S. P. Langley — New Spectrum. 40$ 



1^*8, and Abney had, as stated, obtained relatively complete 

 photographs of the upper infra-red extending to about this 

 point (1^-1). 



The writer had already determined for the first time by the 

 bolometer, at Allegheny and on Mt. Whitney, the wave-lengths 

 of some much remoter regions, including the region then first 

 discovered by him and here called "the new spectrum," and 

 was able to state that the terminal ray of the solar spectrum, 

 whose presence had then been certainly felt by the bolometer, 

 had a wave-length of about 2^*8 or nearly two octaves below 

 the " great A " of Fraunhofer. 



He stated in this communication of 1882, that the galva- 

 nometer then responded readily to changes of temperature in 

 the bolometer strip of much less than one ten-thousandth of 

 a degree Centigrade (as has just been said it now responds to 

 changes of less than one one-hundred-millionth ;) and he added 

 " since it is one and the same solar energy whose manifestations 

 are called 'light' or 'heat' according to the medium which 

 interprets them, what is 'light' to the eye is 'heat' to the bolo- 

 meter, and what is seen as a dark line by the eye is felt as 

 a cold line by the sentient instrument. Accordingly if lines 

 analogous to the dark ' Fraunhofer ' lines exist in this invis- 

 ible region, they will appear (if I may so speak) to the bolo- 

 meter as cold bands, and this hair-like strip of platinum is 

 moved along in the invisible part of the spectrum, till the gal- 

 vanometer indicates the all but infinitesimal change of tem- 

 perature caused by its' contact with such a 'cold band.' The 

 whole work, it will be seen, is necessarily very slow ; it is in 

 fact a long groping in the dark, and it demands extreme 

 patience." 



At that time it may be said to have been shown that these 

 interruptions were due to the existence of something like dark 

 lines or bands resembling what are known as the Fraunhofer 

 lines in the upper spectrum, but apart from what the writer 

 had done, no one then surmised how far this spectrum ex- 

 tended, nor perhaps what these explorations really meant. 

 They may be compared to actual journeys into this dark conti- 

 nent, if it may be so called, which extended so far beyond those 

 of previous explorers that the determination of positions by the 

 writer, corresponding somewhat to longitudes determined by the 

 terrestrial explorer in anew country, was by those who had not 

 been so far, but had conceived an inadequate idea of the extent 

 of the country, treated as erroneous and impossible. 



A necessary limit to the farthest infra-red was in 1880 sup- 

 posed to exist near the wave-length 1A Doctor John Draper,* 



* Proceedings of the American Academy, vol. xvi, p. 233, 1880. 



