396 Prof. Leslie on the Light of the Moon and the Planets, 
of the supposition of a perfect reflexion is, that we should never 
have discovered the magnitude of the moon, surveyed her varied 
surface, or distinguished her successive phases. She would have 
had the appearance of only a very diminutive sun, alternately 
approaching to the earth and receding from it, but never ad- 
vancing nearer than 458 times the distance of the real sun. The 
moon, therefore, does not strictly perform the office of a mirror, 
but scatters the incident solar beams in all directions, so that 
every portion of her surface looks more or less illuminated ; and 
she displays, according as the side fronting the sun turns towards 
our equator, all the gradations from a thin crescent to the full orb. 
3. The conclusion is hence incontrovertible, that if the pla- 
nets and our moon shine by light borrowed from the sun, they 
must have a rough or matt surface, like that of paper or plaster. 
It is only through the medium, indeed, of such surfaces, that 
we are enabled at all to distinguish the natural colours of bodies. 
The particles of incident light are not reflected on their mere 
appulse, but suffered to penetrate the external crust, where they 
are mostly absorbed ; while a certain portion of them is reject- 
ed, and again discharged. The rays thus remitted, after un- 
dergoing such internal secretion, serve to indicate the constitu- 
tion of the substance in respect to colour. Even from polished 
bodies, besides the reflected light, there is always a large por- 
tion rejected after penetration. Thus a slab of polished marble 
partly represents the surrounding objects, and partly evolves its 
own native colours ; but hold it in a position extremely oblique, 
and it will reflect almost the whole of the incident rays, and act 
like a colourless speculum. On the other hand, the colours of 
any stone are brought out, or rendered brighter, by wetting its 
surface, the film of water, by its refractive power, bending the 
rays inwards, and thus facilitating their penetration. 
But the quantity of light sent back from a white matt surface, 
is in every case extremely small. According to Bouguer's experi- 
ments, paper and plaster discharge only the I50th part of the per- 
pendicular rays, and a much less portion of the light when the 
incidence becomes oblique. If the moon appeared of an uniform 
whiteness, we could hardly reckon more than the 300th part of 
the solar beams to be again rejected and dispersed ; but its sur- 
face being very irregular, and dimmed over by obscure spaces, this 
estimate may be reduced at least to the 500th part. Hence that 
