ON  THE  CHANGE  OF  REFRANGIBILITY OF LIGHT.          359
mission, the maxima and minima will be much more developed in the first curve, whose ordinates vary as e~&_9 than in the second, whose ordinates vary as (q + r)~^l. 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 considerable. 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 diminished, 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 /' tend to become the same as those of q~^l.
On the nature of False Dispersion, and on some applications of it.
179. Tt 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 fiuids are mixed together, of which one contains in solution a very small quantity of a substance 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 may be essentially connected with