76 Intelligence and Miscellaneous Articles. 



the marginal portions of its disk. Further, Zollner has found that 

 the albedo of Mars (that is, the mean reflective power of the different 

 parts of its disk) is not more than about one-half greater than that 

 of the lunar surface. Now these optical characters are in accordance 

 with the telescopic observation that in the case of Mars the light is 

 reflected almost entirely from the true surface of the planet. Jupi- 

 ter and Saturn, the light from which has evidently come from an 

 envelope of clouds, are, on the contrary, less bright at the margin 

 than at the central part of the disk. These planets have an albedo, 

 severally, about four or three times as great as that of the moon*. 



The anomalous degradation in brightness of the moon at the 

 phases on either side of the full, as well as the greater brilliancy of 

 the limb, may be accounted for by the supposition of inequalities on 

 its surface, and also by a partly regular reflective property of its 

 superficial rocks. Zollner has shown that if these phenomena be 

 assumed empirically to be due to inequalities, then the angle of mean 

 elevation of these inequalities must be taken as 52°. On the same 

 hypothesis the more rapid changes of Mars would require an angle 

 of 76°f. 



It appears to be highly probable that the conditions of surface 

 which give rise to these phenomena are common to the moon and 

 to Mars. The considerations referred to in a former paragraph sug- 

 gest that these superficial conditions represent peculiarities which 

 exist at the true surface of the planet. In this connexion it is of 

 importance to remark that the darker parts of the disk of Mars gra- 

 dually disappear, and the coloured portions lose their distinctive 

 ruddy tint, as they approach the limb. 



The observations of Sir John HerschelJ and Professor G. Bond§ 

 give to the moon's surface a mean reflective power similar to that 

 from a "grey weathered sandstone" rock. Zollner has confirmed 

 this statement. According to him, 



The albedo of the Moon = '1736 of the incident light. 



Mars = -2672 „ „ 



„ ,, Jupiter = *6238 „ ,, 



„ „ Saturn = "4981 „ „ 



,, ,, White paper = '700 „ „ 



White sandstone = '237 



From this Table it appears that Mars takes in for its own use '7328 

 of the energy which it receives as light. Jupiter's cloudy atmo- 

 sphere, nearly as brilliant as white paper, rejects more than six-tenths 

 of the light which falls upon it. Therefore less than four-tenths 



* Photometrische Untersucliungen, von Dr. J. C. Zollner. Leipzig 1865. 



t Ibid, pp.113, 128. 



X Outlines of Astronomy, p. 272. 



§ " On the Light of the Moon and Jupiter," Memoirs of the American 

 Academy, vol. viii. p. 222. In the same memoir Professor G. Bond esti- 

 mates the albedo of Jupiter to be greater than unity. This estimate would 

 require the admission that Jupiter shines in part by native light. — Ibid, 

 p. 284. 



