ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 285 
with which it shines is derived from the Sun. These shadows appear dark enough to 
imply a contrast of probably more than 1 to 3 between the areas covered by them and 
the surrounding bright surface upon which they are projected. Discs of shaded paper, 
having to the unassisted eye an angular diameter equal to the visual angle subtended 
by the shadows of the satellites when viewed through the telescope, projected upon a 
background reflecting three times as much light, may be described as “dusky” or 
“dark” in comparison, but scarcely “black,” which latter term is not unfrequently 
applied to the appearance of the shadows of the satellites, in telescopes with powers 
sufficient to give to their discs a visual angle of 2 or 3’. 
It cannot, with any degree of probability, be supposed, that, in ascribing this high 
albedo to Jupiter, we have been misled by using Lambert's theory in the reductions, 
for a departure from this theory, as respects the distribution of the light reflected from 
the planet, sufficient to account for a large excess of illumination from the full phase, 
would in all likelihood manifest itself in causing the margin of the disc to appear 
brighter than the centre, just as occurs with the Moon; but it is certain that the 
margin of Jupiter is much fainter than its centre, the distribution agreeing, in this 
particular, sufficiently well with the results of Lambert's theory. 
This is proved by the changes which the appearance of the satellites, particularly 
the third, undergoes, as they transit the disc. The third satellite has been repeatedly 
seen as a “dark” or “black” spot projected.upon the white surface of the planet; when, 
however, the entire transit is observed, it is always seen to enter upon the planet as a 
small white disc on a darker ground; very soon its brilliancy fades, and it is lost to 
sight, presently reappearing, farther advanced in its transit, as a dark or black spot, 
and continuing under this aspect until it has accomplished perhaps Hayes fourths of 
its journey, when it again repeats, in reverse order, the changes attending the entrance, 
and before its final egress it is white once more. 
The most curious phenomenon exhibited by the satellite under these circumstances 
is that in its central position it should become so dark as not to be distinguishable in 
blackness from its own shadow projected side by side with it.* 
The explanation commonly offered of this singular phenomenon, viz. that there are 
large spots upon the satellites which become visible when projected upon a bright back- 
* From observations made with the great refractor of the Observatory on a great number of occasions 
ms that the above or analogous changes of aspect are always presented by 
uring the past twelve years, it see : 
nu es du either disappear if projected on 
the third and fourth satellites in their transits. The first and ene — ' d 
dark belts, or if on white ones they become dark or dusky spots, Bu Iaviriebiy of a brilliant white when 
, 
close to the margin. 
vot Yin. — | 37 
