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.

ABSORPTION SPECTRA OF SOLUTIONS OF DIDYMIUM AND ERBIUM SALTS. 303
A well marked triplet at about X 483, 476 and 469, of which that in the middle is
decidedly weaker than the other two.
A broad weak band, with its centre at about X462, and extending nearly down to the
most refrangible band of the triplet above mentioned.
A very broad band with its centre about X444.
A very weak band,,,,, 433.
A strong, narrow, sharply-defined band at about X 427.
A very weak diffuse band with its centre about X 418.
A still weaker one with its centre about 415.
Another weak diffuse band at about X 406.
A very broad strong band with its centre about X403.
A very weak diffuse band at about X 391.
A diffuse band at about X 380.
Another, wider, at about 375.
A weaker band at about X 364.
Four, nearly equally distributed between X 358 and X 350, which in all but the
weakest solutions run into one broad band extending beyond the above-mentioned limits.
A weak diffuse band at about X 338.
And a broad diffuse band with its centre about X 329.
These bands appear all to belong to didymium, or to the metals associated under
that name, for though they may be modified in character, and even in position, by the
solvent and other circumstances, they all disappear in the absence of didymium, and
they retain so much the sane general character under all circumstances, that it is
reasonable to infer that they have the same primary cause. A reference to plate No. 19
(at the end of the volume) on which are reproduced photographs of the spectra of
didymium chloride in solution in water, in alcohol, and in alcohol charged with hydrochloric acid, will make my meaning evident.
The erbium absorption bands of which I have taken notice in this investigation
are as follows:
A group of four bands in the red, of which the most refrangible but one is much
the strongest and has a wave-length about X 653.
A group of four, of which the more refrangible two are much stronger than the
others, lying between À 536 and À 549.