IN QUARTZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
3. The right-angle prism was replaced by one bearing a Thorpe replica of a Rowland 
grating, placed so that the very bright first order diflFraction spectrum on one side 
acted in co-operation with the two extra-dense 30° prisms to give the effect of a 
constant-deviation prism of exceptionally high dispersive power. 
The resolution attained by these methods may be illustrated by giving the actual 
separation of the two mercury lines 5769'45 and 5790’49 ; these were used as test¬ 
lines during the early part of a series of experiments which had as their ultimate aim 
the effective resolution of the green silver doublet 5465'66 and 5471*73, and the yellow 
sodium doublet 5890*19 and 5896*16. 
Actual Separation of Mercury Lines 5769 and 5790 hy Constant Deviation 
Spectroscope :— 
11-inch focus lens and light C.D. prism.0*1 mm. 
99 n’9 
)5 n n 5 5 5 5 5 5 ." 5 5 
22 „ „ „ „ dense „ „.0*3 „ 
22 ,, ,, „ ,, extra dense 30° prisms (l*92). 0*45 ,, 
22 ,, „ ,, ,, grating replica.0*75 ,, 
22 ,, ,, ,, grating replica and extra dense 30° prisms . . 1*2 ,, 
Virtual Separation of Mercury Lines hy Eye-piece Prisms :— 
Ordinary D.Y. prism. 
Extra dense (1*92) D.V. prism. 
Rutherford prism.. 
0*3 
0*4 
mm. 
5 5 
0*65 
5 5 
The maximum separation was therefore 1*2 + 0*65 = 1*85 mm. 
The complete apparatus as used in resolving the green silver doublet is shown in 
fig. 1. 
A photographic method of measuring rotatory dispersion, which is specially valuable 
in the ultra-violet region of the spectrum, will be described in a later paper ; the 
methods used in the red and infra-red regions will also be described later. 
4. Light Sources. 
The principal light sources used in measuring the rotatory dispersion of quartz 
were;— 
(1) The flame spectra of lithium and thallium ; 
(2) The mercury arc enclosed in glass and in silica ; 
(3) The spectrum of sodium in a carbon arc ; 
(4) The open arc spectra of silver, copper, cadmium, and zinc. 
In almost every case it was necessary to produce a light of the greatest possible 
intensity; only in this way was it possible to make use of a dispersive system of 
