674 Prof. S. P. Langley on "Good Seeing" 



of the scale-length, with ease and certainty, although the 

 scale is very short. Under these circumstances, to claim one 

 per cent, accuracy in interpolating is well within the mark. 

 I should like to take this opportunity of emphasizing the 

 remark made some years ago by Professor Threlfall, that it 

 is better to attain sensitiveness in a galvanometer by having 

 a big mirror and first-rate optical conditions, than to push the 

 electromagnetic sensitiveness to an extreme. 



The apparatus described in this and the foregoing paper 

 is intended for temperature measurements in an attempt at 

 determining the Joule-Thomson effect, and was purchased 

 out of a grant made for that purpose by the Royal Societv, 

 and I am glad to express to them my thanks for the liberality 

 which has enabled me to undertake the work. 



LXXII. " Good Seeing:' By S. P. Langley* 



[Plate XVII.] 



Astropliysical Observatory, Washington, 

 November 12, 1902. 



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 Association about a device which I 

 have recently essayed, for diminishing this universally known 

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

 which has existed always and everywhere 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 endeavour 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 precau- 

 tions the " seeing '' is, perhaps, a little better, but very little, 



* Communicated by the Author. — A paper read before the Washington 

 Meeting of the American Association for the Advancement of Science, 

 December 30, 1902. 



