ON  THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.          321
following the ridge forwards, 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^Q. 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 a?, through the point in the axis of y which corresponds to s. in.                                                                         21