Sola?' and Lunar Spectrum. 517 



to 5 M , no heat appears below 5^ anywhere, not even at the 

 maximum near lo* and 1J/ 4 ; because, though heat exists to 

 the bolometer, the highest ordinate which would represent it 

 on our drawing is not so great as the thickness of the thin 

 black line, which denotes the axis of abscissae. I have accord- 

 ingly here been obliged to exhibit it separately by a dotted 

 curve whose ordinates are one hundred times those in the 

 remainder of the spectrum. The points of minima in it are 

 identifiable with absorption-bands, which we have directly 

 observed and which we have independently found to exist in 

 our own atmosphere by studying the radiations from a copper 

 surface, one metre square, at the temperature of boiling water 

 placed in the open air at a distance of one hundred metres 

 from the bolometer. (The description of this will be found in 

 a supplementary research to be given elsewhere.) The prin- 

 cipal lunar heat, then, is found here, at a point of the spectrum 

 corresponding to the maximum radiations from melting ice, 

 but its maximum amount is probably less than 1 per cent, of 

 the corresponding solar heat which we have just found to be 

 itself so small. That we can detect the lunar heat at all under 

 these circumstances is due to the fact that we are here able to 

 employ for it very short-focused mirrors and lenses, which 

 condense it into a very short and relatively hot spectrum 

 (there being no fear of their diffusing extraneous heat, since 

 none worth mention exists). In the case of the sun we must 

 employ a wholly different optical train, forming a far longer 

 spectrum. It would be easily understood that these means, 

 which enable us to determine the position of the solar and 

 lunar heat-maxima here, are not favourable to a determination 

 of the relative amounts of heat received from the sun and 

 moon under such different conditions. We can only say that 

 these ratios are themselves utterly changed from what they 

 are in the visible spectrum, where we all know that the solar 

 light is something like five hundred thousand times moonlight. 

 It is probable that the solar heat received in this part of the 

 spectrum is less than five hundred times the lunar; but the actual 

 ratio is only very roughly determinable by our present means. 



By comparison with the " heat ''-spectra given in a previous 

 memoir, we may also note the fact that some of the wave- 

 lengths given from ice are identifiable in the solar spectrum, 

 nor (in view of the now established facts that the ratios of 

 the heat at different parts of the spectra of two unequally hot 

 bodies are functions of the wave-length) need it surprise us 

 that we have also found that this part of the spectrum of the 

 sun is not incomparably hotter than the corresponding part 

 of the ice-spectrum. 



Phil. Mag. S. 5. Vol. 26. No. 163. Dec. 1888. 2 N 



