Prof. Leslie on the Light of the Moon and the Planets . 395 
2. It may be shewn, that if a planet were a perfect sphere, 
and reflected like a mirror the whole of the incident light, it 
would always exhibit a round image of the sun, extremely small, 
indeed, but equally bright as that luminary, and only varying in 
size according to its relative position. On this hypothesis, our 
planetary system would have appeared only a group of minute 
suns, and not betrayed their dependence by any distinction of 
phases. From the principles of catoptrics, we learn that each re- 
flected ray would appear to flow from a virtual focus situate on 
the chord of each circular section of the planet, and at the dis- 
tance of one-fourth of that chord behind the refringent surface. 
The image so formed by the reflexion of the light of the sun, sub- 
tends the same angle at the middle of the chord as that luminary 
itself. We may deduce this simple theorem, That the diameter of 
a planet is to that of the solar image which it would refect at its 
superior conjunction , as its distance from the sun to the fourth 
part of his diameter. Hence, at her opposition, Venus would 
have appeared a lucid circle, bright as the sun, but with only 
the 317th part of her actual diameter. In approaching to the 
inferior conjunction, this circle, still preserving the same bright- 
ness, would gradually shrink into a point. For a like reason, Ju- 
piter, as a perfect mirror, would appear, at the superior conjunc- 
tion, a sun of only the 2712th part of its diameter, and at the 
inferior conjunction with the 1797th part of its diameter. It 
may hence be calculated, that if Venus, at her opposition, were 
to reflect the whole of the light received from the sun, her illu- 
minating power would not exceed the 3100 millionth part of 
that refulgent mass. By a similar computation, it will be found, 
that the quantity of light reflected by Jupiter at his superior 
conjunction, is only the 988 millionth part of the direct illu- 
mination of the sun, but at his inferior conjunction the 432 
millionth part. 
It is more interesting, however, to trace the operation of our 
moon. Had her surface acted as a perfect mirror, she would 
have appeared, after Change, a mere lucid spot, and gradually 
opened with the splendour of the sun, till it appeared at Full, to 
shine with a diameter equal to the 458th part of its real dimen- 
sion. Supposing the moon, therefore, to send back the whole 
of the incident light, this would be still attenuated 210,000 
times more than the solar beams. But the most singular result 
