276 DR. T. M. LOWRY; NATURAL AND MAGNETIC ROTATORY DISPERSION 
when working with rotations of 3,000° to 4,000° ; a specially narrow slit was used in 
conjunction with a Rutherford eye-piece and extra dense 30° prisms (without the 
grating) on the constant-deviation spectroscope. The half-shadow angle was 4°. 
(6) (b) Cadmium Green. Cd 5085’8240. 
This magnificent line, if more easily produced, would be the most suitable for use in 
place of the green mercury line as principal standard in polarimetric and refractometric 
work. It was read with the same dispersing system as the preceding line but with 
a half-shadow angle of only 3°. 
(6) (c) Cadmium Light Blue. Cd 4799‘9107. 
This line was read with the same dispersive system but with the half-shadow angle 
increased to 5° or 6°. 
(6) (d) Cadmium Dark Blue. Cd 4678‘37. 
This line is difiicult to read on account of its smaller intensity and diminished visual 
efiiciency ; the presence of the silver line Ag 4668'70 presents a further difiiculty in 
the case of large rotations, as it acts as a somewhat close satellite. The line was read 
with considerable dilficulty, using the ordinary dense constant-deviation prism on the 
spectroscope, a dense direct-vision prism on the eye-piece and a half-shadow angle 
of 8°. 
(7) Zinc. 63637, 481071, 4722*26, 4680*38. 
The methods used to read the zinc lines were similar to those followed in the case 
of cadmium. Two alloys were used, a silver-zinc alloy and a brass specially prepared 
from pure zinc and electrolytic copper; the latter alloy was the easier to burn; it 
gave the best results with a small current of about 3 amperes. 
(7) (a) Zinc Red. Zn 6363*7. 
This line was more difficult to read than the corresponding cadmium line on account 
of the greater amount of continuous light in the spectrum—an unimportant factor 
with readings of ordinary magnitude but of vital importance when reading large 
rotations. It was finally read with the ordinary dense constant-deviation prism, 
Rutherford prism, specially narrow slit and a half-shadow angle of 7°. 
(7) (b) Zinc Blue. Zn 4810*71, Zn 4722*26, Zn 4680*38. 
These lines were read under the same conditions as the blue cadmium lines. The 
third zinc blue line presents practically the same difficulties as the second cadmium 
o 
blue line, from which it is separated by only two Angstrom units ; it was read with 
brass electrodes, thus eliminating the silver line, and a half-shadow angle of 6°. The 
extreme difficulty of reading these lines serves to emphasise the extraordinary value of 
the violet mercury line, which is 320 units further on in the violet region and yet can 
