Prof. Leslie on the Light of the Moon and the Planets , 893 
milk ; and that it is sufficiently delicate for all medico-legal pur- 
poses, as it will detect satisfactorily a quarter of a grain of arse- 
nic dissolved in 8000 parts of any of the foregoing fluids. 
Art. XXVI. — Pemarks on the Light of the Moon and of the 
Planets. By John Leslie, Esq. Professor of Natural Phi- 
losophy in the University of Edinburgh, and Corresponding 
Member of the Royal Institute of France. 
J^Iodern astronomers have generally assumed it as an evident 
truth, that all the planets and their satellites shine merely by the 
reflected light from the sun, the great source of heat and illumi- 
nation. But this conclusion, however probable and consistent, is 
too hastily drawn, and would require some discussion to establish 
it. Are those celestial bodies rendered luminous by the rays of 
the sun, or by the emission of their own native light ? Are the 
whole of the incident solar beams reflected by them, or a part only 
of those rays ? Is the light simply reflected which the planets 
transmit to us, or is it rejected and dispersed in all directions, 
after having entered their surface ? Or, lastly, is this light all 
emitted from their internal substance, in consequence of the ab- 
sorption and calorific action of the sun’s rays ? These are so 
many curious questions, which it is necessary to solve before we 
can form a correct opinion on the subject. 
1 . Since the rays of light are darted in straight lines, their 
mutual divergency increases continually as they proceed, and 
consequently their illuminating power must be inversely propor- 
tional to the square of the distance from the point of emission. 
The quantity of light which the pupil of our eye can receive 
from any shining body will, therefore, be in the inverse duplicate 
ratio of the distance ; but its visual magnitude, or the size of 
the image impressed on the retina, follows the same ratio ; and 
hence the object must appear to us with exactly the same degree 
of brightness at every distance. Thus, a candle removed 50 
yards from the eye, will seem just as brilliant as when placed 
only 10 yards from it ; because, though the eye receives 25 times 
fewer luminous particles, these are concentrated into a space of 
impression likewise 25 times smaller. 
