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.