46 Prof. S. P. Langley on the 



passing the rajs through, glass before measurement, the rays 

 from the upper maximum passing freely, as rays belonging to 

 the maximum of solar reflected heat should do ; those in the 

 lower maximum, on the contrary, being absolutely cut off by 

 the glass, as rays of this wave-length from a source at a 

 temperature below that of boiling water are known to be. 

 This test of the glass employed by Lord Rosse in the direct 

 lunar beam, is here, it will be observed, applied at different 

 parts of the lunar heat-spectrum ; and its result in the latter 

 case, corroborating that already obtained by the respective 

 wave-lengths of the maxima, brings evidence of a radiation 

 of heat from the lunar soil at a temperature at any rate below 

 that of boiling water. 



Other observations furnish the means of computing the 

 relative absorption of the earth's atmosphere as exhibited in 

 extended cold bands in the region of the lower maximum. 

 Then, from a combination of the two, we are enabled to reach 

 a certain approximation to the position and magnitude of this 

 maximum as it would appear if the atmosphere had not inter- 

 vened. The existence of some of the principal atmospheric 

 cold bands in this region, due to the absorption exercised by 

 an atmospheric column 100 metres in length, has been quite 

 independently determined by means of the great radiator 

 already referred to. During moist summer weather two prin- 

 cipal maxima have been found in its spectrum — the larger at 

 deviation 37° 15', nearly agreeing with the lunar curve in 

 summer, a second smaller maximum at deviation 38° 45', and 

 between them a cold band with its minimum at deviation 

 38° 20'. Remembering that the unabsorbed spectrum- from a 

 radiating surface of lampblack, at the temperature of boiling 

 water, has its maximum at 38° 25', or very near the deepest 

 depression of the cold band, it will be recogized that we have 

 evidence of a considerable absorption at this point. 



To this must be added the fact, shown by our observa- 

 tions, that in the case of solids the greater part of the whole 

 heat is always found below the maximum of the (unabsorbed) 

 prismatic curve. If this law hold in the case of the sun, 

 since little heat is found below its actual prismatic maximum 

 (near deviation 39° 40'), the inference is that absorption in 

 that region (i. e. the extreme infra-red), must have been 

 great. 



Arguments on these different lines, combined with another 

 derived from a direct comparison of sun and electric-arc 

 radiation, which will be described farther on, enable us to 

 present a curve (fig. 1) showing, with the degree of ap- 

 proximation compatible to the first attempt in such a field, 

 the atmospheric absorption in all parts of the spectrum. 



