Anomalous Rotatory Dispersion of Neodymium. 271 
succeeded in photographing the spectrum of the restored 
light. 
The neodymium nitrate was prepared in the form of a 
solid film, amorphous and isotropic, pressed between two very 
thin cover-glasses. The nitrate was prepared from the 
double ammonium salt by precipitation with oxalic acid, 
ignition, and treatment with nitric acid. The nitrate, freed 
from acid, was boiled down to the consistency of syrup, in 
which state a drop placed on a strip of glass will immediately 
solidify into a clear glass bead without crystallization, a fact 
found by Anderson in the course of his investigations upon 
the absorption spectra of the rare earths. By forming a 
prism of about 40 degrees of this substance between plates 
of heated glass, I have detected anomalous dispersion at the 
absorption-band at wave-length 5790, something which I 
have never been able to do with solutions, even with com- 
pensated prisms. 
The films are not at all difficult to prepare. The cover- 
glasses are heated and a small drop of the molten substance 
pressed out between them. If crystallization occurs, it in- 
dicates that the solution has not been sufficiently boiled down. 
Films varying in thickness from 0*1 to 0'5 mm. were used, 
the best results being obtained with a moderate thickness. 
The light from an arc-lamp was passed through a nicol 
and the cores of a large Ruhmkorff magnet, between the 
poles of which the film was mounted, then through a second 
nicol, after which it was concentrated on the slit of a spectro- 
scope. The nicols were set for extinction, which could be 
done to within a tenth of a degree, after which the magnet 
was excited. A restoration of the light immediately occurred 
throughout the whole range of the spectrum in the case of 
the thickest films, the intensity being greatest however in 
the vicinity of the absorption-band. With thinner films the 
restoration was confined to the immediate vicinity of the 
band, a bright and very narrow line shining out exactly in 
coincidence with the centre of the absorption-band, with 
fainter bands on each side (PI. XIII. fig. 1). The direction of 
the rotation was the opposite of that produced by a plate of glass 
placed between the poles of the magnet, and was about four 
times as great. It immediately occurred to me that what 
we may term the rotation of the substance as a whole (that 
is the rotation due to infra-red and ultra-violet electrons) 
could be practically compensated by employing glass plates 
of the requisite thickness, leaving an outstanding effect due 
solely to the *absorption-bands which fell within the limits of 
the visible spectrum ; and this method was successfully 
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