By Dr. F. OGDEN STOUT.
I know it is the custom with a great many if not the majority of
opticians to fit a customer without knowing whether he has presbyopia,
hypermetropia, or any of the other errors of refraction. Their method
is first to try a convex, and if this does not improve, a concave,
etc., until the proper one is found. This, of course, amounts to the
same thing if the right glass is found. But in practice it will be
found both time saving and more satisfactory to first decide with what
error you have to deal. It is very simple, and, where you have no
other means of diagnosing (such as the ophthalmoscope), it does away
with the necessity of trying so many lenses before the proper one is
found. You should have a distance test card placed at a distance of
twenty feet from the person you are examining, and in a good light.
A distance test card consists of letters of various sizes which it has
been found can be seen at certain distances by people with good
vision. Thus the largest letter is marked with a cc, meaning that this
should be seen at two hundred feet, and another line, XX, at twenty
feet, which is the proper distance for testing vision for distance,
for the reason that a normal eye is at rest when looking at any object
twenty feet from it or beyond, and the rays coming from it are
parallel and come to a focus on the retina. You must also have a near
vision test card with lines that should be seen by a normal eye from
ten to seventy-two inches, and a card of radiating lines for
astigmatism. With this preparation you are ready to proceed. To
illustrate, the first customer comes and tells you that up to six
months ago he had very good vision, but he finds now that, especially
at night, he has trouble in reading or writing, and that he finds he
can see better a little farther away. His head aches and eyes smart.
You will of course say that this is a very simple case. It must be old
sight (presbyopia). Probably it is if he is old enough (45), but you
must prove this for yourself, without asking his age, which is
embarrassing in the case of a lady. If you direct him to the distance
card twenty feet away, and find that he can see every one down to and
including the one marked XX, his vision is up to the standard for
distance, and you know that he can have no astigmatism worth
correcting, nor any near sight, as both of these affect vision for
distance, but he may have far sight or old sight or both combined. You
must find which it is.
If, while he is still looking at the twenty-foot line, you place in
front of the eyes a weak convex and he tells you he sees just as well
with as without, it proves the existence of far-sight or
hypermetropia, and the strongest convex that still leaves vision as
good for distance as without any, corrects the manifest. But if the
weak convex blurs it, it shows that there is some defect in focusing,
if the near vision is below normal. You therefore know that you have a
case of old sight or presbyopia, requiring the weakest convex to
correct it, that will enable your customer to see the finest line on
the near card at the required distance.
The next customer that comes to be fitted with glasses can only see
the line marked XL on the distance card at 20 feet or about one-half
of what he should see, which leads you to think that there is no far
sight, for vision for distance is good except in very high degrees of
this error. Nor can there be old-sight, for vision for distance is
good in old-sight until after the fifty-fifth year, but it can be near
sight (myopia) or astigmatism, or both. We next try the near card and
find that even the finest line can be seen clearly if held
sufficiently close to the eyes. We now know that this is a case of
near sight, and we must fit them with glasses for distance. The
weakest concave that will enable him to see the line that should be
seen on the distance card at 20 feet is the proper one to give him for
use.--_The Optician._
* * * * *