406 jS. P. Langley — New Spectru?n. 



by Sir John Herschel* to show that the heat was not continuous) 

 till the first drawing of the energy curve by Lamansky,f in 1871, 

 which, on account of its great importance in the history of the 

 subject, is given on the map. It consists of the energy curves 

 of the visible spectrum, and beyond it, on the right, (and in 

 illustration of what has just been said it will be seen how 

 relatively small these latter appear) of three depressions indi- 

 cating lapses of heat in the infra-red. It is almost impossible 

 to tell what these lapses are meant for, without a scale of so me 

 kind (which he does not furnish), but they probably indicate 

 something, going down to near a wave-length of 1A It is 

 obvious that the detail is of the very crudest, and yet this 

 drawing of Lamansky's was remarkable as the first drawing of 

 the energy spectrum. It attracted general attention, and was 

 the immediate cause of the writer's taking up his researches 

 in this direction. 



It seems proper to state here that the true wave-lengths 

 were at that time most imperfectly known, but that in 1884, 

 and later in 18854 they were completely determined by the 

 writer as far as the end of what he has called " the new spec- 

 trum" at a wave-length of 5^*3. 



The upper portion of the infra-red is quite accessible to 

 photography, and the next important publication in this direc- 

 tion was that of Captain Abney,§ which gave the photographic 

 spectrum down to about 1^*1, much beyond which photography 

 has never gone since. 



From the time of seeing Lamansky's drawing, the writer had 

 grown interested in this work, but found the thermopile, the 

 instrument of his predecessors, and the most delicate then 

 known to science, insufficient in the feeble heat of the grating 

 spectrum, and about 1880 he had invented the bolometer) and 

 was using it in that year for these researches. This may per- 

 haps seem the place to speak of this instrument, though with 

 the later developments which have made it what it is to-day, 



a wave-length of less than If 1 . (Its true wave-length was not determined till 

 much later.) Later, Fizeau seems to have found further irregularities of this heat 

 as long ago as 1847, and of its location, obtaining his wave-lengths by means 

 of interference bands. His instrumental processes, though correct in theory, 

 were not exact in practice ; and yet it seems pretty clear that he obtained some 

 sort of recognition of a something indicating heat, as far down as the great region 

 immediately above i2 on our present charts. Mouton (Comptes Rendus, 1879) 

 confirmed this observation of Fizeau's, and contrived to get at least an approxi- 

 mate wave-length of the point where the spectrum (to him) ended, at about 1H"8. 



* Philosophical Transactions, vol. cxxx, p. 1, 1840. 



f Monatsberichte der k. Akademie der Wissenschaften zu Berlin, Dec, 1871. 



% This Journal, March, 1884, and August, L886. 



S Philosophical Transactions, vol. clxxi, p. 653, 1880. 



I Actinic Balance, this Journal, 3d series, vol. xxi, p. 187, 1881. 



