PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 501 
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 the refrangibility of light be measured as before, and suppose for simplicity’s 
sake the intensity of the incident light to be independent of the refrangibility, so 
that dy may be taken to represent the quantity of incident light of which the refran- 
gibility lies between 3/ siwd.y-\-dy. Considering the effect of this portion of the inci- 
dent light by itself, let x be the refrangibility of any portion of the dispersed light, 
and zdxdy the quantity of dispersed light of which the refrangibility lies between x 
and x-\-dx. Then the curved surface, of which the coordinates are x, y, z, will 
represent the nature of the internal dispersion of the medium. We must suppose 
the intensity of the incident light referred to some standard independent of the eye, 
since the illuminating power of the rays beyond the violet, and even of the extreme 
violet, is utterly disproportionate to the effect which in these phenomena they 
produce. 
From the nature of the case, the ordinate of the surface can never be negative. 
The law mentioned in Art. 80 may be expressed by saying, that if we draw through 
the axis of % a plane bisecting the angle between the axes of x and y, at all points 
on the side of this plane towards x positive, the curved surface confounds itself with 
the plane of xy. 
85. Let us consider the form of this surface in two or three instances of internal 
dispersion. For facility of explanation, suppose the plane of xy horizontal, let x be 
measured to the right, y forwards, and ^ upwards. Let a line drawn in the plane of 
xy through the origin, and bisecting the angle between the axes of .r and y, be called 
for shortness the line L. In all cases the surface rises above the plane of xy only to 
the left of the line L. 
In the case of a solution of leaf-green, the surface consists as it were of two 
mountain ranges running in a direction parallel to the axis of 3/, or nearly so. The 
first range, if prolonged, would meet the axis of j? at a point corresponding to the 
place of the dark band No. 1 in the red, or nearly so. The second would meet it 
somewhere in the place corresponding to the green. The green range is much 
broader than the red, but very much lower, and is comparatively insignificant. The 
ridge of the red range is by no means uniform, but presents a succession of maxima 
and minima. The range commences at the end nearest to the axis of x with a very 
high peak, by far the highest in the whole surface. In following the ridge forwards. 
