90 S. P. Langley—"Good Seeing." 



directed it with a high power on the distant "south mark, " 

 which I expected to find almost indistinguishable 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 extraordinary exception to 

 ordinary experience, and led me to take an interest in the sub- 

 ject. 1 have since pursued an inquiry to which this circum- 

 stance first directed my attention, and I have done so at all alti- 

 tudes, at one time residing on Aetna for this purpose, noting 

 that even on high mountains telescopic 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 analyze here, which 

 are caused not only by lateral vibrations of the cone of rays, 

 but by its elongation and contraction. 



For this purpose I have, within the last few months, been 

 making experiments at the Smithsonian Astrophysical Obser- 

 vatory; 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 principally concerned 

 with rays best studied in an image formed by reflection, has 

 no large dioptric telescope, on which account these experiments 

 have been made with a reflector. I have no reason to suppose, 

 however, that they will not be equally successful with a diop- 

 tric telescope. 



A large part of the "boiling" of 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, was found 

 to have little effect on the ordinary "boiling" of the image, 

 which so seriously prejudices the definition. An image-form- 

 ing mirror, fed by a coelostat, was placed at the end of this 

 triple-walled tube, which was itself sheltered by a canvas tent, 

 aud contained the stillest air of the most uniform temperature 

 which could be obtained. The "boiling" was but little 

 diminished merely by enclosing the beam by this tube, which 

 was only what had been anticipated from the ordinary experi- 

 ence of all astronomers. 



The device which I had determined to try was one of a 

 paradoxical character, for it proposed to substitute for this still 

 air which gave the usual troubled image, agitated air which it 



