732 Dr. Thomas Martin Lowry [April is, 



exhaust the usefuhiess of the mercury arc. In addition to the lines 

 of the visible spectrum, there is a powerful series of ultra-violet lines 

 which are freely transmitted by the glass of the silica lamps. These 

 are of value for a number of scientific purposes, and have found an 

 important technical application in the sterilization of water. 



To touch on a subject that is perhaps less generally familiar, I 

 have found that, on the other side of the spectrum, the magnificent 

 though invisible line at wave-length 10,14u is of unique value as a 

 starting point for calibration work in the infra-red. It will also be 

 remembered that some of the longest waves of light that have yet 

 been detected were discovered by Rubens in the radiation from a 

 mercury lamp. 



Resolution of the Mercury Spectrum. 



One merit of the mercury arc as a source of light consists in the 

 readiness with which the three main components may be separated. 

 A direct vision prism of quite moderate dispersive power, placed in 

 front of the eye-piece of a polarimeter, produces a separation of the 

 three images which is sufficient for most purposes. The lines may 

 also be separated by means of coloured screens prepared from gelatine 

 films stained with suitable dyes. Tlie efficiency of these screens may 

 be shown by interposing them one by one in the path of the beam 

 which forms the images which are now thrown on the screen. The 

 lines selected for transmission are weakened in intensity, but the 

 others are absorbed so completely that the transmitted light can be 

 obtained spectroscopically pure. 



Occasionally it is desired merely to reduce the glare of the green 

 line in order to render the violet line more distinctly visible. An 

 ideal dye for this purpose, which was suggested to me a year ago by 

 Dr. Mees, is xylene red. 



This substance fluoresces so beautifully under the light of the arc 

 that I cannot resist the temptation to show it to you in the striking 

 form which Sir James Dewar has used to exhibit this effect in the 

 case of other dyes. 



If I have dwelt at some length on the merits of the mercury arc, 

 I may perhaps be excused on the ground that to one's scientific 

 colleagues the description of a novel tool, which they may be able to 

 use in their own work, is usually of greater interest and importance 

 than any record of the work for which it may already have been used. 



B. — Rotatory Polarization. 



The phenomenon of rotatory polarization was first discovered in 

 the case of quartz. Arago in 1811 (Mem. Inst., isil, pp. 93-134) 

 found that a plate of quartz interposed between a polarizer and 

 analyser was capable of depolarizing the light in such a way that 

 transmission took place where previously there had been complete 



