The first thing after the family history, personal history, inspection
of the eye and the osteopathic examination, is to find out how well the
patient can see. To test the acuteness of vision certain test letters
are used. Snellen’s Test Letters are good. The normal eye can read 3-8
inch letters at twenty feet. The test letters on the cards usually
range in size to be read at 10, 15, 20, 30, 40, 50, 70, 100 and 200
feet. The most desirable distance is 20 feet. If at the distance of
twenty feet he reads the 3-8 inch letters his acuteness of vision would
be marked 20-20 or normal. Always use the distance between patient
and chart as the numerator of the fraction and the number above the
letters which he reads as the denominator. If he is twenty feet away,
the numerator remains twenty and the denominator changes according to
the line of letters seen on the test cards thus: 20-15, 20-30, 20-70,
or 20-200 may express the vision. If the patient could not see the 200
feet letters at 20 feet he must be brought nearer, say 10 feet, for him
to see the large letters; his vision would be 10-200. These fractions
representing the acuteness of vision may be expressed in meters. Some
charts have letters numbered that way.
If the vision is good enough for small objects to be clear, the near
point should be taken. This would show the amount of accommodation of
the eye. This is expressed in diopters.
A =diopter= is the unit of measurement of the =refractive= power of
lenses. =Lenses= are numbered by their refractive power in diopters. A
lens that has a curvature that will refract parallel rays of light and
bring them to a focus at one meter distance is said to be a one diopter
lens. This unit of measurement for the refractive power of lenses was
proposed by Nagel in 1866. It soon became quite generally used.
The focal distance of a lens decreases as the strength of a lens
increases. One diopter lens (written 1 D) has a focus of one meter (1
M) or 100 cm distance. A 2 D lens has a focal distance of ½ M or 50
cm. A 4 D lens has 25 cm focal distance and a ½ D lens has 100 cm ÷
½ = 200 cm distance or 2 M. Trial cases have in them lenses varying in
strength from .12 D or .25 D to 20 D of the spheric form. We will not
discuss the trial case here.