﻿S. P. Langley — Unrecognized Wave-lengths. 89 



fusing them, owing to their feebleness compared with that of 

 the first spectrum in which we are searching. 



We usually, however, employ a bolometer of not more than 

 l mm aperture, and this demands excessive delicacy in the heat- 

 measuring apparatus, since the heat here is, approximately 

 speaking, about 10 ^ 0o of that in the region between the sodium 

 lines in the direct spectrum of a rock-salt prism. This is near 

 the limit of our present measuring powers with the grating, 

 even when every possible device is used to increase the ex- 

 tremely feeble heat in this part of the spectrum. 



We commenced by using an electric arc with carbons 12 mm 

 in diameter in the position indicated. These were supplied by 

 an engine of three-horse power; but even in this case the pit of 

 the crater did not nearly cover the very short slit (its length is 

 8 mm ). For these last and most difficult measurements, we 

 have been obliged to procure the use of an engine of twelve 

 horse-power and carbons 25 mm (one inch) in diameter. With 

 this enormous current the hottest part is not easily maintained 

 in place. To keep it directly in front of the slit we have tried 

 various plans, such as boring out the carbons lengthwise, so as 

 to form hollow cylinders of them, and filling the core with a 

 very pure carbon tempered to the requisite solidity. Ordinarily 

 it will be sufficient however to first form the central crater by a 

 drill. This gives us a persistent crater, whose light, in the 

 position shown in the engraving, filled a slit whose vertical 

 height is 8 mm . It is probably the intensest artificial heat ever 

 subjected to analysis. 



Bolometer. 



The changes in the bolometer since it was first described 

 (Proceedings American Academy, 1881) are superficial rather 

 than radical, and refer chiefly to the form of the case, and 

 facilities for its accurate pointing. Tbe linear bolometer is 

 now made to expose to the radiant heat a vertical tape or wire 

 of platinum, iron or carbon. This is usually about 10 mm 

 long and only from T -oVo to ro"o mm thick, but according to its 

 special purpose it is made from l mm to 0'04 mm wide. In 

 the latter case it appears like the vertical strand of an ordinary 

 reticule in the focus of a positive eye-piece attached to the case 

 and is movable b} 7 a micrometer screw. It is in fact in appear- 

 ance a micrometer thread, controlled in the usual way, but 

 which is connected with the galvanometer and endowed with 

 the power of feeling the radiations, visible or invisible, from 

 any object to which it is directed. For very feeble sources of 

 heat, such as those with which we are here concerned, the strip 

 is made as much as a millimeter in width, and is not provided 

 with a micrometer screw, but moves with the arm carrying it, 



