OF ARTS AND SCIENCES. 67 



exists in the optical brightness of this planet. The chemical albedo of 

 Jupiter, supposing the planet to reflect light after the usual manner 

 of opaque substances, exceeds that of the Moon in the proportion 

 of fourteen to one, the optical, in the proportion of eleven or twelve 

 to one. 



The experiments are open to the large uncertainties to which pho- 

 tometric comparisons are ordinarily liable ; but, assuming their correct- 

 ness, and that, as there is good reason to suppose, the proportion of 

 sunlight incident on the Moon which is absorbed at its surface, com- 

 pared with the amount reflected, is less than the smallest of the above- 

 mentioned ratios, it would follow that the planet shines in part by 

 native light, agreeably to the old notion of its phosphorescence. It is 

 difficult to put any other construction upon the experiments, provided 

 that Lambert's theory of the quantity of sunlight reflected to the 

 Earth from a planet is applicable in the case of Jupiter. Perhaps a 

 more acceptable explanation is, to suppose that its surface has the 

 property of returning towards the Sun a disproportionate amount of 

 the whole quantity reflected, taking ordinary opaque substances as a 

 standard. 



That this condition obtains with the Moon may be inferred from 

 the fact, that at the full, the mai'gin of its disk is brighter than the 

 central regions, indicating a peculiarity in the constitution of its surface 

 which would be likely to produce an excess of brightness at full moon. 

 It is, moreover, placed beyond question from a consideration of the 

 observed variations of the illuminating power of the different phases 

 of the Moon, of which a detailed account is given in the memoir, — 

 showing that the theoretical representations of the intensity of moon- 

 light, in its changes from new to full and vice versa, as investigated by 

 Euler and Lambert, bear no resemblance to the actual variations in 

 the amounts transmitted to us. As Jupiter always presents a nearly 

 full phase to the Earth, a similar property of reflection in its surface 

 would tend to explain the anomaly. There is, however, this objection 

 to that hypothesis : while the superior marginal brightness of the 

 full Moon, whatever may be its cause, would naturally lead us to 

 anticipate just that deviation from Lambert's theory of the amount of 

 illumination derived from it which is actually observed to occur, the 

 reverse order in the distribution of light over the disk of Jupiter, 

 namely, its regular increase from the margin to the centre, in very 

 good accordance with the same theory, is a strong argument for adopt- 



