ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 261 
If the value y = 2 be adopted, the albedos of Venus and Jupiter will be nearly 
= 1, which can scarcely be true, since in this case the ‘planets must reflect the whole 
light incident upon them, while the whitest substances known on the Earth reflect less 
than one half of the incident light.* As regards Venus, we have mentioned reasons to 
show that a tendency to a regular reflection of sunlight from its surface may possibly 
explain its excess of brightness; but the case is quite otherwise with Jupiter, for here 
the planet is presented nearly at full phase, and in this position the quantity of light 
reflected from an opaque globe should be at least twice as great as from one of equal 
albedo reflecting regularly. Moreover, it is quite certain that the disc of Jupiter pre- 
sents gradations of brightness from its centre to the circumference, conforming suffi- 
ciently well with Lambert’s theory, and this’ circumstance gives additional confidence 
in the use made of his formula in obtaining the result before us. 
As the foregoing calculations cannot be readily followed through their details, and 
on this account may be open to some doubt as to their correctness, the following ex- 
periments of a different kind, though less precise, will be useful in confirming the 
results which have been obtained. 
The light from a small area of the Moon was compared with that of Jupiter by 
receiving the pencil from the focus of the object-glass of the 23-foot refractor upon a 
screen, or upon the silvered glass globe B,, described on p. 247, equalizing each in turn 
with the image of the light of a lamp provided with a diaphragm, and placed at a 
suitable distance. The relative brightness of the images reflected from the globe was 
easily varied by moving it towards or away from the focus of the telescope. "The min- 
utes of some other comparisons, principally relating to the absorption of the light of 
Jupiter by colored glass screens, are also added. 
1860, March 2d. Clear. Moon 41 days before full (v = 119°) compared with Ju- 
piter. Both near the meridian and at the same altitude. A brass diaphragm plate 
having a hole with an aperture of 0.0425, admitting a pencil of light 32.47 in diam- 
eter, was placed in the focus of the object-glass of the 23-foot refractor, and the tele- 
scope, with its aperture of 14".94, was directed alternately upon Jupiter and the Moon; 
the image of a disc of each object subtending the above-named angle was received, 
without the intervention of any eye-piece, upon a screen of black book-binder's muslin 
fastened to a square of pasteboard, and washed with a thin shade of indigo to destroy 
the gloss, and give a dead blue-black surface. The screen was moved from the focus 
until the light of the image projected upon it could just be discerned ; its distance from 
the focus was then noted, its position being fixed by three observers. 
* Beer, Grundriss des Phot. Cal., p. 70. 
