VOL. XLIII.] PHILOSOPHICAL TRANSACTIONS. 57 



accordingly it will be found, that all such people have skins of a thickness or 

 density proportional to the whiteness or darkness of their colours. The particular 

 manner in which this opacity, or imperfect pellucidity of bodies is brought about, 

 Sir Isaac Newton explains in his Opt. prop. 2, where he shows, that the opacity 

 of bodies depends on 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 must suffer in passing through it, by which they will be extinguished, 

 in proportion to the number of such reflexions; that is, the more opaque, or 

 less white, it must appear: so that, though the particles, of which white and 

 dark skinned people are composed, may not be very different from each other, as 

 they seem not to be ; yet a greater number of such combined particles, or more 

 strata of them, in thick skins, and the smallness of their intervals in skins of a 

 dense texture, will increase the number of reflexions made in their internal parts, 

 or the opacity of them, which renders them less white, since their whiteness 

 proceeds from the number of the transmitted rays. 



In the same manner, by which we have accounted for the colours of tawny 

 people, may we account for the colour of those that are black ; for, if the skin 

 appears darker and darker coloured, the more the, rays of light are intercepted by 

 it, of course it must follow, that when the rays of light are entirely intercepted 

 by a body of the same structure, which the skins of negroes seem to be, it must 

 be quite black; for blackness always proceeds from a suffocation of the rays of 

 light, as those versed in the doctrine of light and colours are well acquainted 

 with; but we have shown above, prop. 2, that the skins of negroes transmit no 

 colour or rays of light through them, on account of the thickness of their sub- 

 stance, and density of their texture, in the same manner as they are imperfectly 

 transmitted in some white or tawny people, whose skins appear to be of the same 

 structure with those of negroes, and to differ from them in nothing but in degree 

 of thickness and density, and in colour; which different density may therefore 

 probably be one, if not the only cause of this difference of colour. So that the 

 thickness and density of the skins of negroes seems to be the grand cause of their 

 colour, in the same manner as it is of Indians, Moors, &c. Which may be 

 further confirmed by the following considerations: 1. In the cicatrices of their 

 ulcers, the thin and tender new-formed skin appears whitish, nay perfectly white 

 in some, especially on the shins, or those places where these cicatrices are thin; 

 but, where the skin is thick, or when these cicatrices turn more thick and cal- 

 lous, they grow blacker in proportion; as in those places where the scars grow 

 thicker than the rest of the skin, they are likewise blacker. 2. The colour of 

 the water, contained in the blisters of white people, may be plainly seen through 

 their cuticles, especially if tinctured yellow, which cannot be perceived in the 

 blisters of negroes : a plain proof, that their cuticles are not transparent, as those 



VOL, IX. I 



