528 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
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 solu- 
tion 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 absorb- 
ing 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 surface, 
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 direc- 
tion 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, 
I the intensity of the light at that depth, or rather the intensity of a given kind of 
light, so that the whole intensity may be represented by f\d^, being the refractive 
index in some standard substance. In passing across the stratum whose thickness 
is dt, suppose the fraction qdt of the light to be absorbed, and the fraction rdt to be 
reflected and scattered in all directions, then 
d\-= — {q-]rr)^dt. 
Integrating this equation, and supposing lo to be the initial value of I, when #=0, 
we have 
(a.) 
For the sake of simplicity, suppose the body viewed in a direction nearly perpen- 
dicular to the general surface ; and of the light reflected and scattered in passing 
across the stratum whose thickness is dt, suppose that the fraction n would enter the 
eye if none were lost by absorption, &c. Then the intensity of the light coming from 
that stratum would be nr\dt. But in getting back across the stratum whose thick- 
ness is t, the intensity is diminished in the ratio of lo to I. Hence if F be the inten- 
sity of the light actually entering the eye, 
dV=nri;^rdt=nrUe--^^'^^^^*dt. 
If we suppose the thickness of the body sufficient to develope all the colour which 
the body is capable of giving, the superior limit of t will be 00 , and we shall have 
r 
:L 
(b.) 
'2{q-\-r) 
177- The colour which accompanies ordinary reflexion being usually but slight, I 
shall neglect the chromatic variations of r. It is q which is subject to extensive and 
apparently capricious variations, depending upon the refrangibility of the light. 
