IN QUARTZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
273 
which are picked out are, except in the case of the yellow doublet, really monochro¬ 
matic. But in spite of these merits this method of working has no advantage over the 
use of a direct-vision prism, which gives an even more efficient separation at about the 
same cost; the economy of light, as compared with the absorbent screens, more than 
compensates for the glare of the adjacent lines.* 
(4) (b) Mercury Yellow. Hg 5790'49, Hg 5769‘45. 
The yellow doublet of the mercury spectrum can be resolved with the utmost ease 
by the apparatus described above, giving wide blocks of pure monochromatic light in 
the field of the polarimeter. Unfortunately, the vertical silica lamp, which was 
required to illuminate the field with the system of highest dispersive power, was found 
to yield appreciable quantities of continuous light in this part of the spectrum ; the 
actual measurements were therefore made with the extra dense prisms (but no 
grating) on the spectroscope, and a Rutherford prism on the eye-piece, using a glass 
Bastian lamp and a half-shadow angle of 5°. For small rotations the two lines can 
be read as one, but there is no advantage in this, as so many purer light-sources are 
now available. 
(4) (c) Mercury Violet. Hg 4358'58. 
Owing to the weak visual intensity of the blue and violet portions of the spectrum, 
economy of light is of the utmost importance ; thus, in spite of its extreme brightness, 
the violet mercury line must be read with a prism at the eye-piece as the only 
dispersive system. The line is accompanied by two more refrangible satellites 43480. 
and 4341'O ; these are not usually seen in the polarimeter, but become important when 
using a silica lamp as a source of light for reading optical rotations of several 
thousand degrees. 
The Rutherford prism, which had been constructed for these measurements, could 
not be used, as the violet line was covered by a strong glare of green stray light. 
The observations were therefore made with a dense direct-vision prism, the slit in 
front of the triple field being narrowed until the line was completely separated from 
the two satellites. In order to make the line at extinction of about the same intensity 
as the unextinguished satellites, it was necessary to increase the half-shadow angle 
to 7°, when an excellent series of readings was obtained. 
( 4 ) (d) Optical Mass-centre of the Violet Mercury Line. 
On account of its extreme brightness and the ease with which it can be produced, 
[■*■ Note added August 6, 1912.—In the case of the violet line, Dr. Mees has suggested to me that a 
screen of pure xylene red may be used with a direct-vision spectroscope to reduce the glare of the green. 
Although it would not, if used alone, give a pure monochromatic light, it absorbs much less of the violet 
light and is therefore much more suitable for the present purpose than a screen of density sufficient to 
absorb completely all the other lines of the mercury spectrum.] 
VOL. CCXII.—A. 2 X 
