(or Rain-band Spectroscope) in Meteorology. 125
spectroscope are to be depended upon, even when the rain-
band alone is taken into account in making an observation.
What is the rain-band ? and where is it situated in the
spectrum ?
The principal rain-band, for there are others which I will
refer to subsequently, is situated on the red side of the D
lines, involving them, and at times increasing or decreasing
in intensity, according to the nearness or quantity of rain.
In spectroscopes of small dispersion it exhibits itself as a
dark shading; in larger instruments this band will be split
up into a quantity of fine black lines, which are due to the
absence of rays of light of certain refrangibilities.
It would be as well now to take you back to the time when
physicists first became sensible of varying lines in the
spectrum, due to the presence in the air of vapour of certain
densities, and of peculiar states of equilibrium. To quote
from Mr. Capron's excellent little pamphlet, 'A Plea for the
Rain-band'1:—
"Its history may be said to date from the time when
Angstrom's maps of the solar lines were found to present
different aspects, according to the condition of moisture of
the atmosphere at the time of observation, and when that
pioneer of spectroscopy proved the presence and absence of
certain lines forming bands in the spectrum, more especially
a set near D, to depend on that condition. No practical
meteorological result followed, however, until, as Prof. Piazzi
Smyth tells us, the subject was first presented to him as a
marked feature in sky spectrum at Palermo before and after
a sirocco in 1872. Next, when in July, 1875, destructive
1 I may be permitted to make a few remarks as to the origin of the
word "Hygro-spectroscope." A short time since it occurred to me that
the name "Rain-band Spectroscope" was a decided misnomer, as there
are other moisture-bands of equal importance besides the rain-band.
The instrument is essentially a hygroscope or hygrometer (i. »., an
apparatus for seeing or measuring moisture) combined with a spectro-
scope. With these reasons in view, I was led to coin the word Hygro-
spectroscope (ύγρός moist, Spectrum; and σχοπεω, to spy), which
compound is more applicable to the necessities of the case, and, what is
very satisfactory, has gained the approval of several scientific men of
high standing.