OF ARTS AND SCIENCES. 299 



XIX. 



CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 



LABORATORY. 



SELECTIVE ABSORPTION OF METALS FOR ULTRA 



VIOLET LIGHT. 



By John Trowbridge and W. C. Sabine. 



Presented March 14, 1888. 



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 Professor Pickering, 

 Director of the Harvard University Observatory, we were provided 

 with a number of metallic surfaces prepared by Professor Wright of 

 Yale College. These metallic surfaces were deposited upon glass by 

 means of electricity. The surfaces were of gold, platinum, tellurium, 

 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 specu- 

 lum metal. Since our heliostat arrangement 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 the 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 color 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 reflection, 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 pre- 

 sented by the intensity of the photographs of the solar spectrum ob- 

 tained by light reflected into the spectroscope by these various metallic 



