Rule a short line upon a slate, and upon another slate rule another
line, one-eleventh longer than the first: a person possessing what is
called “a true eye,” may perceive the difference in length, even though
fifty or sixty seconds elapse between looking at the first and the
second lines. If they differ only one-twentieth, then an interval of
thirty-five seconds may elapse without destroying the judgment; but,
if it be longer, the estimate will be incorrect. When the difference
between the lines amounts only to one-fiftieth, an interval of three
seconds between the examination of each, is the longest that can be
allowed without interfering with the correctness of the comparison.