408 S. P. Langley — New Spectrum. 



us in the visible, then the bolometer, whose sensitive strip 

 passes over a dark line in the spectrum, visible or invisible 

 (since what is darkness to the eye is cold to it), gives a deflec- 

 tion on the side of cold, and in the warmer interval between 

 two lines a deflection on the side of heat ; these deflections 

 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 draw- 

 ing, 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 to 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 invis- 

 ible 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 labors 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 Southampton meet- 

 ing of the British Association for the Advancement of Science 

 in 1882.* During these two years, very many thousand gal- 

 vanometer readings were taken by a most tryingly slow pro- 

 cess, to give the twenty or more interruptions shown at that 

 time, below the limit at l^'l of Abney's photographs. The 

 bolometer has been called an eye which sees in tlie dark, but 

 at that time the " 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 elim- 

 inated. 



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 he then pre- 

 sented (at the Southampton meeting of the British Associa- 

 tion) as having been mapped for the first time. Mouton had 

 two years before obtained crude indications of heat as far as 



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



