412 S. P. Langley — New Spectrum. 



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

 fall, to one to the left ; for in this way the deflections 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 which 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 Mt. Whitney in the Sierra 

 Nevada, 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 be- 

 lieved they had found 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 noth- 

 ing, 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 con- 

 cerned, to about 1^*1 : that everything beyond this is due to the 

 bolometer, except that early French investigators had found 

 evidence of heat extending to 1^'S. Still beyond that ultima 

 thule, this region, which he has ventured to call the "New 

 Spectrum,'' extends. It will be found between wave-lengths 

 1^*8 and 5^*3 on the map. 



The speaker had been much indebted to others for the per- 

 fection to which the apparatus, and especially the galvanom- 

 eter, had been brought. He was under obligations particu- 

 larly to Mr. Abbot, for assistance in many ways, which he had 



