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POPULAR SCIENCE MONTHLY 



the invisible rays below the red) ; and Brashear undertook the solution 

 of the problem of producing accurate surfaces upon lenses and prisms 

 made of this substance. Its deliquescent character was a factor very 

 difficult to contend with, but a method was discovered by which sur- 

 faces were produced within a half light wave, answering to the most 

 critical demands of Langley's bolometric research. During a visit to 

 the Chicago World's fair, in 1893, some unusually fine rock-salt crystals 

 from mines in Poland were found in the Eussian exhibit, which were 

 secured for the Smithsonian Institution, from which some of the 

 largest and finest lenses and prisms were made. Langley's joy in 



Smithfiei.u Street 



Brashear's success was boundless — and naturally so. The proportion 

 of invisible radiated heat, underneath the red part of the sun, is many 

 times greater than the visible rays. It is through the curious quality 

 of rock-salt lenses and prisms that it became possible to concentrate 

 the dark, lower heat rays, and to achieve the splendid results with the 

 bolometer. 



Professor Langley's life work and the honors that have been heaped 

 upon him are so well known that to enumerate them would be super- 

 erogatory. The secretaryship of the Smithsonian Institution is the 

 highest gift of science in America, and this honor he held for nearly 

 twenty years, until his death in 1906. He was given the Eumford 

 Medal of the London Eoyal Society, and elected to a membership in 

 that body; was president of the American Association for the Advance- 



