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Side-Lights on Astronomy and Kindred Fields of Popular Science by Simon Newcomb
page 89 of 331 (26%)
balance the tube.]

When mounted in this way, an object can be followed in its diurnal
motion from east to west by turning on the polar axis alone. But
if the greatest facility in use is required, this motion must be
performed by clock-work. A telescope with this appendage will
commonly cost one thousand dollars and upward, so that it is not
usually applied to very small ones.

We will now suppose that the reader wishes to purchase a telescope
or an object-glass for himself, and to be able to judge of its
performance. He must have the object-glass properly adjusted in
its tube, and must use the highest power; that is, the smallest
eye-piece, which he intends to use in the instrument. Of course he
understands that in looking directly at a star or a celestial
object it must appear sharp in outline and well defined. But
without long practice with good instruments, this will not give
him a very definite idea. If the person who selects the telescope
is quite unpractised, it is possible that he can make the best
test by ascertaining at what distance 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
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