'.GOOD sep:ing."« 



Bv S. p. LAN(!r.KY 



Evciyone who has used a teh^scopc knows that our atmosphere is 

 forever in pulsating" motion, and troiiltlini;' oiii- \ ision of tiie hea\enlv 

 ])odies, during- the most ck)udh'ss day or niglit, so that observatories 

 are ])ut even on higli mountains, to get rid of th<> (listurl)ances in this 

 atmosphere, whieli tend to make the image of eveiy object trenudous 

 and indelinite, and to prevent what the astronomer terms "'good >^eeing/' 



I desire to speak to the academy about a device which I hav(^ recently 

 essayed for preventing this universally known and dreaded •"boiling"'' 

 of the telescopic image, a dilticulty which has existed always and every- 

 where since telescopes have been in use, and which has seemed so insur- 

 mountable that I l)elieve it has hardly ever been thought of as subject 

 to correction. 



Hitherto it has l»eeii the endeaAor of astronomers, so far as I know, 

 to secure a more tranfjui! image l)v keeping the aii in the telescope 

 tube, through which the rays pass, as (|uiet as possible, and for this 

 purpose the walls of tiic tube have been made nonconducting, and 

 extreme j)ains hax'e l>een t;d<en not to set up currents in the lube. 

 With these precautions the '•seeing" is perha})s a little better (but 

 very little) than if noiu:- were used at all. the main dirticulty having been 

 always found insurmounta])le. 



I ha\'(^ l)een led for some years to considin' the conditions under which 

 this •• boiling'''' presents itself. It is not necessarily due to a high tem- 

 perature of the (External air. for the most perfect definition I ha\'e ever 

 seen of any terrestrial object was obtained l)y me long since in the 

 Harvard College ()i»ser\atory at Cambi-idge. with its great e((uatorial 

 telescope, when, on the hottest day that I e\'er knew in a New Kngland 

 sununer. I directed it with a high power on the distant '"south mark,"" 

 which 1 exp(H'ted to find almost indistinguishable from the '" boiling."'' 

 I icmeniber my extreme surprise when, under a magnifying powei' of 

 o'H), 1 found the im;ige as still as the lines of an (Uigi'aving. This was 

 an extraordinary exc(>ption to ordinaiy experience, and led me to take 



"A paper presented to the National Academy of Sciences, Novenilicr li', 1902. 

 SM 1002 18 193 



