ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 231 
Jupiter and the Moon, the aperture of the object-glass was reduced, on the 12th of 
April, 1860, from 14.94 inches, with which the latter were taken, to 0.021 inch. 
The Sun's image then photographed in six fifths of the time required for the full 
Moon, the sky being clear and the conditions in other respects being alike for both. 
From this experiment it would appear that sunlight surpasses that of the Moon in 
actinic energy in the proportion | 
5 1494) E 
$ (oon x 0.805 — 340000. 
The altitude of the Sun was 50° and that of the Moon 28°. The coefficient 0.805 
has been applied as a correction for extinction by the atmosphere, assumed to be the 
same for the photographic as for the visual rays. ‘The aperture used for the Sun was 
‘a hole turned in a plate of brass; the edges were clean-cut and free from dust or ob- 
structions, but not polished. It is perhaps possible that the smallness of the aperture 
employed is an objection to this result. 
On the 7th of April, 1860, the sky being clear and the Sun’s altitude about 45”, it 
was found that a collodion plate exposed at the focus of the great refractor received 
images of equal intensity with the lunar photographs, under the following exposures 
and apertures. 
l. Side of a house painted of a pure white and presented to the Sun's rays at an 
angle of incidence of 45^; distant 1300 ft. Aperture reduced to 5 inches; time of 
exposure ¿ of that for full Moon's picture of equal intensity. 
2. Experiment repeated, and the ratio found to be ¿, comparing with the brighter 
parts of the Moon. 
3. Aperture changed to 9 inches. Exposure the same as for the Moon's average 
surface. Picture too faint. 
4. Aperture 9 inches; images of equal strength require 13 longer exposure than 
for the Moon. 
5. The landscape in the azimuth opposite to that of the Sun required three or four 
times as long an exposure as the Moon, but the sky in the horizon was much brighter 
than the Earth's surface. 
The above experiments are not entirely conclusive, on account of the uncertainty 
of the action of the chemicals; but there seems to be no doubt that the reflective 
properties of the Moon's surface for the chemical rays rather exceed than fall short of 
those of the main features of a landscape view on the earth. The only object which 
decidedly surpassed the Moon in photographic power was a very white surface, which 
