See eS 
“ 
S. P. Langley—Selectiwe Absorption of Solar Energy. 171 
when the atmospheric absorption is very different, we are able 
has just been said, at best, less than one-tenth that in the pris- 
matic, the latter is itself, when taken in portions so narrow as 
e€ approximately homogeneous, almost insénsible, The 
admirable gratings of Mr. Rutherfurd, one containing 17,296 
lines to the inch, or 681 to the millimeter; and one, half that 
number, both ruled upon speculum metal. 
have used a slit at a distance of 5 meters, without an 
collimator, and have kept the grating normal to the optical axis. 
t will be seen then that the rays have passed through no ab- 
sorbing medium whatsoever, except the sun’s atmosphere and 
our own 
narrow spectrum, which passes down the case of the instru- 
* Through these measures the unit of wave-length will be the micron (u)= 
Tiss millimeter, or 10,000 times the unit of Angstrém. Thus the wave- 
length of Frauenhofer’s “A” is here written 0-76. 
