Page 312 - My FlipBook

176 PATHOLOGY OF THE HABD TISSCES OF THE TEETH.
power of the artificial lens is required to adjust the balance of
muscular effort.
Some persons, on a false hypothesis, fight against the use
of lenses as aids to near vision, believing that the habit contrib-
utes to early changes in the eye. This is incorrect. The eyes
are relieved and made better. The correct statement is that the
person finding relief will continually find the desire for it. He
should have it.
For the near vision used by the dentist both sets of muscles
may be relieved artificially to advantage. The movement of the
eyeballs in converging from distant vision to near vision differs
among persons, but may, as a general statement, be placed at
ten degrees. If prisms are placed before the eyes that will do
this instead of requiring the muscular apparatus to do it, and if
combined with this there is a lens which gives the near focus of
the eye without effort, the eyes may see at ten inches at ease and
obtain a much larger retinal image than can be had with the
unaided eye. As a statement of fact, this is literally true, but in
its realization there are certain serious difficulties that must be
understood and avoided. "When the eyes are adjusted to distant
vision, the person can not by any effort adjust his eyes so that
he will have divergent vision or crossed vision. All of these
movements are the coordinates of the one effort — to see — and
can not be separated by the will. One may cross his vision by
an effort to fix his eyes on an imaginary near point, but he will
do it much easier by placing his finger before his eyes. But he
has no power of will to fix his eyes in divergent vision. There-
fore, if, by aid of the prism and lens, the eye is placed at ease
for near vision, there is no power of will to change this into more
distant vision. The only way in which the person may see at
a greater distance is to remove these aids, or look over them.
Therefore, if these aids are combined, the adjustment must lie
only partial, relieving in part but giving some opportunity for
more distant vision. In this way the dentist may have relief for
both sets of muscles for his purposes at the operating chair. The
use of the prism before the normal eye must be confined to use
in seeing small objects. The prism distorts all portions of objects
outside of a small area of central vision and is therefore totally
unfit for reading or other work in which any considerable form
is prominent before the eye. While letters in the center of vision
may be very clear, the book page will be very annoyingly dis-
torted. In the mouth, where there are no straight lines, these
distortions soon pass out of notice. These combined aids must