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X. -DESCRIPTION OF THE INSTRUMENTS AND PRO- 

 CESSES EMPLOYED IN PHOTOGRAPHING ULTRA- 

 VIOLET SPECTRA, BY W. N. HARTLEY, f.r.s.e., &c.. 

 Professor of Chemistrv, Royal College of Science, Dublin. 

 Plates 12, 13, 14, and 15. 



[Read, April 11th, 1881.] 



Introduction. 



The first successful pliotographs of spectra were executed by M. 

 Edouard Becquerel (Bibliothe'que Universelle de Geneve, t. xl. 

 1842). Dr. J. W. Draper first photographed the sun's spectrum 

 with a ruled diflfraction grating in 1843 (Philosophical Magazine, 

 June, 1845, and March, 1857). The work thus commenced has 

 been continued in late years by his son, Dr. Henry Draper, of 

 New York. In the year 1852 it was discovered by Professor 

 Stokes that quartz absorbs the ultra-violet rays less than glass, 

 and in 1853, when experimenting previous to the delivery of a 

 lecture at the Royal Institution, he found the length of the 

 spectrum of electric light yielded by the powerful discharge of a 

 Ley den jar and analysed by quartz apparatus, extended no less 

 than six or eight times the length of the visible spectrum. The 

 obscure rays were rendered visible by receiving them on a 

 fluorescent screen. — (" On the Change of Refrangibility of Light," 

 Phil. Trans., 1852, and" On the Long Spectrum of Electric Light," 

 Phil. Trans., 1863). Professor Stokes studied the ultra-violet 

 spectra of metals and executed drawings of the lines exhibited by 

 aluminium, zinc and cadmium. He discovered the fact that 

 certain solutions show light and dark bands in the spectra of rays 

 transmitted by them, the solutions being colourless the bands are 

 invisible unless they fall upon a fluorescent screen. The late Dr. 

 W. A. Miller, of King's College, London, in 1863, simultaneously 

 with Professor Stokes, described his method of examining the 

 photographic transparency of various saline solutions and organic 

 substances, and of depicting metallic spectra. A sensitised photo- 

 graphic plate was used for the reception of the rays of the spec- 

 trum, so that they were made to register their own position and 

 intensity by means of chemical action. The photographed spectra 



SciEN. Pboc, R.D.S. Vol. III., Pt. iir. I 



