316 Intelligence and Miscellaneous Articles. 



For the details of experiment, for the discussion of its results 

 and of those previously obtained, the original must be referred to. 

 — Wiedemann's Annalen, vol. xxxv. p. 137 (1888). 



SELECTIVE ABSORPTION OF METALS FOR ULTRA-VIOLET LIGHT. 

 BY JOHN TROWBRIDGE AND W. C. SABINE * 

 The question of the absorption of the ultra-violet rays by metallic 

 surfaces possesses considerable interest, both from a practical and 

 a theoretical point of view. By the kindness of Prof. Pickering, 

 Director of the Harvard University Observatory, we were provided 

 with a number of metallic surfaces prepared by Professor Wright 

 of Tale College. These metallic surfaces were deposited upon glass 

 by means of electricity. The surfaces were of gold, platinum, tel- 

 lurium, palladium, copper, silver, and steel. A preliminary trial 

 had shown us that a heliostat mirror of the same composition as 

 that upon which the grating was ruled did not absorb light of 

 greater wave-length than 2900. We resolved, therefore, to compare 

 other metals with speculum metal. Since our heliostat arrange- 

 ment required two mirrors to direct the light upon the slit of 

 the spectroscope, we employed a speculum mirror for the movable 

 mirror of the heliostat, and replaced the fixed mirror by mirrors 

 of metals whose selective absorption we wished to compare with 

 that of speculum metal. To our surprise, the metallic mirrors of 

 gold, copper, nickel, steel, silver, tellurium, and palladium all 

 reached the same limit as speculum metal. Here was a complete 

 experimental proof that colour in no way influences the selective 

 absorption of metals for the ultra-violet rays ; for the copper 

 mirror, which gave a strong yellow light by reflexion, was as 

 capable of reflecting light of as short wave-length as the brilliant 

 white surface of polished silver. Although the metallic surfaces 

 we employed were bright, slight differences in polish undoubtedly- 

 existed, and therefore we are not justified in placing much reliance 

 upon the evidence presented by the intensity of the photographs of 

 the solar spectrum obtained by light reflected into the spectroscope 

 by these various metallic surfaces. The photographs, however, can 

 be classified according to intensity, in order of numbers as follows, 

 number 1 indicating the greatest intensity: — 1, steel; 2, gold ; 

 3, platinum ; 4, palladium ; 5, silver ; 6, tellurium ; 7, copper. 



It was evident from these experiments that selective absorption 

 of metals is far less than the absorption exercised by the earth's 

 atmosphere. We therefore resolved to employ the light of the 

 electric spark between metallic terminals, in order to ascertain 

 whether any limit of absorption could be reached. For this pur- 

 pose, the light of the spark between copper terminals was reflected, 

 by means of a mirror of the metal whose selective absorption we 

 wished to examine, upon the slit of the spectroscope. To protect the 



* From an advance proof from the ' Proceedings of the American 

 Academy.' Communicated by the Authors. 



