ON THE USE AND CHOICE OF SPECTACLES. 
139 
not suspected, no remedy for it was known. Sufferers were 
advised to change their mode of life, to give up reading and 
writing, and handicrafts requiring accurate vision, and to turn 
sheep farmers in the colonies. It was reserved for Professor 
Bonders, of Utrecht, to find out the cause of the malady ; and 
to know the cause was to know the cure. The necessity to 
strain the accommodation is removed by convex spectacles, 
and their use restores the sufferer to good vision and perfect 
comfort. They should be the strongest that can be borne for 
distant objects ; and, when first used, will usually require to be 
strengthened after a time, perhaps more than once, if any of 
the old symptoms return. They must be worn always ; and 
only laid aside at bed-time. 
Before leaving hypermetropia, it is as well to mention that 
it is the ordinary cause of squint ; although space does not 
allow any explanation of its influence. 
The existence of the three conditions mentioned may be 
known by the following tests. In presbyopia, we' have an in- 
creasing necessity to remove a book farther and farther from 
the eyes, and convex spectacles allow it to be brought back to a 
convenient distance. In myopia, or short sight, distant vision 
is improved by weak concave glasses. In hypermetropia, it is 
improved, or at all events not made worse, by weak convex 
glasses. 
Besides the above, which may be called simple defects, there 
are others of a more complicated character. In astigmatism ” 
the surface of the eye is differently curved in different direc- 
tions, and horizontal lines are more plainly seen than vertical 
ones, or vice versa. Sometimes the surfaces are altogether 
irregular, sometimes the two eyes are dissimilar. Dr. Scheffler 
of Brunswick, whose treatise on ocular defects I have lately 
translated, states that there are 729 'possible defects of each 
eye singly ; and hence more than half a million of possible 
combinations in the pair. These step beyond the boundaries 
of popular science. 
In the same work Dr. Scheffler has fully described a new 
form of spectacles of his invention, to which he has given the 
name of “ orthoscopic spectacles.” They are made by cutting 
the glasses out of the margin of a larger glass, and are adapted 
to relieve the feeling of fatigue, or ‘‘ strain,” which spectacles 
often produce. For presbyopia of moderate degrees, not re- 
quiring glasses of higher power than eighteen inches focal 
length, I think they are likely to be extremely valuable. 
It still remains to say a few words about the materials of 
which spectacle lenses are made, and about the frames in which 
they are set. 
The lenses should be of the best crown glass, free from all 
