162 Prof. S. P. Langley on 



We now make a Second Series, and though the two series 

 follow each other at a brief interval on a day described as 

 " clear," the values of the deflections in the second series 

 on the same points indicate nearly twice the heat in the 

 first. The change is due to the altered diathermancy of 

 the apparently clear sky in the brief interval. It is one of 

 the difficulties already signalized by the writer and by others *, 

 and is to be eliminated only by repeated observation. The 

 second series, however, gives the same value as the first, and 

 hence we conclude (as far as this day's observation goes) 

 that this was the index of refraction, for the wave-length in 

 question and for a certain flint prism. Such observations 

 must generally be repeated on many days before a reliable 

 result is reached; and in the case of the glass prism, which 

 grows rapidly athermanous just beyond the limit of the solar 

 spectrum, they are limited by the nature of the substance to 

 little more than the wave-length in question. 



We now consider, as our second example, a measurement 

 at 5 x XD 2 , where glass can hardly be used, and where its 

 place is supplied by the rock-salt prism, and that of the sun 

 by the arc. 



Second example of Measurement. With Rock-salt Prism. 



(Extract from original record.) 



Station, Allegheny. 



Date, April 15th, 1886 (p.m.). 



Wet bulb at 4 h 30 m (inside dark room artificially heated), 



= 21° C. 

 Dry bulb at 4 h 30 m (inside dark room artificially heated), 



= 26°-4C. 

 Aperture of slit Si =4 millim. 



•>•> » S 2 =1'25 „ 



Prism used, Hastings No. 1 (rock-salt). 

 Lenses used, focus = 760 millim. for visible rays. 

 Grating used, Rowland No. 2. 



Galvanometer used, No. 3, damping magnet at 40 centim. 

 Time of single vibration = 17*5 sec. (with current passing). 

 Bolometer, No. 16 (aperture 1 millim.). 

 Setting on slit = 0° 00' 00". 



„ D 2 (spectrum thrown east) =41° 03' 00". 

 D 2 ( „ „ west) =39 57 40. 

 Current of battery = 0'037 ampere. 

 Reader at circle, J. P. 



„ galvanometer, F. W. V. 



* See Oroya, Comptes Bendus, tome ci. p. 418 (August 10, 1885). 



