Concerning the second part of this paper, namely, the causes which often affect mirrors so as to prevent their showing objects distinctly, though it be well known to astronomers that telescopes will act very differently at different times, yet no particular inquiry had yet been made respecting the cause of this imperfection. The experience our author has acquired during his long series of observations. in which he never lost sight of this circumstance, has enabled him to combine a set of facts, from which he thinks himself authorized to deduce in- ferences which will be found to throw a considerable light upon the subject.
These observations are here described at length, and arranged under different heads, chiefly according to the state of the atmosphere at the time they were made. Their results will in some measure point out the nature of them. They seem to establish, as a general principle, that in order to see distinctly with "telescopes, it is re- quired that the temperature of the atmosphere and mirror should be uniform, and that the air be impregnated with moisture." Hence it appears that a frost after mild weather, or a thaw after frost, will sensibly derange the performance of our mirrors, till either the frost or the mild weather are sufficiently settled that the temperature of the mirror, and indeed of the whole telescope, may accommodate itself to that of the air. That when a frost, though very severe, be- comes settled, the mirror will soon accommodate itself to the tem- perature, and the telescope will be found to act well. That no tele- scope brought into a cold atmosphere out of a warm room, can for a time be expected to act properly; and that no delicate observations, with high magnifying powers, can well be made when looking through a door, window, or slit in the roof of an observatory. It equally ap- pears that windy weather in general, which must occasion a mixture of airs of different temperatures, cannot be favourable to distinct vision: and that the aurora: boreales, when they induce, as they often do, a considerable change in the temperature of the different regions of air, are likewise detrimental as to distinctness. ,
Sometimes the weather may be perfectly serene, and yet the tele- scopes will act imperfectly. This may be owing to the dryness oc- casioned by easterly Winds, or by a change of temperature arising from an agitation of the upper regions of the atmosphere, or perhaps by both these causes combined together.
Dry air, it seems, is by no means proper for vision; and hence dampness, haziness, and fogs, to a certain degree, will generally be found favourable to distinctness: damp situations, therefore, and the neighbourhood of lakes or rivers, need not be objected to in choosing a spot for an observatory. As the warm exhalations of the roof of a house in a cold night must disturb the uniformity of the temperature of a certain contiguous portion of air, it is to be expected that the appearance of stars seen over a house, and at no considerable distance from it, will be affected by that emanation.
Lastly, one of the most essential causes of the want of uniformity in the performance of telescopes must, it seems. be ascribed to the
