Prof. S. P. Langljy on the Sew Spectrum. 125 



The writer had already determined for the first time by 

 the bolometer, at Allegheny and on Mount Whitney, the 

 wave-lengths of some much remoter regions, including, in 

 part, 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 i 2'8 fl , or nearly two octaves below the " great A " of 

 Fraunhofer. 



He stated in this communication of 1882 that the galvano- 

 meter 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 bolometer, 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 invisible region, they will appear (if 1 

 may so speak) to the bolometer as cold bands, and this 

 hair-like strip of platinum is moved along in the invisible 

 part of the spectrum till the galvanometer indicates the 

 all but infinitesimal change of temperature 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 extended nor, perhaps, what these explorations 

 really meaut. They may be compared to actual journevs 

 into this dark continent, 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 a new country, was, by those who had not been 

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

 the region, treated as erroneous and impossible. 



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

 supposed to exist near the wave-length l/\ Doctor John 

 Draper*, for instance, announced in other terms that the 



* Proceedings of the American Academy, vol. xvi. p 233(1830). 



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