Spectra of some of the Fixed Stars. 413 



should expect to discover indications of an atmosphere about the 

 moon, if such exist, agree in proving the non-existence of a lunar 

 atmosphere of sensible amount. From the absence of appreci- 

 able refraction at the moon's limb, and from the sudden extinc- 

 tion during a total lunar eclipse of stars of even the tenth and 

 eleventh magnitude at the limb of the moon, "we are," writes 

 Sir John Herschel, "entitled to conclude that no amount of 

 appreciable vapour is suspended near the surface of the moon, 

 and .... the non-existence of an atmosphere at the moon's 

 edge having the 1980th part of the density of the earth's 

 atmosphere"*. 



As by direct observation we know that the solar light is 

 reflected from the surface of the moon, the light which reaches 

 the earth after having undergone this reflexion must have passed 

 through, a length of lunar atmosphere, if such exist, at least 

 equal to double the height of such atmosphere above that sur- 

 face of the moon which is visible to iis. From some parts of the 

 moon, when the whole or a large part of its illuminated surface 

 is turned towards the earth, the length of the column of lunar 

 atmosphere which the solar light would have to traverse would 

 be considerably greater. 



The examination of lunar light by the spectroscope, and the 

 comparison of the light reflected from different portions of the 

 moon's illuminated surface with each other by this method, 

 would take place under conditions favourable to the detection of 

 an atmosphere of considerable extent, if such exist. 



The moon was examined by us on April 12 and November 26, 

 1862, March 31 and December 31, 1863, March 15 and 19, 

 and April 12, 1864. The solar lines were perfectly well seen, 

 appearing exceedingly sharp and fine. The line D was well 

 divided, and its components were observed to coincide with those 

 of sodium. Coincidence of the magnesium group with the three 

 lines forming b was also observed. The lunar spectrum is indeed 

 full of fine lines ; and they were well seen, from B to about half- 

 way between G and H. On all these occasions no other strong 

 lines were observed than those which are visible in the solar 

 spectrum when the sun has a considerable altitude. 



Previously to the observations of March 15 and 19, and April 

 12, 1864, the apparatus was directed to the sun when near the 

 horizon, and the relative positions and characteristic appearances 

 of the atmospheric lines in the orange and red were carefully 

 observed. These portions of the spectrum were closely scruti- 

 nized when the moon's light was afterwards examined; but no 



* Outlines of Astronomy, 7th edition, par. 431, p. 284. See also a paper 

 by Professor Challis in the Monthly Notices of the Royal Astronomical 

 Society, vol. xxiii. p. 254, June 1863. 



