22 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



For the transmission of tlie large pane for lunar radiation were obtained the values: 



Per cent. 



Meaa of observation of November 20 12. 2 



Mean of observation of December 2 14.5 



Mean of observation of December 3 14. 9 



Mean of all 13. 9 



(the moon's zenith distance being about 45°.) 



Observations on the variation of the coetficient of transmissibility of the lunar rays at differ- 

 ent altitudes of the nioou have been made at every opportunity, Ijut the results are so dependent 

 on fluctuations in our atmospheric conditions that they are at i)re.sent only to be interpreted as 

 showing that, if there be a difference in transmissibility of glass for lunar rays at ditt'erent altitudes 

 of the moon, this difference is not a conspicuous oiu^. 



Among tlie substances, whose power of transmitting the lunar i-adiation was tested, was a very 

 thin disk of polished ebonite (thickness = 0.28 millimeters) through which the moon, when viewed 

 with the naked eye, ai)|ieared of a dark-red color. It was found by experiment with tlie bolometer 

 that tliis ebonite disk transmitted CO per cent, of the moon's rays. Its transmission of tlie solar 

 ra,')S was 32.4 per cent. The transmission of the large pane for solar rays was also carefully rede- 

 termined, with the following results: 



Per cent. 



Mean of observation of November 26 75. 6 



Mean of observation of December 3 75. 1 



These values are quite in accordance with those iireviously given. 



All of the later and more careful observations show, therefore, that, whereas nearly 70 per 

 cent, of the total apparent solar radiation is transmitted by the large pane of glass, only about 14 

 l)er cent, of the total ajjparent lunar radiation is transmitted. 



PRELIM IXAHY VBOTOMEiniC OBSERVATIOXS IN 18S:i. 



The low transmissibility by glass whicli the lunar rays have been shown to possess by the 

 experiments described in the first part of this paper is quite conftrmatory of the experimental 

 results of Lord Rosse, though not necessarily of his inferences from them. As we shall see, it 

 may be partly accounted for by the supposition that the rays which reach us have sufiered selective 

 reflection at the surface of the moon. It is quite evident that, if selective absorption of heat take 

 place, we ougUt to see it iu the study of those heat rays whiiih are also seen as " light." Moreover, 

 as rays emitted from a source eveu of the temi)erature of boiling water can have nothing to do with 

 vision, we shall not be liable to confound what we see with any efi'ect due to radiation from'the lunar 

 soil, for what we thus observe must be due to retlected heat only (since "light" and "heat "are 

 but names given to different manifestations of the same energy). Accordingly, if photospectro- 

 metric observ^ations on homogeneous rays show a progressive selective reflection such that rays of 

 low wavelength (such as are more absorbable by glass) are present in greater |)roportion after 

 reflection from the moon than l)efore, we shall undoubtedly be justified iu concluding that the 

 ettect observed by Lord Rosse is in part, at any rate, due to this cause, and not necessarily to the 

 presence of heat of low refrangibility radiated from the lunar soil. From the fact that the lunar 

 light is not white like the sun'.s, but yellowish (Sir J. Herscliel compares the moon's surface to that 

 of sandstone rock), it was antecedently probable that such was the case. The fact has indeed been 

 independently determined, but the writer was not familiar at this time with the work of others iu 

 this direction. The following apparatus, which was fitted u]) in June, 18S3, was employed in the 

 months of June, July, and October of that year for photometric comparisons of moon and sun 

 light. It is not described more minutely because all the observations were afterward rei)eated 

 with an imi)roved form of it hereafter described. 



The lunar beam, reflected from the siderostat mirror, passed into the dark roonr, fell on a 

 silveron-glass min-or of seven inches aperature and five feet focirs which formed a lunar image on 



