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



surfaces on which the Rowland gratings are ruled. On this 

 portion of the apparatus alone very great labor has been 

 expended. 



We have procured through Mr. Brashear's skill, by means 

 previously described, a rock-salt prism having a field filled 

 with Fraunhofer lines and showing distinctly the nickel line be- 

 tween the D's. This is true when first polished, as it comes from 

 the maker's hands ; but owing to the deliquescent nature of the 

 material, with the utmost care, the surfaces rapidly deteriorate. 

 As it is necessary for the precision of these researches to deter- 

 mine the refracting angle of the prism, and also the indices of 

 refraction of some of the principal lines in its visible and invis- 

 ible spectrum with a high degree of accuracy, and as all these 

 labors have to be repeated when the prism is re-polished, some 

 idea of the work in this portion alone may be understood, 

 when it is stated that the prism has been sent to the maker 

 and entirely refigured, and its principal constants redetermined 

 by us, no less than thirteen times in the past fourteen months, 

 or since the first of January, 1885.* 



We now give two examples of actual measurement of wave- 

 lengths: the first, that in sunlight in a flint prism, which we 

 have designated as No. 2; the second, in the rock-salt prism 

 just mentioned. 



First example of measurement. With flint glass prism. 

 (Extract from original record.) 

 Station^ Allegheny. 

 Date, March 3d, 1886. 

 Temperature of apparatus z=. — 1°*8 Cent. 

 State of sky, clear overhead, cirrus clouds near the horizon. 

 Aperture of slit (S,) = 2 mm . 

 Aperture of slit (S 2 ) = l mm . 



Lenses of glass (non-achromatic) focal length = 800 ram for vis- 

 ible rays. 



Grating, Rowland No. 1 (concave). 



Prism, H. No. 2 (glass). Refracting angle = 62° 15' 03*. 



Spectrum thrown east. 

 Galvanometer No. 3, damping magnet at 40 cm . 



Time of single vibration = 21 sec. (with current passing). 

 Bolometer No. 16 (aperture — l mm ). 

 Reading on slit (before mounting prism), 0° 00' 00". 

 Beading on Z> 2 (through prism), 52° 52' 40". 

 Current of battery = 0036 Amperes.f 



* For a full description of the constants of this prism, see this Journal, Dec, 

 1885. 



f It must be understood that this is the total current of the battery. The dif- 

 ferential current which passes through the galvanometer is of an altogether 

 different order of magnitude, in this case probably not exceeding 0-00000001 

 Ampere. 



