Prof, S. P. Langley on "Good Seeing: 3 675 



than i£ none were used at all, the main difficulty having been 

 always found insurmountable. 



I have been led for some years to consider the conditions 

 under which this " boiling " presents itself. It is not neces- 

 sarily due to a high temperature of the external air, for the 

 most perfect definition I have ever seen of any terrestrial 

 object was obtained by me long since in the Harvard College 

 Observatory, at Cambridge, with its great equatorial telescope, 

 when, on the hottest day that I ever knew in a New England 

 summer, I directed it with a high power on the distant 

 " south mark," which I expected to find almost indistinguish- 

 able from the "boiling.'" I remember my extreme surprise 

 when, under a magnifying power of 300, I found the image 

 as still as the lines of an engraving. This was an extra- 

 ordinary exception to ordinary experience, and led me to take 

 an interest in the subject. I have since pursued an inquiry 

 to which this circumstance first directed my attention, and 

 I have done so at all altitudes, at one time residing on Aetna 

 for this purpose, noting that even on high mountains vision 

 was so far from being always clear that it was sometimes even 

 much worse than at sea-level. 



I have since come to the important conclusion that while 

 the ordinary " boiling " is due to all the air between us and 

 the sun or star through which the rays pass, the greater 

 portion of it is due to the air immediately near us, probably 

 within a few hundred yards, or even feet, from the telescope, 

 and this has led me to ask whether it was not possible that 

 some way to act upon this air could be found. Its non- 

 uniformity leads to deformations of the image too complex 

 to analyse here, which are caused not only by lateral vibra- 

 tions of the cone of rays, but by its elongation and contraction. 



For this purpose 1 have within the last few months been 

 making experiments at the Smithsonian Aslrophysical Ob- 

 servatory, first with a horizontal tube having three successive 

 walls with air-spaces between, which was intended to give 

 the maximum security which freedom from changes of 

 temperature could afford. This Observatory being princi- 

 pally concerned with rays best studied in an image formed 

 by reflexion, has no large dioptric telescope, on which ac- 

 count these experiments have been made with a reflector. I 

 have no reason to suppose, however, that they will not be 

 equally successful with a dioptric telescope. 



A large part of the " boiling 9i o£ the image is due to air 

 without the tube, but a not unimportant part to the air 

 within it and, in the preliminary experiments the air. kept 

 still in the tube by treating it with the ordinary precautions, 



