421 
on the Colour of Blood. 
among whom may be placed the learned Mr. Delaval. This 
gentleman has lately published * 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 an- 
terior surfaces, is reflected by the opake particles which are 
diffused through their substance. But had the learned author 
not believed, that no European writer upon colours, before Sir 
Isaac Newton, contained any valuable information upon 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 upon experiments 
similar to his own. The merit of the invention 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 difficulties, which, as far as I 
know, admit of an explanation from no other source. Among 
these I regard the phaenomenon which is the subject of the 
present inquiry. 
To shew 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 upon 
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 posterior surface, and be 
thence returned through the first surface, must appear black ; 
* Manchester Memoirs, Vol. IT. 
3 1 2 
