394 Prof. Leslie on the Light of the Moon and the Planets. 
This unexpected but important result is subject to no modifi- 
cation, except what may arise from the accidental loss of light 
during its passage through an intervening medium. No such 
obstruction, however, can occur in the celestial expanse, and, 
consequently, the proposition that a shining body, however re- 
mote, will appear always equally bright, is rigidly true in re- 
ference to the planets and fixed stars. Nor will it alter the ef- 
fect, whether such bodies derive the radiating power from their 
own substance, or from the mere influence of external illumina- 
tion. When the object lies so remote, however, that we can dis- 
cern no longer its visual magnitude, which seems contracted in- 
to a mere lucid spot, we confound the intensity of its brightness 
with the quantity of light received from it. Supposing the fixed 
stars to have all the same constitution, their lustre, as judged by 
our unaided sight, would be in the direct ratio of the squares 
of their diameters, and the inverse ratio of the squares of their 
distances. But in the case of a planet, the application of a 
powerful telescope will expand the radiating point into a broad 
surface, and thus enable us to distinguish easily the density from 
the quantity of illumination. If Venus and Jupiter were con- 
stituted alike, and shone by their native light, the apparent lustre 
of the former, at the period of their superior conjunction with 
the sun, would be ten times less than that of the latter, though 
their relative brightness, as disclosed by the telescope, would 
continue the same. But neither of these inferences will agree 
with observation. On the other hand, if those two planets de- 
rived their luminous quality from the sun, Venus would have 
had only the twenty-fifth part of the lustre of Jupiter, though 
the brightness of her surface, when viewed by the telescope, 
would have been five times greater. This conclusion approaches 
nearer to the actual appearances. 
But the different phases which the planets exhibit, according 
to the relative position of the sun, clearly prove that their light is 
merely dependent on the action of this great luminary. The ob- 
scuration which they periodically suffer, from the intervention 
of their satellites, indicates the same conclusion. The only 
question then is, to determine what changes the rays of light 
transmitted from the sun undergo at the surface of the planet. 
