ON  THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.          319
83.     Although the law mentioned in Art. 80 is the only one which I have been able to discover, relating to the connexion between the intensity and the refrangibility of the component parts of the dispersed beam, which appears to be always obeyed, and which admits of mathematical expression, there are some other  circumstances  usually attending the  phenomenon  which deserve notice.
When dispersion commences almost abruptly on arriving at a certain point of the spectrum, the dispersed beam is very frequently almost homogeneous at first, and of the same refrangibility as the active light. If the dispersed beam, when first perceived, be decidedly heterogeneous, its refrangibility extends almost, if not quite, to that of the active light, so that it is difficult, if not impossible, to separate the beams of truly and falsely dispersed light. On the other hand, when dispersion comes on gradually, it is generally found that the refrangibility of even the most refrangible part of the dispersed beam does not come up to that of the active light.
Thus in the cases of the red dispersion exhibited by a solution of leaf-green, and of the orange dispersions exhibited by solutions obtained from archil and from the Mercurialis perennis, the dispersed light was at first nearly homogeneous, and of the same refrangibility as the active light. In the case of the green disper-                         !f
sions shown by a solution obtained from archil, and by canary glass, the dispersed light was heterogeneous from the first; but still, when it first commenced, a portion of it had nearly the same                        \i\
refrangibility as the active light. In a solution of sulphate of quinine the dispersion came on gradually, being perceptible when the active light belonged to the middle of the spectrum; and in this case the dispersed light consisted of colours of low refrangibility. The bright part of the dispersion however came on pretty rapidly, when the active light approached the extreme limit of the visible spectrum, and accordingly the dispersed beam consisted in that case chiefly of light of high refrangibility.
84.    The mode of absorption of any medium may very conveniently be represented by a curve, as has been done by Sir John Herschel.    To represent geometrically in a similar manner the mode of internal dispersion, would require a curved surface.    Let