Mercury Vapour at the 2536 Absorption Line, 437 



region under investigation, is reproduced on PL III. rig. 7. 

 The mercury line which gives rise to the selective dispersion 

 is indicated by a long arrow, and the fainter companion line 

 which borders it on the short wave-length side by a short 

 one. Curiously enough, this faint companion line does not 

 show any evidence of its existence in the absorption spectrum, 

 though there is a faint absorption line on the other side of 

 the main line at about the same distance from it, which I 

 have found represented by a bright line in the fluorescence 

 speccrum of the vapour excited by the light of the cadmium 

 spark. A photograph of this spectrum will be found in the 

 second edition of my 'Physical Optics,' on p. 580, in which 

 the 2536 fluorescent line will be seen to be accompanied by 

 a faint companion on the long wave-length side. The 

 negative, of which fig. 7 is a sixfold enlargement, was made 

 with a large quartz spectrograph by Fuess, which was placed 

 at my disposal by Professor Trowbridge, of Princeton Uni- 

 versity. It has lost much in the process of enlarging, for on 

 the original the iron line immediately to the right of the 

 mercury line is a beautifully resolved double line. The 

 wave-lengths of the two mercury lines were measured with 

 reference to Rowland's values given for the iron lines, the 

 main line being 2536*5, and the faint companion 2534*8. 



The wave-lengths of the iron lines immediately to the left 

 and right of the main mercury line, which are the lines in 

 which the greatest displacement of the interference fringes 

 occur, are 2535*6 and 2536*9. Inasmuch as the mercury 

 line is much nearer the latter, it is in this line that we should 

 expect to observe the greatest displacement of the fringes, 

 for a given vapour density. Over 100 plates were exposed 

 with the tube at various temperatures between 23° and 60°. 

 It was found impossible to work at higher temperatures, for 

 the fringes became very indistinct in the vicinity of the 

 absorption line. This results from the circumstance that one 

 of the interfering beams is weakened by absorption, and 

 interference then takes place between two beams of unequal 

 intensity : moreover it was more difficult to hold the tempe- 

 rature constant during the time of exposure at the higher 

 temperatures. The pictures are scarcely suitable for repro- 

 duction as they were made with a small quartz spectro yraph, 

 the only instrument at my disposal, nevertheless tJ e five 

 enlargements produced on PI. III. fig. 10 will serve to give 

 a general idea of the distortion of the interference fringes in 

 the vicinity of the 2536 mercury line. The temperature of 

 the mercury dispersion tube is recorded on each photograph. 

 The position of the mercury 2536 line is indicated on the 



