194 GOOD SEEING. 



an interefst in the subject. I have since pursued an iiK^uiry to which 

 this circumstance first directed my attention, and I have done so at all 

 altitudes, at one time residing on ^Etna for this purpose, noting- that 

 even on high mountains telescopic vision was so far from V)eing always 

 clear that it was sometimes even nuich worse than at sea level. 



I have since come to the important conclusion that while the ordinary 

 " )»oiling"' 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 imme- 

 diately 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 pos- 

 sible that some way to act upon this air could l)e found. Its nonuni- 

 formity leads to deformations of the image too com];)lex to analyze here, 

 which are caused not onh' by lateral vibrations of the cone of rays l)ut 

 by its elongation and contraction. 



For this purpose I have, within the last few months, l)een making 

 experiments at the Smithsonian Astrophysical Observatory; tirst with 

 a horizontal tube having three successive walls Avith 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 reflection, 

 has no large dioptric telescope, on which account these experiments 

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

 ever, that they will not be equally successful with a dioptric 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 pre- 

 liminar}^ experiments the air, kept still in the tube by treating it with 

 the ordinary precautions, was found to have little effect on the ordinary 

 ''l)oiling" of the image, which so serioush' prejudices the detinition. 

 An image-forming mirror, fed by a coelostat, was placed at the end of 

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

 contained the stillest air of the most uniform temperature which could 

 be obtained. The " boiling" was but little diminished merely by inclos- 

 ing the beam by this tube, which was only what had been anticipated 

 from the ordinary experience of all astronomers. 



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

 character, for it proposed to substitute foi- this still air. which gave 

 the usual troubled image, agitated air which it Avas hoped would give 

 a still image." B^'or the purpose of this new expei'inuMit, the horizontal 

 telescope using a reflector of 40 feet focus, fed by a coelostat through 

 the above tube, was connected with a fan iiiii by an electric motor, 

 which was arranged to draw out the air from the iinuM' tube at the 



same time that it forced air in at diffcicnt ])()iiits in its length, so as to 



^ 



"I may mention here that my laiiicnti'd I'riiMul, lleurv (.'. Draper, once showed me 

 that agitating the oontentH of a hisulphide of carbon prism improved its detinition. 



