458 Proceedings of Royal Society of Edinburgh. [sess. 
On Astronomical Seeing. By Dr J. Halm, 
Lecturer in Astronomy in the University of Edinburgh. 
(Read May 6, 1904. MS. received October 14, 1904.) 
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 observa- 
tions 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. 
This result has led me to investigate the question as to whether 
a similar conclusion may perhaps be drawn with regard to the 
great mass of atmosphere which is traversed by the luminous rays 
of the celestial object before they reach our telescopes. Is there 
any reason for assuming that stirring this mass of air would 
improve the definition, sharpness, and steadiness of the star 
images 1 The question, I think, has not been asked before ; and 
