410 S. P. Langley — New Spectrum. 



for instance, announced in other terms that the extreme end 

 of the invisible spectrum might from theoretical considerations 

 be probably estimated at something less than the wave-length 

 of 1^*0, whence it followed that the above value of l/*'8 was 

 impossible, .and still more that of 2^'S. If in this connection 

 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 less than 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, 

 founded on the then universally accepted formula of Cauchy, 

 which was later discredited by the direct experimental evi- 

 dence given of its falsity by the bolometer. 



The bolometer then, 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. 



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 re- 

 membered that when an invisible band or line is suspected its 

 presence is revealed by the change of temperature in the bolo- 

 meter 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 right if from heat. The writer's 

 first method was to have one person to note the exposure; an- 

 other to note the extent of the deflection, 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 approximate 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 existed, it was apparent that 50 years 

 would be required to denote them. 



The writer then devised a second apparatus to be used in 

 connection with the bolometer. This apparatus was simple in 



