Nineteenth Annual Meeting. 



117 



series of rays which comprise the visible spectrum. It is affected by all wave-lengths 

 in proportion to their intensity, and in the skillful hands of its inventor it has been 

 the means of bringing to our knowledge a vast and important series of heat-waves 

 hitherto undetected. By its use it is now possible to analyze and study the radia- 

 tion from bodies of low temperature — even below the freezing point of water — 

 with almost the same certainty and precision as that from substances in a high state 

 of incandescence. The diagram now on the screen (Fig. 2) shows the results of 

 some measurements by Professor Langley of the radiation from substances at 100° C. 

 and — 2°C. The curves show the energy of each wave-length emitted, and present 

 to our view spectral regions far beyond the infra-red of the solar spectrum. The 

 very limited region lying between ^4 and H, comprises those rays which are capable 

 of affecting the human retina. The most refined methods previously known to sci- 

 ence had extended our acquaintance with the longer wave-lengths only to the point 

 marked -Tl. Beyond lay the still longer waves due to radiation at low temperatures, 

 the wave-lengths and intensities of which are indicated in these curves. 



Armed with this wonderful instrument, and possessed of skill and perseverance 

 given to few, Professor Langley made exhaustive studies of the quality of the sun's 

 rays after they had 

 been sifted by pass- 

 ing through the 

 miles of atmosphere 

 above his observa- 

 tory at Alleghany. 

 He then took his in- 

 struments and a 

 corps of trained as- 

 sistants across the 

 continent to a re- 

 mote point in the 

 American desert 

 which was known to 

 possess an atmos- 

 phere of unrivaled 

 purity, established a 

 temporary station at 

 the base of Mt. Whit- 

 ney, and repeated his 

 measurements there. 



The party then ascended the almost inaccessible peak to an elevation of 14,000 feet, 

 and with nearly half the earth's atmosphere below them, succeeded in making a de- 

 termination of the quality of sunlight at this high station. Of the dangers and 

 difficulties of bringing across the desert and up the mountain all the delicate appa- 

 ratus necessary to such a research, only those who accompanied the exjjedition can 

 form any adequate conception. The published accounts furnish one of the most 

 romantic passages in the history of science. 



The energy curves of sunlight obtained at these three stations (Fig. 3) are of 

 the highest scientific interest. They show beyond dispute that the earth's atmos- 

 phere absorbs much more of the violet and other more refrangible rays than of the 

 red wave-lengths; so that could we ascend entirely beyond the atmosphere, sun and 

 stars would appear much bluer than they do at the surface of the earth. It seems 



From Langley's monograph ; 1. c. 



