[ 270 1 
XXII. Anomalous Magnetic Rotatory Dispersion of Neo- 
dymium. By R. W. Wood, Professor of Experimental 
Physics in the Johns Hopkins University *. 
[Plate XI1I.1 
THERE has been a good deal of discussion as to whether 
anomalous dispersion of the rotatory polarization occurs 
in the vicinity of the absorption-bands of the rare earths. 
The work of Bates, as well as my own, appears to have proven 
beyond much question that the aniline dyes do not exhibit 
the phenomenon, as was claimed by Schmauss, who published 
curves which apparently showed very pronounced anomalies 
at the absorption-bands. Schmauss also investigated the 
magnetic rotation of solutions of the rare earths (Annalen 
der Physik, x. p. 853, 1903), and found that there was, 
in every case, an abnormal increase of the rotation on 
the red side of the bands, and an abnormal decrease on the 
blue side, as in the case of ordinary anomalous dispersion. 
Bates was unable to find any anomalies whatever, but an 
examination which I made of a solution of praseodymium 
appeared to confirm, at least in a measure, the results of 
Schmauss (Phil. Mag. May 1905 ; Phys. Zeit. vi. p. 416, 
1905). 
As I stated in my earlier paper, it was difficult to get 
results in which one could feel absolute confidence, owing to 
the great experimental difficulties. I had no polarimeter of 
precision, and was obliged to work with a pair of nicols only, 
and determine the rotation of the plane by the method of 
extinction. 
If one could deal with rotations of sufficient magnitude to 
enable- one to obtain a dark band in the spectrum of the 
transmitted light, one could feel sure of the results, and this 
I have at last succeeded in doing. To my great surprise, 
however, in the case of the one band which has been carefully 
studied, the anomaly is the same as with sodium vapour, that 
is the rotation increases very rapidly as the absorption-band 
is approached from the short wave-length side. It has been 
found possible to set the nicols for extinction and cause a 
restoration of the light in the vicinity of the absorption-band 
by the excitation of the magnet, as can be so beautifully done 
with the non-luminous vapour of metallic sodium f. This is 
proof positive that the phenomenon exists, and I have even 
* Communicated by the Author. 
t Macaluso and Corbino, Hend. Heal. Ac. del Lincei (5) vii. p. 293. 
Wood, "Magnetic Rotation of Sodium Vapour/' Phil. Mag. Oct. 1905 ; 
and July 1907. 
