S. P. Langley — "Good Seeing" 91 



was hoped would give a still image.* For the purpose of this 

 new experiment, the horizontal telescope using a reflector of 

 40 feet focus, fed by a coelostat through the above tube, was 

 connected with a fan run by an electric motor, which was 

 arranged to draw out the air from the inner tube, at the same 

 time that it forced air in at different points in its length, so as 

 to thus violently disturb and churn the air along all the path 

 of the beam from the coelostat to the solar image. 



This first experiment gratifyingly reduced the "boiling" 

 and produced an incontestibly stiller image than when still air 

 was used. As a further test, a series of artificial double stars 

 was now provided, and the concave mirror, acting both as 

 collimator and objective, brought the images to focus, where 

 they were examined with an eyepiece. With the stillest air 

 obtainable, the images were not sharp and only the coarsest 

 doubles were resolvable. Then the blower was started and the 

 definition immediately became sharp. Violently stirring the 

 air in the tube, then, eliminates all or nearly all the " boiling" 

 of the stellar image which arises within the tube itself when 

 using ordinary still air. This experiment concerned the air 

 within the horizontal tube only. 



I have next taken up the solar image formed by the mirror 

 in the above tube. This is clearly improved by the stirring. 

 I have also wished to try a tube something like a prolonged 

 dew cap, but which is arranged to be inclined toward the sun. 

 The air in both can be stirred together, but experiment has 

 not yet gone far enough to demonstrate whether it has, as is 

 hoped, any superiority commensurate with the special mechan- 

 ical difficulties involved. 



I am not prepared to give quantitative estimates, which I hope 

 to furnish later ; but all observers to whom I have shown these 

 early results on the sun have agreed, that if the " boiling" was 

 not wholly cured, what remained was but a small fraction of that 

 obtained with still air. I have not completed these experi- 

 ments, which I am still pursuing at the observatory, but they 

 seem to me to give promise of an improvement of universal 

 interest to observers, which justified the making of this early 

 announcement. I had hoped to have shown the Academy some 

 photographs of the sun taken, first, in the ordinary way, and 

 again, with the churned air, but the condition both of the sun 

 and of the sky, of late, has prevented my obtaining them. I 

 can, to my regret, only give here a photograph of the images 

 of the artificial double stars as seen through ordinary condi- 

 tions, as distinguished from those here mentioned, of artificial 

 " good seeing." 



Astrophysical Observatory, Washington, November 12, 1902. 



* I may mention here that my lamented friend, Henry C. Draper, once 

 showed me that agitating the contents of a bisulphide of carbon prism 

 improved its definition. 



