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.

PROF. LIVEING, ON THE EFFECTS OF DILUTION, TEMPERATURE, ETC. 299
inclined to the axis of the telescope so that, as far as the doubly refracting character
of the calcite prisms allows, the image might be in tolerably good focus across the whole
width of the plate, two and a half inches.
To concentrate the light, and make it, for the parts of the spectrum not subject to
absorption, nearly uniform whatever the thickness of the absorbent stratum of liquid,
a quartz lens of three inches focal length was fixed at that distance in front of
the slit, and a similar lens fifteen inches further off, and three inches beyond the
second lens was fixed a screen with a circular hole in it about one-eighth of an inch
in diameter, and beyond that was of course the source of light. The centres of the
hole in the screen and of the two lenses were aligned with the axis of the collimator.
The distance between the lenses was fixed so as to allow of the interposition of the
longest trough, used as a water bath for maintaining the temperature of the tubes
containing the solutions. These troughs were of brass fitted with a plate of quartz at
each end, and each had in it two V-shaped septa on which the tube with solution
rested, and thereby took up at once its right position in the course of the pencil of
light between the lenses. The tubes holding the solutions were of glass, fitted at the
ends with quartz plates. These plates were held in position by outer brass plates with
central circular perforations, connected by three wires passing along the outside of the
tube and furnished with screw nuts by which the plates could be firmly pressed against
the ends of the tube. The joint between the quartz plate and the end of the tube
was made water-tight by a washer of thin rubber. The washers all had the same
sized circular opening which determined the cross section of the pencil of rays falling
on the slit. This seemingly complicated arrangement was adopted because it was
necessary to have joints which would not be affected by a temperature of 100~, or by
dilute acids, or by alcohol, and could be easily taken to pieces for cleaning the tube
or plates.
Each tube had a branch on its upper side which was left open for the purpose of
filling the tube, and to allow of expansion of the liquid when it was heated. Tubes
of four lengths in geometrical progression, namely of 38mm., 76 mm., 1525 mm., and
305 mm., and a cell with quartz faces having an interval of 6'7 mm. between then,
were used to hold the solutions; and for a few observations a cell of only 5 mm. thickness
was used.
For observations on the effects of temperature, the trough containing the tube with
solution was filled with water and a photograph of the spectrum taken at the temperature of the room; the trough was then heated by one or more gas lamps until
the water boiled, the gas lainps were then lowered so as to maintain the bath 3 or 4
degrees below the boiling point, bubbles adhering to the quartz plates swept off with
a feather, and when the whole appeared to be in a steady condition another, photograph
was taken. Unless the solution in the tube were a very dilute one there was not much
trouble with bubbles in the solution, but bubbles in the bath were very troublesome,
and had to be removed because they impeded the passage of the light, and thereby
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