Of the atmosphere

In descending to the lower strata of the atmosphere, the reader should be informed that we here restrict the term to the portion of ambient air next the Earth, which receives vapours and exhalations, and increasingly refracts the rays of light. Ptolemy had already remarked that the light of the stars underwent a change of direction in the course of passing through this terrestrial envelope ; and every observer is aware of its darkening effect, especially under high magnifying power. It therefore follows, that the pervading consequence of angular altitude is a paramount condition to be considered before the amateur expends much time upon objects near the horizon, wherever he may happen to be located. On this account the diatribe was uttered against low stars (see ante, page 46) ; though perhaps an occasional dip among the remote regions of the South may hereafter yield comparisons in aid of further investigations of the phenomena—visible and invisible—of vapours.

A conviction of this will very soon come home to the intelligent observer, who will shape a course accordingly, trimming agreeably to his means and intentions ; and his judgment must guide him in selecting objects for a systematic attack. Not even will a tropical climate free the spectator from these tremulous deceptions. On the contrary, the absence of aqueous vapour may be perceived by anybody to diminish the apparent steadiness of objects, in proportion to the amount of heat and distance from the eye.