DETERMINING ACUTENESS OF VISION. 29
an angle of five minutes. The thickness of the lines and smaller parts of the letters is exactly one-fifth their height, so that the dif- ferent parts of the letters are included in an angle of one minute, while the whole letter is included in an angle of five minutes. The letter C, for example, in comparison with the letter O, shows an interruption with a visual angle of one minute. Accordingly this series of letters is so arranged that the test letters, No. I, one foot distant, No. II, two feet distant, No. XX, twenty feet distant, and No. C, one hundred feet distant, are all included in the same visual angle of five minutes. If, for iustanee. No. XX is dis- tinctly seen at twenty feet, but not No. I at one foot, it shows normal acuteness of vision, but that probably the accommoda- tion is insufficient to adjust the eye for a distance of one foot. This probability becomes a certainty if by the use of the proper convex lenses No. I is also plainly seen at a distance of one foot.
Again, if No, I is seen distinctly at a distance of one foot, but not No. XX at twenty feet, there is normal acuteness of vision, but there is reason to suspect the existence of myopia. This suspicion is confirmed if with the proper concave lenses No. XX is seen distinctly at twenty feet.
If after the correction of any existing anomaly of refraction the patient cannot see No. XX at a distance of twenty feet, but only at a distance of perhaps ten feet, there exists a deficiency in the acuteness of vision. The degree of this deficiency may be expressed by a fraction whose numerator is the number of feet at which the letters are seen, and whose denominator is the number of feet at which they should be seen. In the example just given there is an acuteness of vision of 1|- = i.
The examination of the condition of vision is conducted as follows. The test letters are set up in a well-lighted position in a room twenty feet long or more. If now, for example, No. XX is read at a distance of twenty feet, the acuteness of vision is |-|, that is = 1. Persons who see No. XX at more than twenty, say at twenty-five feet, possess an acuteness of vision of ff , or one exceeding the normal average ; such cases are not uncommon among young individuals. If, on the contrary, No. XX is not seen at a distance of twenty feet, but the patient must ajiproach nearer in order to distinguish the different letters, there is a dimi-
