To do this he should have an eye-piece magnifying about fifty
times for each inch of aperture of the telescope. For instance, if
his telescope is three inches clear aperture, then his eye-piece
should magnify one hundred and fifty times; if the aperture is
four inches, one magnifying two hundred times may be used. This
magnifying power is, as a general rule, about the highest that can
be advantageously used with any telescope. Supposing this
magnifying power to be used, this page should be legible at a
distance of four feet for every unit of magnifying power of the
telescope. For example, with a power of 100, it should be legible
at a distance of 400 feet; with a power of 200, at 800 feet, and
so on. To put the condition into another shape: if the telescope
will read the print at a distance of 150 feet for each inch of
aperture with the best magnifying power, its performance is at
least not very bad. If the magnifying power is less than would be
given by this rule, the telescope should perform a little better;
for instance, a three-inch telescope with a power of 60 should
make this page legible at a distance of 300 feet, or four feet for
each unit of power.
The test applied by the optician is much more exact, and also more
easy.
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