THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. YI.— " Good Seeing";* by S. P. Langley. (With Plate I.) 



Every one who has used a telescope knows that our 

 atmosphere is forever in pulsating motion, and troubling our 

 vision of the heavenly bodies, during the most cloudless day or 

 night, so that observatories are put even on high mountains, 

 to get rid of the disturbances in this atmosphere, which tend 

 to make the image of every object tremulous and indefinite, 

 and to prevent what the astronomer terms "good seeing. " 



I desire to speak to the Academy about a device which I have 

 recently essayed for preventing this universally known and 

 dreaded "boiling" of the telescopic image, a difficulty which 

 has existed always and every where since telescopes have been in 

 use, and which has seemed so insurmountable that I believe it 

 has hardly ever been thought of as subject to correction. 



Hitherto it has been the endeavor of astronomers, so far as I 

 know, to secure a more tranquil image by keeping the air in 

 the telescope tube, through which the rays pass, as quiet as 

 possible, and for this purpose the walls of the tube have been 

 made non-conducting, and extreme pains have been taken not 

 to set up currents in the tube. With these precautions the 

 "seeing" is perhaps a little better (but very little) than if 

 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 necessa- 

 rily 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 Observa- 

 tory at Cambridge, with its great equatorial telescope, when, on 

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

 * A paper presented to the National Academy of Sciences, November 12, 1902. 



A.M. Jour. Sci. — Fourth Series, Yol. XV, No. 86.— February, 1903. 



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