VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 231 



among whom may be placed the learned Mr. Delaval. This gentleman has lately 

 published, (in the Manchester Memoirs, vol. 2) a very elaborate treatise to prove, 

 that the colours of opake bodies do not arise from the rays of light which they 

 reflect from their anterior surfaces; but from that portion of it, which, having 

 penetrated their anterior surfaces, is reflected by the opake particles which are dif- 

 fused through their substance. But had the learned author not believed, that no 

 European writer on colours, before Sir Isaac Newton, contained any valuable infor- 

 mation on that subject, he would probably have discovered, that both Kepler and 

 Zucchius had long ago maintained the very opinion which he now advances, and 

 that they had built it on experiments similar to his own. The merit of the inven- 

 tion of this theory belongs therefore to the great Kepler; but still much praise is 

 due to Mr. Delaval, both for reviving and confirming it; since, though it be not 

 free from defects in some of its parts, it affords solutions of several optical diffi- 

 culties, which, as far as I know, admit of an explanation from no other source. 

 Among these I regard the phenomenon which is the subject of the present inquiry. 



To show then, from the theory of Kepler, Zucchius, and Delaval, how common 

 air and the neutral salts may brighten the appearance of blood, without producing 

 any change on its colouring matter, I shall first suppose that all its parts have the 

 same reflective power. The consequence will be, that a mass sufficiently thick to 

 suffocate the whole of the light which enters it, before it can proceed to the pos- 

 terior surface, and be thence returned through the first surface, must appear black; 

 for the rays which are reflected from the first surface are without colour, and by 

 hypothesis none can be reflected from its internal parts. In the next place, let 

 there be dispersed through this black mass a small number of particles, differing 

 from it in reflective power, and it will immediately appear slightly coloured; for 

 some of the rays, which have penetrated its surface, will be reflected by those par- 

 ticles, and will come to the eye obscurely tinged with the colour, which is exhi- 

 bited by a thin layer of blood, when placed between us and the light. Increase 

 now by degrees the number of those particles, and in the same proportion as they 

 are multiplied, must the colour of the mass become both stronger and brighter. 



Having thus shown that a black mass may become highly coloured, merely by a 

 considerable reflexion of light from its internal parts; if I should now be able to 

 prove, that both common air and the neutral salts increase the reflexion of light 

 from the internal parts of blood, at the same time that they brighten it, great pro- 

 gress would certainly be made in establishing the opinion, that the change of its 

 appearance, which is occasioned by them, depends on that circumstance alone. 

 But the following observations seem to place this point beyond doubt. I compared 

 several pieces of crassamentum of blood, which had been reddened by means of 

 common air and the neutral salts, with other pieces of the same crassamentum, 

 which were still black, or nearly so; on which 1 found, that the reddened pieces 

 manifestly reflected more light than the black. One proof of this was, that the 

 minute parts of the former could be much more distinctly seen than those of the 



