ually
both to permanent errors of curvature and to temporary distortions
produced by strains and by inequality of temperature. The perfect
achromatism of a reflector is, of course, a great advantage, but the
chromatic aberration of refractors is now so well corrected that their
inferiority in that respect may be disregarded. It must be admitted that
reflectors are cheaper and easier to make, but, on the other hand, they
require more care, and their mirrors frequently need resilvering, while
an object glass with reasonable care never gets seriously out of order,
and will last for many a lifetime.

Enough has now, perhaps, been said about the respective properties of
object glasses and mirrors, but a word should be added concerning
eyepieces. Without a good eyepiece the best telescope will not perform
well. The simplest of all eyepieces is a single double-convex lens. With
such a lens the magnifying power of the telescope is measured by the
ratio of the focal length of the objective to that of the eye lens.
Suppose the first is sixty inches and the latter half an inch; then the
magnifying power will be a hundred and twenty diameters--i. e., the disk
of a planet, for instance, will be enlarged a hundred and twenty times
along each diameter, and its area will be enlarged the square of a
hundred and twenty, or fourteen thousand four hundred times. But in
reckoning magnifying power, diameter, not area, is always considered.
For practical use an eyepiece composed of an ordinary single lens is
seldom advantageous, because good definition can only be obtained in the
center of the field. Lenses made according to special formulae, however,
and called solid eyepieces, give excellent results, and for high powers
are often to be preferred to any other. The eyepieces usually furnished
with telescopes are, in their essential principles, compound
microscopes, and they are of two descriptions, "positive" and
"negative." The former generally goes under the name of its inventor,
Ramsden, and the latter is named after great Dutch astronomer, Huygens.
The Huygens eyepiece consists of two plano-convex lenses whose focal
lengths are in the ratio of three to one. The smaller lens is placed
next to the eye. Both lenses have their convex surfaces toward the
object glass, and their distance apart is equal to half the sum of their
focal lengths. In this kind of eyepiece the image is formed between the
two lenses, and if the work is properly done su