Temperature of the Moon. 51 



memoir for reasons for estimating the total amount of the 

 reflected radiation as little more than \ of that emitted *. 



The memoir, of which an abstract is here given, contains 

 numerous subsidiary researches and observations for which 

 we must refer the reader to the original, only here mention- 

 ing two : — 



First. Temperature-correction of a rock-salt prism. This 

 investigation of the change of refrangibility due to tempera- 

 ture was carried only far enough to give such approximate 

 accuracy as served our immediate purpose, giving a value of 

 — 13" of arc for each degree Centigrade; but subsequent 

 measurements have materially modified this, and I give the 

 value now adopted, which is — 9 //# 5, the formula being 



d t o = d 20 o-(t-20) 10", 



where d = deviation, and t = temperature in Centigrade 

 degrees. 



Second. Comparison of the intrinsic intensity of solar 

 radiation with that of the electric arc in different parts of the 

 spectrum. This observation of the comparative intensity of 

 the sun and the electric light is given in the original memoir 

 only so far as to show that it brings independent evidence of 

 a large atmospheric absorption of the extreme infra-red rays 

 and enables us to estimate approximately the amount of this 

 absorption at each point in the spectrum. The observations 

 were not repeated to obtain such a thorough comparison as 



* Lord Rosse found that 87 per cent, of all the solar rays were trans- 

 mitted by a particular piece of glass which allowed 92 per cent, of solar 

 light to pass, and 12 per cent, of the total lunar beam. He attempted 

 from this to determine the relative amounts of the solar and lunar heat, 

 but felt obliged, in the then state of knowledge, to make the assumption 

 (which our subsequent researches have shown to be erroneous) that the 

 glass absorbed all the invisible rays, or that the lunar radiation contains 

 12 per cent, of luminous rays, instead of less than 5 per cent., which is 

 more nearly the actual case ; but when Lord Rosse's own observations 

 are reduced with the aid of the facts determined by the writers, and 

 representing the actually large trausmissibility by glass of the invisible 

 rays of shorter wave-length in the infra-red, his expression for the relative 

 value (which we will call oc) of the emitted part of the lunar radiation 

 (whose transmission by glass is presumed from observations on a Leslie 



0-87+0-016.r 

 cube to be 1*C per cent.) becomes TJT^ = TW) from which .r=7*2 



times the reflected solar part ; so that if Lord Rosse could have possessed, 

 at the time his reductions were made, knowledge as to the diathermic 

 properties of glass which has only been acquired since, his own observa- 

 tions would have given results for the relative amounts of reflected and 

 radiated heat in somewhat remarkable accordance with our own. 



E2 



