538 Anomalous Magnetic Rotatory Dispersion of Rare Earths. 
If the pressure in the thermometer, namely it or p , is 
less than 1000 mm., as it usually is in practice, the corrections 
are to be reduced in the same ratio. This process of reduction 
is not rigorous, but it is correct to a much higher degree of 
approximation than that to which the corrections for 1000 mm. 
are known. 
National Bureau of Standards, 
Washington, D.C., November 9, 1907. 
XLIX. On Anomalous Magnetic Rotatory Dispersion of Rare 
Earths. Remarks on Prof. R. W. Wood's recent paper. 
By G. J. Elias*. 
PROF. R. W. Wood having recently published an inves- 
tigation on anomalous dispersion of magnetic rotation 
in neodymiumf, I should like to direct attention to expe- 
riments which I made in 1906 J. 
For measuring the rotation I used a Lippich polarimeter, 
illuminated by light having passed through a spectrometer, 
and monochromatic to within \ fifju about. The rotation 
varying very rapidly within the absorption-band and near its 
edges, a high degree of homogeneity of the light is necessary; 
otherwise it may occur that the most important points of the 
curve are overlooked, as I actually found. Of course through 
the light not being absolutely homogeneous small errors 
will come in ; nevertheless it seems to me that this direct 
method should give much better results than that used by 
Prof. Wood, which I rejected on account of its inaccuracy 
and the errors occurring necessarily in the neighbourhood 
of absorption-bands. Moreover, I consider it impossible to 
make measurements within the absorption-band in that way ; 
but this is most desirable in order to detect many peculiarities 
of the band's structure and to find the sense of rotation on 
both sides of the maximum. 
In the case of the absorption-band of a solution of chloride 
of erbium, at about A = 520 (jl/j,, to which I confined my first 
measurements, I found considerable anomalies, and the 
sense of rotation in the immediate neighbourhood of the 
maximum positive, farther away negative, as is shown by 
* Communicated by Prof. PL du Bois. 
t K. W. Wood, Phil. Mag. [6] xv. p. 270 (1908) ; also E. W. Wood, 
Physik. Zeitschr. vi. p. 416 (1905) ; ix. p. 148 (1908). 
% G. J. Elias, Physik. Zeitschr. ix. p. 931 (1906). See also H. du Bois 
and G. J. Elias, Proc. Eoy. Acad. Amsterdam, Feb. 29, 1908. 
