S. P. Langley—Selective Absorption of Solar Energy. 187 
so that every feature which is not common to them all is re- 
jected or rééxamined, and in this manner the whole spectrum 
is studied. ese original charts are on a scale four times as 
large linearly as that the reader now sees (Plate ITI). 
In addition to this, on some clear days, ae have been 
made upon the chart directly corresponding to the movements 
of the galvanometer needle; that is to say, the observer at the 
spectro-bolometer has moved the bolometer through the whole 
spectrum by means of the tangent screw; the slit has been left 
permanently open so that the bolometer has been constantly 
exposed; and the observer at the galvanometer, seeing the 
action of the selective absorption of the atmosphere in every 
part of the spectrum, as the rays of the sinking sun pass through 
greater depths of air. This third method—very useful when, as 
in this case, many observations have to be taken in a short 
time—is nevertheless ise accurate than his before described. 
A careful bolometric and also optical setting ade on 
the invisible rays measured. This lens and the slit are fitted 
into opposite ends of a tube, T, 44 meters long, held by suita 
ble y’s. The beam of rays from the slit, now rendered parallel 
by the collimator, next falls upon a prism,* P, of the same 
ee bee _ as the tk ed tlin on a cireular ade 
salt prism of near co size and great puri as 8 as prisms of quartz an 
Spar, have been u sued to determine the pron ava es the glass for sah ray, visible 
_ and invisible.) 
