688 THE NEW SPECTRUM. 



3'ear.s very many thousand o-ahanonieter readings wore taken, by a 

 most tryingly slow process, to give the twenty or more interruptions 

 shown at that time. l)elow the limit of 1.1'" of Abney's photographs. 

 The bolometer has been called an eye which sees in the dark, l)ut at 

 that time the "eye" w^as not fairly open, and having then not been 

 brought to its present rapidity of use, the early results were attained 

 only b}' such unlimited repetition, and almost intinite patience was 

 needed till what was inaccurate was eliminated. 



Several luindreds at least of galvanometer readings were then taken 

 to establish the place of eacJt of the above twenty lines during the two 

 years when they were being hunted for, and this patience so far found 

 its reward that they have never required any material alteration since, 

 but only additions such as the writer can now give. The part ))elow 

 1.1^ he then presented (at the Southampton meeting of the British 

 association) as having been mappc^d for the first time. Mouton had 

 two years before obtained crude indications of heat as far as LS*^, and 

 Abney had. as stated, ol)tained relatively complete photographs of 

 the upper infi-a-red extending to about this point (1. 1'"). 



Tiie writer had already determined for the first time by the l)olometer, 

 at Allegheny and on Mount Wliitney, 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 tJiev l)een 

 certaiid^' felt by the bolometer, had a wave-length of al)out y.S'^, or 

 nearly two octaves below the "great A" of Fraunhofer. 



He stated in this conununit'ation of 1882 that the galvanometer 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 

 l)een said, it now responds to changes of less than one one-hundred- 

 niillionth), and he added: '"'Since it is one and the same solar energy, 

 wh()S{> manifestations are called 'light' or 'heat' according to the 

 medium which intcrpn^ts tiuMU, what is 'light' to the eye is 'heat' to 

 the bolomet(M-. and what is seen as a dark line by the eye is felt as a 

 cold line ])v the sentient instrument. Accordingly, if lines analogous 

 to the dark 'Fraunhofer' lines exist in this invisible region, they will 

 appear (if I 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 ])ut infinitesimal 

 change of temperature caused by its contact w^ith such a 'cold band.' 

 The whole work, it will be seen, is necessarily veiT 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 interrup- 

 tions were due to the existence of something like dark lines or bands, 

 resembling what are known as the P^'raunhofer lines in the upper spec- 

 trum; but, apart from what the writer had done, no one then surmised 



