"The actual illumination of the lunar surface is not much superior to that of weathered sandstone rock in full sunshine. I have frequently compared the Moon, setting behind the gray, perpendicular façade of the Table Mountain, illuminated by the Sun just risen in the opposite quarter of the horizon, when it has scarcely been distinguishable 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 estimate 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 Jupiter, and different objects on the Earth.

=

1860, March 17th. 0h 30m, 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 24 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 screenglasses AK, the eye having been protected from daylight for about half an hour. After applying corrections similar to those employed in reducing the comparisons between Jupiter and the Moon, and allowing for the difference of aperture of the objectglass, we find,

* Outlines of Astronomy, (417), note, p. 272.

This is not entitled to much confidence for several reasons, but especially on account of the difference in color. The side of the house was also strongly illuminated by light reflected from surrounding objects, by which its brightness must have been considerably increased; the illumination of the atmosphere intervening would also have a similar tendency. Hence it seems probable that the denominator of the fraction expressing the relative albedos is too small; the experiment taken in connection with (51) will therefore be in keeping with previous results.

A distant hill-side of withered grass was not sensibly brighter than the bricks; a tract of woodland, without leaves, was darker. The brick was supposed to reflect about one fifth as much light as an equal surface of pure white paper.

1860, March 19. 0h 30m to 1h 0m, m. s. t. Clear, with haze near horizon. The same adjustments and screen-glasses as were used in viewing Jupiter.

Repeated comparison with brick wall as on 17th. The image seemed slightly more intense than that of Jupiter.

The side of a wooden house, painted white, and by subsequent trial found to be barely distinguishable from the purest white paper, gave,

Albedo of side of white house

Albedo of Jupiter, central element

A variety of objects in the landscape, — fields, bare or covered with withered grass, trees, rocks, fences, &c.,- in the azimuth opposite to the Sun's, and again nearly in the same azimuth with it, gave,

1860, March 23d. 1 m. s. t. Sky perfectly clear,- between cumuli. Adjustments, eye-piece, &c., as usual, but the aperture of the object-glass was reduced until the objects viewed through the colored screens were judged to have less brightness than Jupiter. From five trials, after applying the proper corrections, I find,

A sheet of white paper placed against the side of the house was found to be a good match in point of brightness. A disc of chalk, ground flat and painted with "Flake white," was, however, sensibly whiter.

As this was considered the most satisfactory of the trials made with the colored screens, the corrections which have been applied in the reductions are here added.

The intensity of the light of the two objects admitted by the focus diaphragm of 0in.0425 diameter, was judged to be the same when the ratio of the exposed areas of the object-glass was

The actual illumination of the side of the house by the direct light of the Sun at an incidence angle of 30°, added to the light received by reflection from surrounding objects, was estimated, experimentally, to be to the illumination which it would receive directly from the Sun, at perpendicular incidence, as 4 to 3; and the coefficient & has been accordingly applied to the ratio of the areas of the object-glass, so as to refer the comparisons to the maximum illumination of the object by direct sunlight.

For distinct vision of the house, the eye-piece, with the focus diaphragm, was moved 4in.4 from the object-glass, contracting the angle subtended by the diaphragm at the object-glass in the proportion 274.4 The coefficient for the correction of

270.0

1

1.016 *

the light of Jupiter for phase is 1.016. The altitude of Jupiter was 67°, and that of the Sun 45°. We must apply the coefficient to reduce the light of the

зов R2

1 1.045

planet for atmospheric extinction to the altitude of 45°. The ratio of the intensity of sunlight at the Earth and Jupiter is 27.7. The average brightness of the whole disc of Jupiter is less than that of the part included within the diaphragm, in the proportion 1: 1.23. Lastly, the central element of Jupiter, which receives sunlight at perpendicular incidence, is brighter than the average of its disc in the proportion 1.43 1.

The experiments were continued, and varied in the following way. The daylight was excluded from the dome, leaving only the object-glass exposed. The telescope being directed upon the object, the eye-piece lenses were removed, the focus diaphragm only remaining. The pencil of light admitted by the aperture in the diaphragm was then compared with the standard lamps, by equalizing the images of the two reflected

from glass globes, in the manner already described. It will be noticed that the aperture of the object-glass of the telescope has no influence in these experiments, and that no correction for its absorption is required, the proportion of light extinguished in passing through it being the same in all cases.

1860, March 26th. A fine clear sky. A few cumuli appeared before the close of the experiments. Aperture of object-glass, 5 inches. Focus aperture, 0.0425; angular diameter at solar focus, 32".47.

Telescope directed upon the silvered glass globe B2, diameter 3in.436, placed at a distance of 1336 feet from the object-glass, in a nearly horizontal line, azimuth 10° east of north, and the image of the Sun reflected from it, compared with the lamp S, diaphragm Z.

Telescope directed to southeast side of a white house, at nearly the same distance and azimuth. This side of the house receives the direct sunlight at a small angle of incidence.

Telescope pointed at the southwest side of the house, on which the direct light of the Sun falls at an angle of about 40°.

A screen of white paper, painted with "Flake white" and "Chinese white," was placed at the same distance, perpendicular to the sunlight.

Telescope pointed at a mass of trees without leaves, distant about 1000 feet, and 180° in azimuth from the Sun.

h

m

ft.

ft.

The flames of the lamps were occasionally examined and found in good adjustment. The globe B, was placed within a hollow tin cone, near its apex. The cone was well blackened to cut off dispersed light, and the globe was frequently altered in position so as to expose different parts to the Sun.

1860, March 28th. Sky nearly, though not perfectly clear, especially in the later Adjustments as on 26th. Globe B, in same position. Lamp S, dia

experiments. phragm Z.

At the same distance, a screen of "Whatman's" drawing-paper painted with “Flake white” was set up, perpendicular to the Sun's rays, affording a surface of a pure white without gloss, sensibly more brilliant than new-fallen snow.

Silvered globe B3, diameter 1.760, placed in the position described above for B2 .

A screen of black cloth, washed with a thin size of indigo, to destroy the gloss and afford a dead blue-black surface, was set up at the same distance.