676 Prof. S. P. Langley on li Good Seeing" 



was found to have little effect on the ordinary " boiling " of 

 the image, which so seriously prejudices the definition. An 

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

 end of this triple-walled tube, which was itself sheltered hy 

 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 inclosing 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 for this 

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

 w T hich it 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 by 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, Vio- 

 lently stirring the air in the tube, then, eliminates all, or 

 nearly all, the u 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, 

 but, as a supplementary improvement, I have wished to try 

 a tube something like a prolonged dew-cap, but which is 

 arranged to be inclined toward the sun, so that the beam 

 may pass down it before being reflected by a mirror into the 

 horizontal tube to form the image. This inclined tube is to 

 be connected with the blower like the horizontal one, and 

 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 me- 

 chanical difficulties involved in the research. Again, there 



