REPORT OF THE SECRETARY. 31 



scope of such an establishment, touching upon some of the essential 

 features of the work which had been undertaken under my direction. 



An explicit description of the specially important investigation which 

 has been continued during the last year, and which is now drawing to 

 a completion, can not so well be given here as in a later portion of 

 the rei^ort to which the reader is referred, but the general object 

 of the immediate work is, as has already been stated, the detailed 

 investigation of that great spectral region, still nearly unknown, where 

 yet the greater portion of the solar energy is known to be displayed; 

 or, in other ^rords, of that invisible portion of the solar spectrum which 

 lies beyond the limit of the red. 



That the solar spectrum did not cease at the limit of visibility has 

 long been known, and the attention of many distinguished physicists 

 has been directed to the investigation of this invisible part, Avhose 

 presence is manifest neither to the eye nor to any ordinary process of 

 photography, but which nevertheless comprises more tban three-fourths 

 of the energy which the sun sends to us. Were the range of the human 

 eye vastly extended so as to enable us to receive impressions corre- 

 sponding in character to the kind of energy which is present in this 

 infra-red region we should see in it phenomena of precisely the same 

 character as we now see in the limited spectral region to which we are 

 physiologically limited. It was probably this idea which led Melloni 

 to the use of the term " heat color " to convey to the mind some idea 

 of this similarity between the invisible and the ordinary visible spec- 

 trum, and this term expresses by the force of association the charac- 

 teristics distinguishing one portion of this region from another, char- 

 acteristics which, although unrecognized by the eye or by any of our 

 senses directly, are yet more striking in their various physical results 

 than the various colors which mark out to the eye the great divisions 

 of the visible sj)ectrum. 



The invisible spectrum, then, is marked by narrow bands or lines, 

 which are almost entirely devoid of energy, quite like those which appear 

 in the visible spectrum as black lines and are known as the " Frauenhofer 

 lines." It is to the study of these Prauenhofer lines of the visible spec- 

 trum, that we owe nearly all those recent advances which have not only 

 given us definite information as regards the constitution and nature of 

 the heavenly bodies, but have been of immense advantage in the study 

 of meteorological and atmospheric phenomena on the earth. The prac- 

 tical importance of the study of the character of these lines in the invis- 

 ible spectrum (where their intensity and probably their number is far 

 greater than in the visible) then is evident. 



Here, however, all ordinary methods of spectroscopic investigation 

 fail ; but long since the writer devised a method which has in the course 

 of the last two years been perfected to such a degree as to enable us 

 to search out the lines in this invisible spectrum and to map them with 



