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 surfaces 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 Russian 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
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 supererogatory. 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 rumford Medal of the London Royal Society, and elected to a membership in that body; was president of the American Association for the Advance-