338 



Capt. Abney and Col. Festing. [June 21 y 



"We submit that a comparison of the thermograms of the solar 

 spectrum and those of the electric light points to the conclusion that 

 much the greater portion of the absorption in the infra-red of the 

 former is due to the presence of water-stuff. In the case of the 

 electric light it will be seen how large a proportion of the energy is 

 absorbed by even If inches of water, especially between \10,000 and 

 A21,000. If water in the state of vapour, as we believe to be the 

 case, absorbs as much energy as the same amount in a liquid state, 

 the sun's rays are absorbed to at least a corresponding extent in 

 passing through our atmosphere, which must, we believe, always con- 

 tain more than an equivalent of If inches of water. 



Now Langley's normal solar curve may be considered representa- 

 tive of a thermogram taken under the most favourable conditions 

 for dryness of atmosphere, and it cannot be supposed, even under 

 these circumstances, that between the spectroscope and the solar 

 atmosphere there could have been less than ^-inch of water (or its 

 equivalent in vapour), judging by the depression in it, Fig. 2, 

 Diagram IY, shows the general form of his curve corrected for the 

 absorption due to j- inch of water ; from which it appears that there 

 must be a large proportion of energy emanating from a source of com- 

 paratively low temperature. 



The prismatic thermogram of the spectrum from a source having a 



