1*28 1*1*0 f. S. P. Langley on the New Spectrum. 



Taxed from a mean of at least six independent determinations 

 with the accuracy stated above. 



The reader will perhaps gather a clearer idea of this action 

 if he imagines the map before him hung up at right angles 

 to its actual position, so that a rise in the energy-curve given 

 would be seen to correspond to a deflexion to the right, and 

 a fall, to one to the left ; for in this way the deflexions were 

 written down on the moving photographic plate from which 

 this print has been made. The writer was now speaking of 

 the refinements of the most recent practice ; but there was 

 something in this retrospect of the instrument's early use 

 which brought up a personal reminiscence w r hich he asked 

 the Academy to indulge him in alluding to. 



This was that of one day in 1881, nearly twenty years 

 ago, when being near the summit of Mount Whitney, in the 

 Sierra Nevadas, at an altitude of 12,000 feet, he there, with 

 this newly invented instrument, was working in this invisible 

 spectrum. His previous experience had been that of most 

 scientific men — that very few discoveries come with a surprise, 

 and that they are usually the summation of the patient work 

 of years. 



In this case, almost the only one in his experience, he had 

 the sensations of one who makes a discovery. He went 

 down the spectrum, noting the evidence of invisible heat 

 die out on the scale of the instrument until he came to the 

 apparent end even of the invisible, beyond which the most 

 prolonged researches of investigators up to that time had 

 shown nothing. There he watched the indications grow 

 fainter and fainter until they too ceased at the point where 

 the French investigators believed they had fo-ind the very 

 end of the end. By some happy thought he pushed the 

 indications of this delicate instrument into the region still 

 beyond. In the still air of this lofty region the sunbeams 

 passed unimpeded by the mists of the lower earth, and the 

 curve of heat, which had fallen to nothing, began to rise 

 again. There was something there. For he found, suddenly 

 and unexpectedly, a new spectrum of great extent, wholly 

 unknown to science and whose presence was revealed by the 

 new instrument, the bolometer. 



This new spectrum is given on the map, where it will be 

 observed that, while the work of the photograph (much more 

 detailed than that of the bolometer, where it can be used at 

 all) has been stated to extend, as far as regular mapping is 

 concerned, to about 1*1'*, everything beyond this is due to 

 the bolometer, except that early French investigators had 

 found evidence of heat extending to 1*8^. Still beyond that 



