This test measures how well you see at v arious distances.
It is the familiar eye chart test.
The eye chart itself -- the usual one is called Snellen's chart -- is imprinted
with block letters that line-by-line decrease in size, corresponding to the distance
at which that line of letters is normally visible.
The letters on Snellen's chart are, not surprisingly, called Snellen's test type.
Each block letter is quite scientific in design (so that at the appropriate distance
the letter subtends a visual angle of 5 degrees and each component part subtends
an angle of 1 minute).
The chart and the letters are named for a 19th-century Dutch ophthalmologist
Hermann Snellen (1834-1908) who came up with them as a test of visual acuity. Visual
acuity refers to the clarity or clearness of the vision, a measure of how well a
person sees. The word "acuity" comes from the Latin "acuitas" = sharpness.
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