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. 305
confounded with a diffuse continuous absorption which extends from the ultra-violet
down the spectrum as the solution becomes more concentrated; but they are common
to the nitrate and chloride, and may be seen with a solution of the former when with
an equivalent solution of chloride the advancing continuous absorption has obliterated
them. The superposition of this continuous absorption, even when it is very weak and
scarcely otherwise perceptible, strengthens and widens the bands.
EFFECTS OF DILUTION.
For observing the effects of dilution equal volumes of the stock solutions were
diluted to 2, 4, 8, 45-5, 61 or 91 times their original volumes, and the absorptions
produced by thicknesses of these solutions proportional to their dilutions observed and
photographed.
In the spectra of either didymium or erbium chloride, starting with solutions half
the strongest, or less strong, in thickness of 38 mm., I can find no change with dilution,
when accompanied by proportional increase of thickness, below X 390: see plate 3, at
the end of the volume. With the strongest solution in a thickness of 38 mm. a
diffuse absorption creeps down from the most refrangible end of the spectrum, as
may be seen in the uppermost spectrum in each of the plates 10 and 11. Above
X 375, or thereabouts, it seems to cut off all the light, but the diffuse edge extends
with the strongest didymium chloride as low as X 415, making the absorption bands
look wider and stronger by its superposition. On comparing with the eye the spectrum
of a thickness of 5 mm. of the strongest solution of didymium chloride, with that of
305 mm. of the same solution diluted to 61 times its volume, both spectra being in
the field of view at the same time, I could detect no difference between them.
Again, photographing the spectrum of a thickness of 6'7 mm. of the strongest
didymiium chloride, and that of 305 mm. of the same solution diluted to 45'5 times its
original bulk, I can find no difference between the photographs, which take in a range
from about X 350 to X600. Plate 7 is a reproduction of these photographs. This identity
of the spectra extends to the intensities, even of the weakest bands that I can see,
as well as to the positions of the bands, and even to the apparent extinction of the
diffuse absorption which is produced by a greater thickness of the strongest solution at
the ultra-violet end.
Also erbium chloride of half the strongest concentration, in a thickness of 5 mm.,
gives a spectrum which cannot be distinguished by my eye from that given by 305 mm. of
a solution 61 times as dilute. And photographs of the spectrum of the same solution, half
the strongest, in a thickness of 6 7 mm., are identical with those of 305 mm. of the same
solution diluted to 45'5 times its bulk, below a wave-length of about X 380. Plate 9 is a
reproduction of these photographs. The triple band at about X 378 comes out more strongly
with the stronger solution, but I am not sure whether this is not an effect due to the
superposition of the diffuse absorption creeping down from the more refrangible end. In
the region above 355, a thickness of 152 mm. of a very dilute solution of didymium
VOL. XVIII. 39