270 
DR. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
sufficient power to guarantee the effective purification of the light. Even when light- 
sources of the highest intensity were used, it was often found to be desirable to work 
with a system of lower dispersive power (using narrower slits and a narrower line in 
the field of the polarimeter) in order to maintain sufficient brightness to render 
possible the use of a small half-shadow angle ; this factor is specially important in the 
case of the open-arc spectra, which are difficult to maintain in a condition to produce 
steady illumination. 
(l) Lithium. 6708*2. 
The flame spectrum of lithium is of such low optical intensity that even under the 
most favourable conditions it could not be read with a half-shadow angle of less than 
7°. In order to economise light, the red lithium line was read with a direct-vision 
prism in front of the eye-piece as the only dispersive system. As Perkin's arrange¬ 
ment of the lithium flame did not give sufficient light, the arrangement shown in 
fig. 2a was used, a long, narrow globule of lithium carbonate being supported on a 
bundle of platinum wire over a jet of oxygen led by a platinum tube through the 
grid of a “ Meeker” burner. 
(2) Thallium. 5350*65. 
Although the flame spectrum of thallium gives a very bright and pure mono¬ 
chromatic green light, its use was attended with very great difficulty on account of 
the extreme volatility of the chloride and the transient character of the light obtained 
by the ordinary methods of procedure. After many unsuccessful efforts, a steady light 
was finally obtained by the method shown in fig. 2b, the chloride being vaporised 
in a silica bulb and carried into the flame through a silica jet by a current of oxygen. 
With this arrangement, a very bright and steady illumination can be maintained 
during a period of several hours; the discovery of an equally efficient method for 
