ON THE  CHANGE  OF  REFKANGIBILITY  OF  LIGHT.
357
example in the case of biniodide of mercury.    For the present I shall neglect the light which may have changed its refrangibility.
1*75. Few, I suppose, now attach much importance to the hold speculations in which Newton attributed the colours of natural bodies to the reflexion of light from thin plates. Sir David Brewster has shown how extremely different the prismatic composition of the green of the vegetable world is, from what it ought to be, according to Newton's theory, and what Newton supposed that it was. It is now admitted that the various colours of natural bodies are merely particular instances of one general phenomenon, namely, that of absorption. Absorption is most conveniently studied in a clear fluid or solid, but it does not the less exist in a body of irregular structure, such as a dyed cloth or a coloured powder.
The green colouring matter of leaves affords an excellent example of the identity of the effect produced on light by natural bodies and of ordinary absorption; for the same very peculiar system of absorption bands which are displayed by a clear solution of the colouring matter may be observed directly in the leaf itself. However, it is needless to bring forward arguments to support a theory now I suppose universally admitted; my present object is merely to point out the mode in which the colours which bodies reflect, or more properly scatter externally, depends upon the absorbing power of the colouring matter, so as to justify the conclusions deduced in Art. 142, from observations made in the manner there described.
176. Let white light be incident on a body having an irregular internal structure, such as a coloured powder. A portion will be reflected at the first irregular surfa.ce, but the larger portion will partly enter the particles, partly pass between them, and so proceed. In its progress the light is continually reflected in an irregular manner at the surfaces of the particles, and a portion of it is continually absorbed in its passage through them. For simplicity's sake, suppose the light incident in a direction perpendicular to the general surface, and neglect all light which is more than once reflected. Let t be the thickness of a stratum which the light has penetrated, / the intensity of the light at that depth, or rather the intensity of a given kind of light, so that the whole