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IX. Optical Rotatory Dispersion.—Part I. The Natural and Magnetic Rotatory 
Dispersion in Quartz of Light in the Visible Region of the Spectrum. 
By Dr. Thomas Mahtin Lowry. 
Communicated hy Prof. H. E. Armstrong, F.R.S. 
Received May 24,—Read June 27, 1912. 
Contents. 
Page 
1. Introduction.261 
2. General review of previous observations and methods.263 
3. New methods and devices for measuring rotatory dispersion.265 
4. Light sources. 269 
5. Quartz.277 
6. Experimental methods.284 
7. Tabulated observations. Form of the dispersion-curve.287 
8. Comparison with earlier observations. Optical mass-centre of sodium light.292 
9. Tests of Wiedemann’s Law.294 
10. Summary. 296 
1. Introduction. 
The phenomena of rotatory dispersion are of special interest from two points of view. 
To the chemist they afford the most hopeful clue to the solution of the difficult and 
much discussed question as to why a particular substance possesses a given rotatory 
power. To the physicist, they contribute important data for the discussion of the 
mechanism involved in the propagation of light through matter, on the lines laid 
down by Maxwell, in 1869, and extended by the subsequent work of Sellmeier, 
Von Helmholtz, Larmor, I)rude, and others. On the chemical side, progress 
depends mainly on the accumulation of data as to the rotatory dispersion of a large 
number of substances for a few selected wave-lengths ; in particular also upon the 
development of simple and effective laboratory methods, permitting of the measure¬ 
ment of rotatory dispersion being made a part of the ordinary routine of every 
laboratory in which polarimetric studies are undertaken. Progress on the physical 
side demands that a few selected substances shall be examined with light of many 
VOL. CCXII.-A 492. Publislied separately, October 28, 1912. 
