ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 239 
hemisphere is more easily photographed than the low lands of the northern hemi- 
sphere, and it has been supposed that this is another instance of a difference in chem- 
ical intensity which does not exist in the visual rays. The so-called lunar seas are cer- 
tainly, optically as well as chemically, fainter than the rest of the surface, and the more 
rugged and mountainous regions are brighter to the eye, just as they are chemically. 
The eye is, however, likely to mislead in such estimates without special precautions to 
aid its judgment. 
There is one peculiarity in the appearance of the full Moon, which deserves atten- 
tion in this connection. 
If the Moon were polished perfectly smooth, we should not see its limb at all, but 
only an image of the Sun, formed at a virtual focus by reflection from its surface; the 
visibility of its outline then is entirely due to its asperities, and the particular way in 
which these are disposed will have a great effect on the distribution of its light. 
Its surface ought, according to Lambert's investigation, to be brightest at the centre, 
and very decidedly faintest at the limb, but it is actually the reverse. We have before 
noticed the readiness with which the full Moon photographs at its margin, indicating 
the greater intensity of chemical light from that quarter; with a little attention, the 
eye recognizes the same property in the optical rays. When viewed at the proper 
time, the bright marginal rim of the full disc will be at once perceived, of nearly 
equal breadth throughout, say from 2' to 3’, and reaching entirely round the circum- 
ference. 
We have seen that Eulers view of the manner in which light emanates from the 
Sun requires that it should appear brightest at the circumference,* while Lambert 
makes the Moon brightest at the centre. In reality, these conditions are reversed, 
the Sun being brightest, and the Moon faintest, in the central regions. The condition 
of the Moon's surface, in this respect, seems to be an exception to the law of reflection 
from opaque substances, inferred experimentally by Lambert and Bouguer. It deserves 
notice, that the anomaly observed in the distribution of light over the lunar disc occurs 
also, though less decisively, in the planet Mars, which is ordinarily brightest at the 
limb when near its opposition. Possibly the Earth presents a like aspect viewed from 
a distance. In the daytime we see the sky near the horizon brighter than most terres- 
trial objects projected upon it. The light reflected from a great depth of dense atmos- 
phere is therefore stronger than that reflected from the general landscape. The appear- 
ance of our Earth from a distance would be that of a dark body, seen through a thin, 
translucent atmosphere, which, if at the time free from clouds, will be at the edges 
* Lambert considers the Sun to be uniformly bright over its disc. 
