ig 6 Movements of Jupiter's Cloud-Masses . [April, 
Mr. Knobel observed the shadow of white cloud-masses, 
and as the shadow had not so much greater a length at that 
time, two months from opposition, as it had when the planet 
was much nearer opposition, he infers that the true explana- 
tion of the appearance has hardly been found. He appears 
to have overlooked the faCt that the assumption made in the 
explanation is not that Jupiter has a semi-transparent atmo- 
sphere always equally translucent and penetrable to the 
same depth by the solar rays. When the shadow was 
shorter than it should have been, had the atmosphere been 
in the same condition as when Mr. Brett made his observa- 
tion, it is probable that a layer of clouds interrupted the 
rays, and thus the shadow was much closer to the cloud- 
mass throwing it than it would have been had that layer 
not been there. Mr. Knobel’s paper contains very striking 
evidence of the variability of Jupiter’s atmosphere, or rather 
of the clouds which float in it. “ The greater distinctness 
of the satellites when near the edge,” he says, “ is a curious 
phenomenon which has been repeatedly observed by astro- 
nomers, but which seems to require explanation.” On an 
occasion described “ the second satellite transited a dark limb 
which was ” [seemed] “ most dark near the centre, and 
fainter towards the edge, yet the satellite was almost invi- 
sible when on the darkest part of the belt, and was bright 
and distinct when the background of the belt was faintest.” 
This practically proved that on the occasion in question the 
dark central part of the belt seemed darker than it really 
was, by contrast with the bright belts on either side, while 
the edge seemed lighter than it really was, by contrast with 
the dark sky on which the planet was projected. In reality 
the part near the edge must have been darker than the part 
near the middle, or the satellite could not have appeared 
brighter when near the edge. No doubt the darkness near 
the edge (which, by the way, my friend Mr. Browning tested 
photometrically, and demonstrated, at my suggestion, eight 
years ago) was due to transparency, the darkness of the sky 
beyond being to some degree discernible through the edge. 
But this transparency is not always to be observed to the 
same degree, or through the same extent of Jovian atmo- 
sphere as to depth. Mr. Knobel proceeds, illustrating 
this the more effectively that he does so unintention- 
ally : — “ The third satellite, on March 25, 1874, appeared 
as a dark spot when in mid-transit, and on nearing the 
edge appeared as a bright spot without trace of duskiness. 
But on March 26, 1873 ” (observe the difference of years), 
“ the fourth satellite made the whole transit as a dark spot, 
