S. P. Langley on the Selective Absorption of Solar Energy. 179 



larger, on the normal scale, than the whole of that (both visible 

 and invisible) previously known. 



We observe that the prismatic spectrum is enormously ex- 

 panded at the violet end. To carry this on the prismatic scale 

 to wave-length 0^*3 would extend it far beyond the limits of 

 our chart. All the actual energy in the entire ultra-violet part, 

 however, is insignificant — how insignificant can best be seen 

 by reference to the normal chart, where the minute area beyond 

 wave-length 0^*4 represents the whole ultra-violet energy. 



We are accustomed to speak of the ultra-violet and 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 CT*3, 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 of the feeble rays 

 of this region by certain salts of silver, with which it can be 

 photographed (while the far greater energy below makes little 

 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 underestimated. Its real extent is so vast that we 

 should accustom ourselves to consider " in the infra-red 

 region " a wholly vague term, needing to be supplemented 

 with a description of the particular part of the infra-red 

 referred to. 



It may be well to epitomize the principal results of all these 

 researches as far as they have been here given. In general 

 they emphasize and extend our first conclusions. 



1st. 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 

 later how similar results are deducible from the prismatic 

 spectrum. 



2nd. By comparing the ordinates for high and low sun 

 in different parts of the spectrum we see that they grow 

 unequally, indicating an enormous systematic absorption, in- 

 creasing toward the ultra-violet and diminishing toward the 



