﻿S. P. Langley — Unrecognized Wave-lengths. 97 



The sodium line of the fifth spectrum fell exactly upon the 

 slit S 2 at the beginning and end of the observations. 



From smooth curves the following positions were deduced 

 for the maximum : 



Spectrum east 39° 15'-7 



Spectrum east . _ - 39 15 '0 



Spectrum west 39 16 '0 



Spectrum west 39 16 *4 



Mean= 39° 15'"8 



The following measurements of wave-length with the. grating 

 have been carried on continuously from December, 1885, to 

 April, 1886. As a rule each of the sixty-two determinations 

 in the following table represents one or more day's labor, 

 though in some cases two or more have been secured in the 

 same day. 



They have been taken under the following conditions: 



(1.) In the case even of the very lowest wave-lengths in the 

 feeblest heat, we have been able to use a linear bolometer of 

 not more than l mm width. 



(2.) About an equal number of observations were intended 

 to have been taken with the prism placed so as to throw the 

 spectrum east and west. In doing this a minute systematic 

 error amounting at the greatest to less than V of arc was found 

 to be caused by flexure of the arm, due to the weight of the 

 bolometer cable, and a correction for this has been applied. 

 Otherwise the observations are given as originally made ; and 

 as the " probable error " here includes all the more or less 

 systematic differences, due to the use of different gratings, and 

 different positions of the apparatus, it may be considered to be 

 in this case, a fair indication of the amount of error to be actu- 

 ally expected. 



We do not know of any determination of the change pro- 

 duced in the refractive power of a rock-salt prism by varying 

 temperature. A rough comparison of the deviations of Fraun- 

 hofer lines, incidentally measured in the progress of the work 

 at different seasons, during which the temperature has varied 

 nearly 30° Cent., together with the results of a single day's 

 measures at temperatures differing by 17° Cent., have concurred 

 in indicating a diminution in the deviations throughout the 

 visible spectrum of about 11" for a rise of temperature of 1° 

 Cent. 



We do not doubt that a temperature correction is also re- 

 quired for the invisible spectrum ; but not having yet been able 

 to satisfactorily determine any, we think it best to leave all the 

 observations as they stand, uncorrected for temperature, and 



