124 Prof. S'. P. Langley on the Xew Spectrum* 



eye is cold to it), gives a deflexion on the side of cold, and 

 in the warmer interval between two lines a deflexion on the 

 side of heat ; these deflexions being proportionate to the 

 cause, within the degree of accuracy just stated. 



The third quality, the accuracy of its measures of position, 

 is better seen by a comparison and a statement, for if we look 

 back to the indications of the lower part of Lamansky's 

 drawing we may see that at least a considerable fraction of a 

 degree of error must exist there in such a vague delineation. 

 Now, in contrast with this early record, the bolometer has 

 been brought io grope in the dark and to thus feel the 

 presence of narrow Fraunhofer-like lines by their cooler 

 temperature alone, with an error of the order of that in 

 refined astronomical measurement ; that is to say, the probable 

 error, in a mean of six observations of the relative position of 

 one of these invisible lines, is less than one second of arc ; a 

 statement which the astronomer, perhaps, who knows what 

 an illusive thing a second of arc is, can best appreciate. 



The results of the writer's labours with the bolometer in 

 the years 1880 and 1881, and in part of his expedition in the 

 latter year to Mount Whitney, were given at the Southamp- 

 ton meeting of the British Association for the Advancement 

 of Science in 1882 *. During these two years very many 

 thousand galvanometer readings were taken, by a most 

 tryingly slow process, to give the twenty or more interrup- 

 tions shown at that time, below the limit of l*!/* of Abney's 

 photographs. The bolometer has been called an eye which 

 sees in the dark, but at that time the iC eye " was not fairly 

 open, and having then not been brought to its present rapidity 

 of use, the early results were attained only by such unlimited 

 repetition, and almost infinite patience was needed till what 

 was inaccurate was eliminated. 



Several hundreds at least of galvanometer readings were 

 then taken to establish the place of each 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 below 1*1 M be 

 then presented (at the Southampton meeting of the British 

 Association) as having been mapped for the first time. 

 Mouton had two years before obtained crude indications of 

 heat as far as l'S* 1 ", and Abney had, as stated, obtained 

 relatively complete photographs of the upper infra-red 

 extending to about this point (l'P 4 ). 



* Report British Association, 1882. Nature, xxvi. 1882. 



