C 5 2 3 ] 
particular Manner in which this Opacity, or imper- 
fect Pellucidity of Bodies is brought about, Sir Ifaac 
Newton explains to us (Opt. ib. Prop. II.) ; where 
he fhews, that the Opacity of Bodies depends upon 
the Multitude of Reflexions that are made in their 
internal Parts; but it is very plain, that the thicker 
the Skin is, the more Reflexions the Rays of Light 
muft fuffer in palling thro' it, by which they will 
be extinguifhed, in proportion to the Number of fucli 
Reflexions; that is, the more opaque, or lefs white, 
it mull appear : So that, ah ho' the Particles, of 
which white and dark skinn’d People are compofed, 
may not be very different from one another, as they 
feem not to be 5 yet a greater Number of fuch com- 
bined Particles, or more Strata of them, in thick 
Skins, and the Smallnefs of their Intervals in Skins 
of a denfe Texture, will increafe the Number of 
Reflexions made in their internal Parts, or the Opa- 
city of them ; which renders them lefs white, fince 
their Whitenefs proceeds from the Number of the 
tranfmitted Rays. 
In the fame Manner, by which we have accounted 
for the Colours of tawny People, may we account 
for the Colour of thofe that are black : For, if the 
Skin appears darker and darker colour’d, the more 
the Rays of Light are intercepted by it, of courfe it 
muff follow, that when the Rays of Light are intirely 
intercepted by a Body of the fame Structure, (which 
the Skins of Negroes feem to be) it muft be quite 
black j for Blacknefs always proceeds from a Suffoca- 
tion of the Rays of Light, as thofe verfed in the 
DoCtrine of Light and Colours are well acquainted 
with: But we have fhewn above (Prop. II.), that 
the 
