unrecognized Wave-lengths. 151 



these circumstances, it has been demonstrated by Prof. Row- 

 land that the wave-length of light, passing through the slit Sj 

 to fall upon the grating and there be diffracted to S 2 , is 

 directly proportional to the distance S x 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 Si. We wish to employ the arc 

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

 where any heat is exclusively 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 millim. square, even in large arcs. The car- 

 bons, 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. Experiments with various forms of incandescent 

 strips and carbons, directed by clockwork in the ordinary 

 position, have proved the necessity of adopting the special 

 device by which we have finally overcome these difficulties. 

 Figure 3, Plate V., shows in section and in front view 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 x ; 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 measurements in the extreme infra-red of the spec- 

 trum from the electric arc, is rendered evident when we state, 

 that for these extreme wave-lengths the arc radiation is com- 

 paratively so small, that the heat from the hottest part of the 

 dazzling bright carbon does not very greatly exceed that from 

 a piece of melting ice. If, then, we are to distinguish here 

 between the radiation which passes through the open slit from 

 the incandescent carbon, and that which comes from the 

 adjacent edges of the slit, which inevitably mingles more or 

 less with the former, the difference between the two tempera- 

 tures must be made as great as possible. 



