1895.] on the Radiant Heat from the Moon during an Eclipse. 633 



mature to put them forward when still unconfirmed. My excuse 

 must be that I have waited in vain for more than seven years for an 

 opportunity to test their accuracy and reliability. 



Commenting on our results on heat through glass, Langley says 

 that " The low transmissibility by glass is quite confirmatory of the 

 results of Lord Rosse, though not necessarily of his inferences from 

 them," and he goes on to say that it may be partly accounted for by 

 the supposition that the rays which reach us have suffered selective 

 reflection at the surface of the Moon, and in support of this he quotes 

 Sir John Herschel. " From the fact," he says, " that the lunar light 

 is not white like the Sun's, but yellowish (Sir J. Herschel compares 

 the Moon's surface to that of sandstone rock), it was antecedently 

 probable that such was the case." Langley, however, seems to have 

 mistaken the meaning of Herschel's remark. He says,* " Nor let it 

 be thought surprising that a solid substance thus illuminated should 

 appear to shine and again illuminate the Earth. It is no more than 

 a white cloud does, standing off upon the clear blue sky. By day the 

 Moon can hardly be distinguished in brightness from such a cloud, 

 and in the dusk of the evening appears with a dazzling splendour, not 

 inferior to the seeming brightness of the Moon at night." And in a 

 note he remarks, " The actual illumination of the lunar surface is not 

 much superior to that of weathered sandstone rock in full sunshine. 

 I have frequently compared the Moon setting behind the gray per- 

 pendicular facade of the Table Mountain, illuminated by the Sun just 

 risen in the opposite quarter of the horizon, when it has scarcely 

 been distinguishable in brightness [the italics in both cases are mine] 

 from the rock in contact with it." His remarks, therefore, appear to 

 refer to brightness rather than to colour. 



Langley, however, with the aid of prisms and lenses of rock salt, 

 obtained an absorption heat spectrum for the lunar rays, but such an 

 investigation is far beyond what we were able to attempt. I am 

 not quite clear at present, whether any bands of absorption were 

 detected in the heat spectrum of the Moon which do not equally 

 exist in that of the Sun. 



I still think that our inference is pretty nearly the correct one, 

 that the radiant heat from the Moon may be treated as consisting of 

 two portions — one, of the reflected rays of various refrangibilities in 

 about the same proportions as they exist in the direct radiation from 

 the Sun, the other of rays absorbed and subsequently emitted by the 

 lunar surface, and containing the rays of various refrangibilities in 

 the proportions due to a body at the surface temperature of the Moon. 

 Langley more recently inclined much to this opinion, and in a letter 

 to me, dated October 6, 1885, he says, " Since then " (that is the date of 

 my visit to Alleghany in 1881) " I have obtained so much additional 

 information by the study of the lunar heat spectrum, that I have 

 concluded to make a second memoir. While these observations 



* Herschtl'cj 'Outlines,' p. 272, edition 1858. 



