502 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
five minima or passes may be observed, with hills intervening. The ordinates y of 
the first four of these minima correspond to the refrangibilities of the bright bands 
Nos. 2, 3, 4 and 5. The last minimum lies a little further on. Whether similar 
minima exist in the green range is not decided by observation, on account of the 
faintness of the green dispersed light. 
In the case of canary glass, the surface consists of five portions like mountain 
ranges running parallel to the axis of y, and having abscissae belonging to the red, 
reddish orange, yellowish green, green, and more refrangible green, respectively. These 
ranges do not all start from the immediate neighbourhood of the line L, but on the 
side towards the axis of x end almost in cliffs, at points at which the ordinate y is 
nearly equal to the abscissa of the fifth range, perhaps a little less. Thus the first 
three ranges are well separated from the line L. The ranges are intersected by a 
sort of valley running parallel to the axis of x, and having for its ordinate y the 
refrangibility of F^G. With the exception of the minima which occur where the 
ranges are intersected by this valley, the ridges run on very uniformly, and it is only 
very gradually that the ranges die away. 
The form of the surface which expresses the internal dispersion of a solution of 
sulphate of quinine, may be gathered from the description of that medium. In this 
case the surface resembles a rising country, not intersected by any remarkable 
mountain ranges or valleys. 
Fig. 4 is a rude representation of the internal dispersion in a solution of leaf- 
green. The curves represented in the figure must be supposed to be turned through 
90° about the lines on which they stand, and will then represent sections of the 
surface already described, made by vertical planes parallel to the axis of x. OL is 
the straight line bisecting the angle xOy. The figure is merely intended to assist the 
reader in forming a clear conception of the general nature of the phenomena, and 
must not be trusted for details. No attempt is made to represent the several maxima 
and minima in the intensity of the red beam of dispersed light. In any such 
figure, if we suppose homogeneous light to be incident on the medium, and wish to 
lay down the place of the falsely dispersed beam, we have only to draw a straight 
line parallel to the axis of x, through the point in the axis of y which corresponds 
to the refrangibility of the incident light, and find where this line cuts the straight 
line OL which bisects the angle xOy. 
On the cause of the clearness of fluids, notwithstanding a copious internal dispersioji 
which they may exhibit. 
86. It has been already remarked, that though water holding a water colour in 
suspension makes an admirable imitation of a highly sensitive fluid, when the latter 
is viewed by dispersive reflexion alone, the two fluids have a totally diflTerent appear- 
ance when viewed by transmitted light. The cause of this difference appears to be 
plain enough. The light due to internal dispersion emanates from each portion of 
