present : — 
Age 
Focal Length ! 
of the Spectacles, j 
in Inches 
Beading Distance 
with the fore- 
going, in Inches 
Age 
Focal Length of 
the Spectacles, 
in Inches 
Beading Distance 
with the fore- 
going, in Inches 
48 
60 
14 
60 
16 
13 
50 
40 
14 : 
62 
12 
13 
55 
28 
14 ' 
65 
10 
12 
58 
20 
13 
70 
n 
10 
In myopia, or short-sight, we have to deal with much more 
complicated conditions ; and it may be laid down as a general 
rule that all myopic eyes are more or less diseased eyes, and 
are liable, by reason of their short-sightedness, to various 
changes that are slowly destructive to vision. Short-sighted 
eyes are, as has been said, oval in shape, or too long from front 
to back; and the necessity to keep them fixed on a near 
object tends continually to increase the malformation, and 
hence to increase the shortness of sight. The eyeballs are 
practically bags filled with fluid, and when they are strongly 
drawn inwards, by muscles that are inserted near the front 
of the inner surface of each eyeball, there is a necessary 
tendency to a corresponding degree of bulging at the back; 
and this bulging, if often repeated, has a tendency to become 
permanent, and so to increase the original defect of shape. 
Moreover, the only portion of the eyeball that can be thus 
stretched without grave injury is the outer or protective coat ; 
and the delicate membranes within this coat, especially the 
retina itself, are apt to be seriously or even irreparably damaged. 
There is a prevailing belief that short-sighted eyes are essen- 
tially good and sound eyes ; and than this there can be no 
more mischievous error. The fact is, that short-sighted people 
can often discern very small objects readily, simply because 
they bring them so near the eye as to get plenty of light from 
them. They also do not require reading spectacles on account 
of presbyopia, either not at all, or not so early as other people ; 
and these two peculiarities have given rise to the popular 
notion. It cannot be too universally known that short sight 
tends to increase ; and that, if it increase at all rapidly, it tends 
also to destructive changes. 
