he can read
ordinary print. 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. He points the instrument at a star, or at the reflection of the
sun's rays from a small round piece of glass or a globule of
quicksilver several hundred yards away, and ascertains whether the rays
are all brought to a focus. This is not done by simply looking at the
star, but by alternately pushing the eye-piece in beyond the point of
distinct vision and drawing it out past the point. In this way the
image of the star will appear, not as a point, but as a round disk of
light. If the telescope is perfect, this disk will appear round and of
uniform brightness in either position of the eye-piece. But if there is
any spherical aberration or differences of density in different parts
of the glass, the image will appear distorted in various ways. If the
spherical aberration is not correct, the outer rim of the disk will be
brighter than the centre when the eye-piece is pushed in, and the
centre will be the brighter when it is drawn out. If the curves of the
glass are not even all around, the image will appear oval in one or the
other position. If there are large veins of unequal density, wings or
notches will be seen on the image. If the atmosphere is steady, the
image, wh