On Astronomical Seeing

In the Annual Report of the Smithsonian Institution for 1902 Prof. Langley has published an important note on “Good Seeing,” in which he describes some experiments undertaken with the view of improving the definition of telescopic images, so far as it depends on the conditions of the air in the vicinity of the instrument. Up to now the belief has prevailed among astronomers that in order to obtain good definitions the air inside the telescope-tubes should be kept as much as possible not only at a uniform temperature but also in a state of perfect tranquillity. Langley, however, shows that this view is not quite correct, and that maintaining constant and uniform temperature inside the tube, while preventing circulation between the air inside and outside the instrument, is not sufficient to produce satisfactory telescopic images. Particularly, this method does not diminish the troublesome boiling which in solar observations proves so often to be a source of grave inconvenience to the observer. But he shows that if the air inside and near the telescope-tube is agitated by stirring, the definition becomes at once markedly better. The improvement has in all cases been so decided that the reality of this beneficial effect of stirring cannot well be doubted.