ON THE CHANGR OF REFRANGIBILITY OF LIGHT.         413
or a secular change takes place in the incident vibrations which is equivalent to a succession of such independent series.
We are far from being able to subject molecular disturbances to strict mathematical calculation; for in the first place we are ignorant of the molecular structure to which the calculations would have to be applied, and in the second place even if we did know the structure, the calculations would very probably prove to be too complicated to be satisfactorily carried out. Nevertheless the calculation in some assumed system of a tolerably simple character which possesses some features in common with that which is supposed to lie at the base of some observed phenomenon may not be without use, as indicating in what direction we are to look, or are not to look, for an explanation of the phenomenon, hi the present cjiao if we vary the problem by merely supposing that the equidistant masses are of two magnitudes, coming Jilt,en»at.ely, a result is obtained having some features strikingly resembling those of absorption.]