2i8 The Smithsonian Institution 



tion of heat in the spectrum was so almost utterly unknown 

 that the remark by Sir John Herschel that its heat was dis- 

 continuous contained almost all our knowledge of the subject 

 up to that time. 



At the time of which we speak, comparatively recent as it 

 is, only a few advanced thinkers held the now universal view 

 that heat and light were not two different things, but differ- 

 ent effects of the same thing, and the investigations now 

 commenced with the bolometer did much to prove the cor- 

 rectness of the latter opinion. By continuous studies involv- 

 ing great labor, and the record of extremely numerous 

 experiments (over one thousand galvanometer readings being 

 taken on the average to a single line), there was in the course 

 of three years' patient work established the material for a map 

 of the principal lines in this hitherto unknown region, and 

 the material for a new method of study of the inter-action of 

 the solar heat and our atmosphere, which latter was shown to 

 be a principal agent in causing them. 



The bolometer has been made much more effective and 

 has been still more recently reinforced by the holograph, in- 

 troduced in 1 89 1 and lately perfected — a device for register- 

 ing by photography the fluctuations of the needle, which thus 

 permanently records the bolometer's indications, while by a 

 further step these tracings are automatically converted into a 

 linear spectrum by the use of a cylindrical mirror, a method 

 of translation by which the fluctuations caused by the infra- 

 red tract are reduced to a form comparable to that of the 

 upper portion of the spectrum, as ordinarily visible. In the 

 infra-red spectrum many hundred lines have since been lo- 

 cated in this manner. 



With these instruments Mr. Langley has opened up a new 

 department of physics. He has not only shown the existence 

 of, but has measured the energy in, rays having a wave- 

 length nearly twenty times that of extreme luminous ones. 



