Memoirs presented to the Cambridge philosophical society on the occasion of the jubilee of Sir George Gabriel Stokes, bart., Hon. LL. D., Hon. SC. D., Lucasian professor.

XIV. On the Effects of Dilution, Temperature, and other circumstances,
on the Absorption Spectra of Solutions of Didynmium and Erbium Salts.
By G. 'D. LIVEING, M.A., Professor of Chemistry.
[Received 15 October 1899.]
IN November 1898 I made a preliminary communication to the Society giving
results of observations on the absorption spectra of aqueous solutions of salts of didymium and erbium in various degrees of dilution. Since then most of the observations
have been repeated with improved apparatus, whereby several anomalies in the photographs have been removed, and a great many additional observations made, so that it
will probably be best to make this communication quite independent of the prelirilinary
one, and, at the risk of a little repetition, complete in itself so far as it goes.
APPARATUS.
The observations were made in part directly by the eye with an ordinary spectroscope, and partly by photography. On the former I rely only for the part of the
spectrum below the indigo, on the latter for the more refrangible part. The spectroscope chiefly used for the former had two whole prisms of 60~ and two half-prisms,
all of white flint glass, telescopes with achromatic object glasses of 12 inches focal
length, and eye-piece of very low magnifying power. It was useless to employ higher
dispersion or magnification, because the absorption bands, even the sharpest of them
which is that of didymium at about X 427, are all diffuse, and higher dispersion or
magnification renders some details invisible. In comparing by eye the spectra produced
by two solutions, one was thrown in by reflexion in the usual way, and, after making
the comparison, the positions of the solutions were interchanged and the observation
repeated, in order to correct any error arising from a difference of intensity between
the light entering directly and that coming in by reflexion.
For photography the spectrum was formed by one prism of 60~ and two halfprisms, all of calcite, the object glasses of the telescopes were quartz lenses of 18'5
inches focal length for the sodium yellow light. The photographic plate was of course