S. P. Langley—Selective Absorption of Solar Energy. 198 
area beyond wave-length (4 represents the whole ultra-violet 
ener 
We are accustomed to speak of the ultra-violet or infra-red 
regions without reflecting on the enormous difference between 
their actual importance. The reader will be able to see by a 
simple inspection of the normal chart and a comparison of the 
little area above wave-length 08, and the great area below 
wave-length 0-7, that the latter is nearly a hundred times as 
great as the former. Yet the former, owing to the prismatic 
expansion, and to the selective absorption by the feeble rays of 
this region of certain salts of silver, with which it can be photo- 
graphed (while the far greater luminous energy below makes lit- 
tle impression on these salts), has occupied more attention than 
the latter. When we observe here how the infra-red region is 
compressed by the prism, we can understand how its extent has 
been under-estimated. Its real extent is so vast that we should 
accustom ourselves to consider “in the infra-red region” as a 
wholly vague term, needing to be supplemented with a descrip- 
tion of the particular part of the infra-red referred to. 
e well to epitomize the principal results of all these 
researches as far as they have been here given. In general 
ey emphasize and extend our first conclusions. 
Ist. In measures now made for the first time on approxi- 
mately homogeneous rays in the diffraction spectrum, we find 
that the maximum energy is above the red and is placed in 
fact near the yellow. The place of this maximum point varies 
With the sun’s altitude, ranging from a wave-length of nearly 
0:55 on a clear day and with a high sun, to a wave-length of 
0-65, or even more before sunset. On the normal scale then, 
the position of the maximum of heat in the spectrum does not 
vary widely from that of the maximum of light. It is shown 
ater how similar results are deducible from the prismatic spec- 
trum. 
ators and of present opinion on the point, we find that 
(sassondings to th 4 
whole Jess and less, as we go down below the red, to a point 
