MOVEMENTS OF JUPITER'S CLOUD-MASSES. 87 



defined at the present time, and which seems to aiford evidence re- 

 specting the physical condition of the phinet. The large, white 

 patches which occur on and about the equatorial zone, and interrupt 

 the continuity of the dark belts, are well known to all observers, and 

 the particular point in connection with them to which I beg leave to 

 call attention is, that they cast shadows that is to say, the light 

 patches are bounded on the side farthest from the sun by a dark bor- 

 der shaded otf softly toward the light, and showing in a distinct 

 manner that the patches are projected 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 conclusion 

 that it is not seen at all in either of them, but that all we see of Ju- 

 piter 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 de- 

 fined 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 abso- 

 lute ; and one of the most rudimentary of them is that every body 

 has its light, its shade, and its shadowy 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 inten- 

 sity is uniform throughout in any given instance, and is not affected 

 by the foi-ra of the surface on which it is cast, whereas the shade is 

 distinguished by attributes of an opposite 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 con- 

 cealed; 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 distinctness toward its edge, according to the acute- 

 ness 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 niglit 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 affect the astronomical in- 

 ferences deducible from what Mr. Brett actually saw. I may note 

 that somewhat later Mr. Knobel observed the shadow of white cloud- 



