﻿S. P. Langley — Unrecognized Wave-lengths. 85 



The following is the same in principle, with certain changes to 

 adapt it either to the solar or electric heat. Let S x be the first 

 slit, (see Plate III). For solar heat it has doubly moving jaws, 

 controlled by a micrometer screw, while in the case of the 

 electric arc we use a special form of slit surrounded by water, 

 to be directly described. Gr is the large concave grating. The 

 massive beam A, already described, carries the large rock-salt 

 train S 3 , L 1? P, L 2 , B. Gr is fixed at the extremity of the beam, so 

 that its colli mating axis coincides with that of L x , and by means 

 of an automatic apparatus, not shown here, the slits Sj S 2 are 

 caused to lie always in the same straight line at right angles 

 to G Sj. Under these circumstances, it has been demonstrated 

 by Professor Rowland that the wave-length of light, passing 

 through the slit S x to fall upon the grating and there be dif- 

 fracted to S 2 , is directly proportional to the distance Sj S 2 . 

 Accordingly, owing to this extremely simple relation, we are 

 able to state at once what invisible ray or rays are at any 

 moment passing through the slit into our rock-salt train. Our 

 engraving represents the arrangement as fitted up for the heat 

 of the electric arc, which is placed immediately in front of the 

 special nozzle n carrying the slit S^ We wish to employ the 

 arc chiefly in the extreme infra-red beyond the solar heat, and 

 where any heat is excessively minute. The hottest part of the 

 electric arc is found in the pit or crater of the positive carbon, 

 concealed from direct vision, and occupying a space of only 3 

 or 4 mm square, even in large arcs. The carbons then must 

 be inclined in order that a horizontal beam may escape from 

 this almost hidden crater, which owing to its small size, should 

 be brought nearly in contact with the slit, in order to utilize 

 the whole of its very minute area, while in this case the 

 inclination of the carbons will prevent such approach. Ex- 

 periments with various forms of incandescent -strips and car- 

 bons, directed by clock-work in the ordinary position, have 

 proved the necessity of adopting the special device by which 

 we have finally overcome these difficulties. Figure 1, Plate 

 III, shows in section and in front view, one-half full size, a 

 special slit, conical in form, around which a current of 

 water is forced to circulate. Figure 2 shows the carbons 

 on a smaller scale and the apparatus which permits them 

 to be set at any height, inclined at any angle to the vertical, 

 drawn back or approached to any distance. They are usually 

 placed, almost in contact with the special slit S lt and the 

 need of the water circulation is obvious, were it only to prevent 

 the sides of the jaws of the slit from melting, as they would 

 otherwise soon do. There is however another necessity for this 

 water circulation. The need of a slit which may be artificially 

 cooled for measures in the extreme infra-red of the spectrum 



