122 Prof. S. P. Langley on the New Spectrum. 



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

 continuous) till the first drawing of the energy curve by 

 Lamansky f, * n 1871, which, on account of its great import- 

 ance 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 indicating lapses of heat in the 

 infra-red. It is almost impossible to tell what these lapses 

 are meant for, without a scale of some kind (which he does 

 not furnish), but they probably indicate something, going- 

 down to near a wave-length of l/\ It is obvious that the 

 detail is of the very crudest, and yet this drawing of Laman- 

 sky'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 1861, 

 and later in 1885 J, they were completely determined by the 

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

 spectrum " at a wave-length of 5*3 M . 



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

 photography, and the next important publication in this 

 direction was that of Captain (now Sir William) Abney §, 

 which gave the photographic spectrum down to about l'l 1 *, 

 much beyond which photography has never mapped 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 mav 

 perhaps seem the place to speak of this instrument, though 

 with the later developments which have made it what it is 

 to-day, it has grown to something very different from what it 

 was then. 



It has, in fact, since found very general acceptance among 

 physicists, especially since it has lately reached a degree of 



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



+ Monatsberichte der k. Akademie der Wissenschaften zu Berlin, 

 December 1871. 



X American Journal of Science, March 1884, and August 1886. 



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



|| Actiuic balance, American Journal of Science, 3rd series, vol. xxi. 

 p. 187, 1881. 



