226 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
Strictly speaking, a value of u^ exceeding unity would indicate that Jupiter shines 
partly at least by native light. We must remember, however, in interpreting these 
results, that neither the observations nor the theoretical conditions assumed in discuss- 
ing them are entitled to implicit confidence. 
Experiments made upon the reflective capacity of various substances on the 
Earth's surface lead to the conclusion that the optical albedo of dark, weather-worn 
rocks, dry earth, sand and gravel, and of the Earth's landscape generally, approaches 
that of the Moon, so that in reflective power there is no considerable difference be- 
tween the two bodies. No substance was found of a whiteness comparable to that 
of the bright belts of Jupiter, still less to that of the brilliant spots which at times 
appear on its surface. 
The distribution of light over the disc of the planet has heen ascertained by means 
of transits of the satellites, which present very curious phenomena, only to be explained 
by supposing a disproportion greater than 6 to 1 between the brightness of the central 
regions and the margin of the planet. The observations which have been made in 
reference to this question are sufficiently well explained by this supposition, though it 
is difficult for the eye, at first sight, to recognize the disparity. The fact is of some 
importance in the determination of the albedo of the planet, since it accords sufficiently 
well with Lambert’s theory, and indicates that the application of the latter to Jupiter 
has not tended to an over-estimate of its reflective power. 
We will now proceed to a more detailed account of the experiments and process of 
reduction upon which the above conclusions have been founded. . 
The following numbers, although obtained from trials made for quite a different 
purpose, and affected to some extent by the unstable condition of the chemicals, the 
degree of clearness of the sky, &c., will serve to give a tolerably correct idea of the 
excess of chemical intensity of the light of Jupiter contrasted with moonlight, after 
due allowance for the respective distances of the two bodies from the Sun. 
The plates compared were taken by Mr. Whipple, or by his partner in business, 
Mr. Black, on the following dates; v representing the angles of elongation of the 
Moon from the Sun. 
1851, March 12, Moon, o= nr Exposure 10° to 15° Daguerrotype. 
s € 2 Jupiter, v = 175 Ki About as long as for the Moon, or a little longer. < 
1852, Feb. 20, Moon, v= 76 = 15° v 
* March 3,* “ v — 151 “ 6° to 8* E 
* Ina memorandum dated March 3, 1852, it is stated, as the result of other sier not coated | in detail, 
WEE EE 
