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:

Subjecere oculis distantia sidera nostris,
Ætheraque ingenio supposuere suo.

In the first place, it is necessary that the beginner should rightly
know what is the nature of the instrument he is to use. And this is the
more necessary because, while it is perfectly easy to obtain such
knowledge without any profound acquaintance with the science of optics,
yet in many popular works on this subject the really important points
are omitted, and even in scientific works such points are too often left
to be gathered from a formula. When the observer has learnt what it is
that his instrument is actually to do for him, he will know how to
estimate its performance, and how to vary the application of its
powers--whether illuminating or magnifying--according to the nature of
the object to be observed.

Let us consider what it is that limits the range of _natural_ vision
applied to distant objects. What causes an object to become invisible as
its distance increases? Two things are necessary that an object should