56 MR. HORNER AND SIR D. BREWSTER ON A SUBSTANCE RESEMBLING SHELL. 
quest. The layers of mother-of-pearl are deposited in succession like those which are 
formed upon the dash-wheel ; and there can be no doubt that the animal whose mu- 
cous secretions form the shell that incloses it, rests occasionally from its toils, and 
affords a sufficient interval for the formation of an iridescent film upon the surface of 
the plate of shell which it daily deposits. Owing to the firm adhesion of the suc- 
cessive layers of the shell, we cannot, as in the more imperfectly formed new substance, 
separate each stratum in order to see the iridescent film upon their surfaces ; but we 
can easily determine what phenomena would be produced if the layers of the new 
substance were as transparent as those of mother-of-pearl. If this were the case, we 
should see, both by reflected and transmitted light, the combined colours of all the 
iridescent films in the plate. When these films are numerous and flat, and of various 
thicknesses, the union of all their colours would form a pearly whiteness by reflected 
light, and when films of a particular colour predominate, both the reflected and the 
transmitted light would exhibit that prevailing colour ; but if their surfaces are un- 
dulated as in mother-of-pearl, from the form of the shell and other causes, — if the 
iridescent films vary in thickness, and consequently in colour, — if they are wanting 
in some parts of the shell, and abound in others, — and if films of equal thicknesses 
occur in several laminae in succession, and films of other thicknesses in other laminae, 
which must necessarily take place from the varying and remitting action of the animal 
agent, then we shall have the very structure which is necessary for the production of 
the incommunicable colours of mother-of-pearl. 
I have no doubt that this is the true cause of the phenomena which had so long 
perplexed me ; and the results which I formerly obtained, though I could then attach 
no meaning to them, are in perfect unison with the preceding views. In order, how- 
ever, to obtain something like an experimental confirmation of this opinion,' I have 
examined the fracture of a mother-of-pearl shell where the laminae have been all 
deposited with considerable regularity, and where their overlying edges are exhibited, 
and I find distinct and positive proofs of the existence of iridescent films, sometimes 
green, and sometimes red in several successive strata. 
I am, my dear Sir, 
Ever most truly yours, 
D. Brewster. 
To Leonard Horner, Esq. 
