unrecognized Wave-lengths. 155 



This is near the limit of our present measuring-powers with 

 the grating, even when every possible device is used to increase 

 the extremely feeble heat in this part of the spectrum. 



We commenced by using an electric arc with carbons 12 

 milliin. 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 millim.). 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 millim. 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 millim. 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. The 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 millim. long and only from y^oo t° j^q millim. thick; but 

 according to its special purpose it is made from 1 millim. 

 to 0'04 millim. wide. In the latter case it appears like the 

 vertical strand of an ordinary reticule in the focus of a posi- 

 tive eyepiece attached to the case, and is movable by a 

 micrometer-screw. It is in fact in appearance 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 micro- 

 meter-screw, but moves with the arm carrying it, and its 

 positions are read by the divided circle of the spectrometer 

 to 10" (ten seconds of arc). It is this simple form of the 

 instrument which has been used in the present investigation, 

 andwhichjis shown in Plate IV. fig. 2. The bolometer is shown 

 in position in the middle of the case, where its central strip 



