1913] on Applications of Polarized Light 731 



a ciuTeut of great mairnitude, flowing through a mass of molten steel 

 enclosed in a circular channel, sometimes causes the metal to pinch 

 together to such an extent that the circuit is actually broken. The 

 '• pinching " apart and running together of the mass of molten metal, 

 which may weigh as much as three or four tons, produce a somewhat 

 thrilling display. 



The Meecury Spectrum. 



The mercury arc differs from the carbon arc in giving an extremely 

 simple line-spectrum, the chief features of which are a yellow, a 

 green, and a violet component. The yellow component contains two 

 lines, separated by about 20 units of wave-length as compared with 

 six units for the yellow sodium doublet ; it shows up well in the 

 spectrum, but on account of its duplex character it is not suitable 

 for use in exact measurements. 



By means of powerful high-resolution apparatus, such as the 

 echelon spectroscope, the green line of the mercury spectrum has 

 also been shown to be complex ; but in this case the components are 

 so close together that they do not in any way reduce the value of the 

 line as a source of monochromatic light. The extreme brilliance of 

 this green line, its high spectroscopic purity, and the ease with which 

 it can be produced, have given to it an unrivalled position amongst 

 the various sources of monochromatic light which are now available 

 for polarimetric work. I can say with confidence that no one who 

 has worked with the mercury lamp will ever wish to return to the 

 sodium flame, which it is rapidly displacing both in scientific and in 

 technical laboratories. 



Dr. (Gladstone directed special attention to the strength of the 

 violet lines in the spectrum, of one of which he said that " this ray 

 is situated far beyond what is ordinarily considered the limit of the 

 luminous spectrum." This deep-violet component contains two lines 

 which are clearly visible in the spectroscope ; but they lie so near to 

 the limit of visibility that their presence can be shown most clearly 

 with the help of a fluorescent screen. The bright violet line is, from 

 the scientific point of view, one of the most valuable features of the 

 mercury spectrum. The main line is accompanied by two satellites 

 of greater refrangibility ; but these are so close to the principal line, 

 and are of so much smaller intensity, that they do not diminish 

 appreciably the unique value of this line, which still remains the 

 most powerful source of monochromatic light for work at the violet 

 end of the spectrum. Actual measurements in which the violet line 

 has been used both with and without the satellites have shown that 

 the errors introduced by the presence of the latter do not exceed one 

 part in 10,000 on the readings of a polarimeter. This error would, 

 therefore, be quite inappreciable in the case of all readings of less 

 than 100°. 



The spectrum that I have shown to you does not by any means 



