and such
powers will exhibit Jupiter's belts, Saturn's rings, and the
continent-outlines on Mars; yet, though perhaps a higher power is
applied, they fail to detect these appearances, and can hardly believe
that they are perfectly distinct to the practised eye.

The expectations of the beginner are especially liable to
disappointment in one particular. He forms an estimate of the view he is
to obtain of a planet by multiplying the apparent diameter of the planet
by the magnifying power of his telescope, and comparing the result with
the apparent diameter of the sun or moon. Let us suppose, for instance,
that on the day of observation Jupiter's apparent diameter is 45", and
that the telescopic power applied is 40, then in the telescope Jupiter
should appear to have a diameter of 1800", or half a degree, which is
about the same as the moon's apparent diameter. But when the observer
looks through the telescope he obtains a view--interesting, indeed, and
instructive--but very different from what the above calculation would
lead him to expect. He sees a disc apparently much smaller than the
moon's, and not nearly so well-defined in outline; in a line with the
disc's centre there appear three or four minute dots of light, the
satellites of the planet; and, perhaps, if the weather is favourable and
the observer watchful, he will be able to detect faint traces of belts
across the planet's disc.

Yet in such a case the telescope is not in fault. The planet really
appears of the estimated size. In fact, it is often possible to prove
this in a very simple manner. If the observer wait until the planet and
the moon are pretty near together, he will find that it is possible to
view the planet with one eye through the telescope and the moon with the
unaided eye, in such a manner that the two discs may coincide, and thus
their relative apparent dimensions be at once recognised. Nor should the
indistinctness and incompleteness of the view be attributed to
imperfection of the telescope; they are partly due to the nature of the
observation and the low power employed, and partly to the inexperience
of the beginner.

It is to such a beginner that the following pages are specially
addressed, with the hope of affording him aid and encouragement in the
use of one of the most enchanting of scientific instruments,--an
instrument that has created for astronomers a new sense, so to speak, by
which, in the words of the ancient poet:

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