126 Prof. S. P. Langley on the JVeic Spectrum. 



extreme end of the invisible spectrum might, from theoretical 

 considerations, be probably estimated at something less than 

 the wave-length of 1^, whence it followed that the above 

 value of 1*8^ was impossible, and, still more, that of 2*8^. 

 Jf, in this connexion, we revert to our map, where the 

 visible spectrum has an extent in wave-lengths of 0*36% 

 then, on that same scale, the length of the entire possible 

 spectrum, visible and invisible, was fixed by Draper at the 

 point there shown near the band par. In still other words, 

 according to him the very end of any spectrum at all would 

 be about 3 on a scale in which the visible spectrum was 1. 

 Doctor Draper's authority was deservedly respected, and 

 this citation of his remarks is made only to show the view 

 then entertained by eminent men of science. 



Now, the writer had proved by actual measurement that 

 it extended far beyond this point, and had announced, as 

 the result of experiment, that it extended at any rate to 

 about three times the utmost length then assigned from 

 theoretical reasons, by Draper, founded on the then universally 

 accepted formula of Cauchy, which was later discredited 

 by the direct experimental evidence given of its falsity by the 

 bolometer. 



The bolometer, which is wholly independent of light as a 

 sensation and notes it only as a manifestation of energy, 

 first lays down the spectrum by curves of energy from which 

 the linear spectrum is in turn derived. Two such curves 

 taken at different times are given to show the agreement. 



There must now be explained, however briefly, the way in 

 which these energy curves, which are the basis of all, have 

 actually been produced here. 



In making the map of the energy curves, it should be 

 remembered that when an invisible band or line is suspected, 

 its presence is revealed by the change of temperature in the 

 bolometer strips affecting the needle of the galvanometer, 

 causing this needle to swing this way or that ; let us suppose 

 to the left if from cold and the light if from heat. The 

 writer's first method was to have one person to note the 

 exposure, another to note the extent of the deflexion, and a 

 third to note the part of the spectrum in which it occurred. 

 For reasons into which he does not enter, this old plan was, 

 as he has already said, tedious in the extreme and required, 

 as has been said, hundreds of observations to fix with 

 appropriate accuracy the position in wave-length of one 

 invisible line. It has been stated that only about twenty 

 such lines had been mapped out in nearly two years of 

 assiduous work prior to 1881, and if a thousand such lines 



