1 877.] Movements of Jupiter's Cloud-Masses. 195 
say, the light patches are bounded on the side farthest from 
the sun by a dark border shaded off softly towards the light, 
and showing in a distinct manner that the patches are pro- 
jected or relieved from the body of the planet. The evidence 
which this observation is calculated to afford refers to the 
question whether the opaque body of the planet is seen in 
the dark belts or the bright ones, and points to the conclu- 
sion that it is not seen at all in either of them, but that all 
we see of Jupiter consists of semi-transparent materials. 
The particular faCt from which this inference would be 
drawn is, that the dark sides of the suspended or projected 
masses are not sufficiently hard or sharply defined for 
shadows falling upon an opaque surface ; neither are they 
sharper upon the light background than upon the dark. 
The laws of light and shade upon opaque bodies are very 
simple and very absolute ; and one of the most rudimentary 
of them is that every body has its light, its shade, and its 
shadow, the relations between which are constant ; and that 
the most conspicuous and persistent edge or limit, in this 
association of elements, is the boundary of the shadow ; the 
shadow being radically different from the shade, in that its 
intensity is uniform throughout in any given instance, and 
is not affeCted by the form of the surface on which it is cast, 
whereas the shade is distinguished by attributes of an oppo- 
site character. Now, if the dark spaces adjoining the light 
patches on Jupiter, which I have called shadows, are not 
shadows at all, but shades, it is obvious that the opaque 
surface of the planet on which the shadows should fall is 
concealed ; whereas if they are shadows their boundaries 
are so soft and undefined as to lead to the conclusion that 
they are cast upon a semi-transparent body, which allows 
the shadow to be seen indeed, but with diminishing distinct- 
ness towards its edge, according to the acuteness of its angle 
of incidence. Either explanation of the phenomenon may 
be the true one, but they both lead to the same conclusion, 
viz., that neither the dark belts nor the bright ones are 
opaque, and that if Jupiter has any nucleus at all it is not 
visible to us. [It is obvious that the phenomena I have 
described would not be visible at the time of the planet’s 
opposition, and the first occasion on which I noticed it was 
the night of the 16th of April last] .” 
This reasoning, so far as it relates to the laws of light 
and shade and shadow, is of course altogether sound. Nor 
are there any points requiring correction which in any degree 
affedt the astronomical inferences deducibis from what 
Mr, Brett aClually saw* 1 may not© that somewhat later 
0 a 
