728 Dr. Thomas Martin Lowry [April 18, 



price, in view of the " spar-famine " which has prevailed for some 

 years. 



Considerable advance has, however, been made in the direction 

 of improved methods of illumination. The solar light, which figured 

 so largely in the experiments of the earlier workers, is too precarious 

 to satisfy the ardent worker of to-day, and in any case could render 

 no direct assistance in illustrating a Friday Evening Discourse. 



When Faraday on Friday, January 23, 1846, delivered his dis- 

 course on " The Magnetization of Light " to an audience of 1 003 

 persons, the source of light in the experiments which he described 

 was an Argand gas-burner. Professor Silvanus Thompson in 1889 

 was able to use the electric arc, which was then just beginning to 

 come to the front as a commercial illuminant. With this unrivalled 

 source of Hght he was able to show for the first time in a public 

 lecture a large number of the properties of polarized light which had 

 been reserved hitherto for individual observation in the laboratory. 



The remarkable effects which are seen when light of one single 

 colour or wave-length is substituted for white light were shown by 

 Spottiswoode in 1878, with the help of a powerful sodium-lamp which 

 had been devised by Sir James Dewar. His lecture was aptly de- 

 scribed as " A Nocturne in Black and Yellow." 



During several years I have taken a special interest in seeking to 

 discover other sources of monochromatic light for use in experiments 

 on polarization, and have been particularly concerned to proclaim 

 the merits of the mercury arc as an illuminant for everyday use in 

 optical investigations. On account of the predominance of three 

 pure colours, the few experiments with the mercury arc, which I hope 

 to show you this evening, might perhaps be described as a " Pastoral 

 in Yellow, Green and Violet." 



The Mercury Arc. 



The spectrum of the light produced by passing an electric dis- 

 charge through mercury vapour was described by Wheatstone in 

 1835 in a report to the British Association " On the Prismatic 

 Decomposition of Electric Light " ; but it was not until twenty-five 

 years later that a real mercury-lamp was invented by Professor Way. 

 This consisted of an intermittent jet of mercury which was directed 

 into a cup half an inch below. The current from a battery of 

 Bunsen cells was passed through the jet and developed an intense 

 light. The spectrum of the light was examined by Dr. J. H. Glad- 

 stone, and described in a paper " On the Electric Light of Mercury," 

 published in the ' Philosophical Magazine ' of 1860 (Vol. XX. 

 pp. 249-253). 



The first use of the mercury arc as a source of light in polari- 

 raetry appears to have been made just ten years ago by two German 

 ■workers, Disch and Schonrock, working independently (Disch, Ann. 



