125 
(or Rciin-band Spectroscope) in Meteorology. 
spectroscope are to be depended upon, even wlien 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 
Kain-band 51 :— 
0 “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 “ Eain-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. e., 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 (bypog, moist, spectrum; and o-xo7Tsco, 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. 
