PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 529 
Imagine two curves drawn whose abscissae are proportional to and ordinates pro- 
portional to the ratio of I to for the first, and the ratio of I' to lo for the second. 
These curves will serve to represent to the mind the composition of the light trans- 
mitted through a stratum of the body having a thickness t, and of that reflected 
from the body when seen in mass. It is plain that the maximum and minimum 
ordinates in the two curves will correspond to the same abscissae ; but unless t be 
very small, so small as to be insufficient to bring out the colour of the medium seen 
by transmission, the maxima and minima will be much more developed in the first 
curve, whose ordinates vary as than in the second, whose ordinates vary as 
(§'-|-r)"^ If, then, the absorbing power be subject to fluctuations depending on the 
refrangibility of the light, the bands of absorption may be observed either in the 
reflected or in the transmitted light, but they admit of being better brought out in 
the latter. 
178. If the nature of the substance be given, q will be given. If now the body be 
of a loose nature, as for example blue glass reduced to a fine powder, r will be con- 
siderable. Hence, in accordance with the expression (b.), the quantity of light 
scattered externally will be considerable, but the tint will be but slight. If the 
powder be now wetted, the reflexions at the surfaces of the particles will be di- 
minished, r will be diminished, and, as appears from (b.), the quantity of light scattered 
externally will be diminished, but at the same time the tint will be deepened, since 
the chromatic variations of I' are increased. If the body be compact and nearly 
homogeneous, r will be small, and therefore very little light will be returned, except 
what is regularly reflected at the first surface. The tint of the small quantity of 
light which is reflected otherwise than regularly, will be somewhat purer than before, 
inasmuch as the chromatic variations of I' tend to become the same as those of q~\ 
On the nature of False Dispersion, and on some applications of it. 
179. It has been already stated that a beam of falsely dispersed light seen in a 
fluid has generally more or less of a sparkling appearance, indicating that it owes 
its origin merely to motes held in mechanical suspension. Sometimes, however, no 
defect of continuity is apparent. This is especially the case when two fluids are 
mixed together, of vvhich one contains in solution a very small quantity of a sub- 
stance which we might expect to be precipitated by the addition of the other, or 
when a slightly viscous fluid has remained quiet for a long time. If some part at 
least of a falsely dispersed beam be plainly due to motes, that does not of course 
prove for certain that there is no part which may have a different origin, and inay 
be essentially connected with true dispersion ; nor do the theoretical views which I 
entertain of the cause of the latter lead me to regard it as at all impossible that a 
beam polarized in the plane of reflexion, and having the same refrangibility as the 
incident light, may be a necessary accompaniment of true dispersion. However, 
observation, I think, points in a contrary direction ; for although more or less of 
3 Y 2 
