128 The Use of the Hygro-Spectroscope
the evening, about 7 o'clock, the storm did come, much to
the astonishment of a great many, including the friends with
whom I was staying.
It must be remembered that the spectroscope is quite
. independent of any threatening appearance of clouds, or the
opposite; frequently in the former case it will tell you—no
precipitation will occur ; and in the latter we must expect
some before long. I will now advert to the dry and moist
air-bands, and principal constant solar lines, the position of
which I have delineated on the chart of the spectrum (Fig. 3).
Fig. 3. — In the above chart the relative positions of the permanent
lines and bands of the spectrum are represented, but not their actual
appearances as seen in a spectroscope. (From want of space the chart
has been cut in two, and the second half placed below the first). The
principal rain-band is figured by means of the two lines to the red side
of D ; with greater dispersion these are split up into numerous lines of
varying intensities. To the blue side of D is shown the position of the
line that is unusually prominent and defined previous to the falling of
snow, when this part of the spectrum is partly, if not wholly, cleared
from vapour-lines. The instrument used in the construction of this
diagram was a Grace's spectroscope, with attached micrometer.
Commencing at the red end we have first a ; in spectroscopes
of moderate dispersion this permanent line is seldom seen,
unless the spectrum at this part is unusually clear ; meteoro-
logically speaking, I have not found it of much value.
At a is the position of a moist air- or rain-band, and at b a
dry air-band; these three, a, a, and b, are best studied by
looking direct at the sun. The band at c is probably due to
water vapour. At c' is a small rain-band.