268 DR. T. xM. LOWRY: NATURAL AND xMAGNETIC ROTATORY DISPERSION 
achromatic lens of 22-inch focus, whereby the linear dimensions of the image were 
doubled, a slit 1 mm. in width now giving an image 2 mm. in width. 
Fig. 1. General arrangement of apparatus for reading 
rotatory dispersion. 
A. Rotating arc. 
B. Condenser. 
C. Collimator (11-inch focus). 
D. Constant-deviation prism with grating replica on reflecting prism. 
E. “ Eye-piece tube ” of constant-deviation spectroscope with lens of 
22-inch focus. 
F. Triple-field polariser. 
G. Polarimeter slit. 
H. Water-jacket with quartz cylinders. 
I. Analyser. 
J. Rutherford prism in front of eye-piece. 
2. The light constant-deviation prism was replaced (a) by 
a dense prism of the same shape (b) by a pair of 30° prisms 
of the extra dense glass of refractive index I'92 with a right- 
angle prism of light glass to act as reflector. 
