DR. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
and 3 / 1 6th inch diameter ; this tube, which is normally in a horizontal plane, was 
tilted into a vertical plane for use with the polarimeter, one of the straight portions of 
the tube being focussed on the slit of the constant-deviation spectroscope. A long, 
straight arc of uniform luminosity, which can be focussed directly upon the slit, is an 
ideal source of light for work of this kind, as the uniform illumination of the triple 
field follows automatically without the careful adjustment that is necessary in the 
case of a spherical source of light. The Bastian lamp, taking a current of less than 
an ampere and capable of being connected directly to the ordinary lighting circuit, is 
the most convenient mercury lamp that has yet been devised and it is a matter for 
regret that it is no longer manufactured commercially. 
(h) A 71 end-on mercury lamp and 
(c) A vertical “ Quartzlite" lamp, both made of silica and taking a current of 3'5 
amperes. These lamps were specially constructed for use with the polarimeter and 
have been described and figured in the ‘ Trans. Faraday Soc.,’ 1912, vol. 7, 267. 
( 4 ) (a) Mercury (dreen. Hg 5460'97. 
In reading rotations of large magnitude it is impossible to use sodium light as a 
standard, owing to the fact that the lines Dj and are not extinguished together, 
the difference amounting in the experiments now described to 8° for the dextro-quartz 
and 11° for the Isevo-quartz column. It was, therefore, necessary to adopt some other 
light for the purpose of making stringent tests on the quartz blocks before attempting 
measurements of rotatory dispersion. For this purpose the green mercury line was 
selected. The choice has been fully justified by practical experience during several 
subsequent years. Not only is the line one of the easiest to produce and read but it 
is the only line of the 24 which can be read as easily and as sharply to 0°'01 on a 
rotation of 12,000° as on one of 5°. Most of the measurements were made with the 
glass Bastian lamp as a source of light, a dense prism on the constant-deviation spectro¬ 
scope and a dense direct-vision prism on the eye-piece. Although the line can be 
resolved on the echelon spectroscope, its spectral purity is incomparably greater than 
that of sodium (Nutting, ‘Bureau of Standards Bulletin,’ 1906 [II.], 249), and it is 
undoubtedly the best line to use for standard measurements of natural and magnetic 
rotation, at least until the manufacture of enclosed cadmium arcs has been sufficiently 
perfected and simplified to render the green line Cd 5085'8240 available for general 
use. The green mercury line has the further advantage that it can be separated 
readily from the other lines of the spectrum by a direct-vision prism of low dispersive 
power and can, therefore, be used in everyday polarimetric work without an auxiliary 
spectroscope. 
The yellow, green and violet mercury lines can also be separated by means of 
gelatine screens. This modification of Landolt’s method is far more convenient than 
the original form in which absorbent solutions were used; it has the further 
advantage that it lends itself to- accurate work, since the portions of the spectrum 
