216 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
* The actual illumination of the lunar surface is not much superior to that of weath- 
ered sandstone rock in full sunshine. I have frequently compared the Moon, setting 
behind the gray, perpendicular facade of the Table Mountain, illuminated by the Sun 
just risen in the opposite quarter of the horizon, when it has scarcely been distinguish- 
able in brightness from the rock in contact with it. The Sun and Moon being nearly 
at equal altitudes and the atmosphere perfectly free from cloud or vapor, its effect is 
alike on both luminaries." * ` 
These observations acquire more weight from the circumstance that the light from a 
distant and elevated object, like the top of a mountain viewed in the same range 
with the Moon, is: modified by aerial perspective, so that the two have a similar 
quality of tone, and may be compared, in regard to their relative brightness, with 
greater confidence than if one were close at hand, and the other distant. This will 
be best recognized in attempting, in the daytime, to compare the light of an opaque 
object, held not far from the eye, with the Moon; the latter, owing to the intervening 
atmosphere, has a shadowy, unsubstantial look, which makes it very difficult to esti- 
mate the intensity of its light. In Herschel’s experiment, the mountain summit (if 
the observation was made from Feldhausen, as was probably the case) must have been 
some miles distant, and high enough to rise well into a pure atmosphere. The Moon 
at the time was a day or two past the full. 
We may infer, then, that there is no marked deficiency in the Moon, contrasted with 
the Earth, as to its capacity for reflection, since the “ gray weathered sandstone" of 
the top of Table Mountain would not, in this respect, be an inadequate representative 
of the Earth's general surface. 
The following experiments relate to the determination of the relative albedo of Jupi- 
ter, and different objects on the Earth. 
1860, March 17th. 0" 30", m.s.t. A thin haze in the sky. Observations were . 
made in the clear intervals. The 23-foot refractor, aperture of object-glass reduced to 9 
inches, was pointed at the side of a brick building, presented at right angles and distant 
21 miles, on which the Sun was shining at an angle of incidence — 50° (the angle which 
a line directed to the Sun makes with the plane of the face of the wall. With the 
same power and focal diaphragm employed in viewing Jupiter, March 15th,16th, 19th, 
&c., the light reflected from the brick wall could just be discerned through the screen- 
glasses A + K, the eye having been protected from daylight for about half an hour. 
After applying corrections similar to those employed in reducing the comparisons be- 
tween Jupiter and the Moon, and allowing for the difference of aperture of the object- 
glass, we find, 
* Outlines of Astronomy, (417), note, p. 272. 
