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, when the eye-piece is pushed