ON  THE  CHANGE  OF REFRANGIBILITY OF LIGHT.         387
times, &c., dispersed. This however must be excessively small; for the mean refrangibility of the dispersed light is usually much lower than the refrangibility of the active light, perhaps lower than that of any light capable of exciting the solution. However, generally some few of the dispersed rays would have a refrangibility sufficiently high to be dispersed again. But practically the intensity of the light twice dispersed in this manner would be so very small that it may safely be altogether disregarded.
224.    But by far the most striking point of contrast between the two phenomena, consists in the apparently instantaneous commencement and cessation of the illumination, in the case of internal dispersion, when the active light is admitted and cut off.    There is nothing to create the least suspicion of any appreciable duration in the effect.    When internal dispersion is exhibited by means of an electric spark, it appears no less momentary than the illumination of a landscape by a flash of lightning.    I have not attempted to determine whether any appreciable duration could be made out by means of a revolving mirror*.
225.    Then* appears to be no relation between the substances which exhibit a change of refrangibility and those which phosphoresce,  either  spontaneously, or  on  the  application  of heat. Thus the sulplnuvts of calcium and barium, on being examined for internal dispersion, were found to be insensible, as was also Iceland spar.    The last substance phosphoresced strongly on the application of heat.    So far as was examined, the minerals which did exhibit a change of refrangibility showed no special disposition to phosphoresce.    Sir David Brewater has remarked, that a specimen of fluor-spar which exhibited a blue light by internal disper-
* [Tlui experiment I had in my mind was to view in a revolving mirror the substance* to be examined while illuminated in a dark room by a succession of sparks from the prime conductor of an electrifying machine, taking one's chance for the caBiml appearance) of images iu the field of view. The experiment was afterwards tried with apparatus kindly lent me, but whether with sparks from a prime conductor or with an induction coil I am not now sure. Notwithstanding what M. Beequerel had in the mean time done with his beautiful phosphoroscope, the results obtained are not perhaps wholly without interest. Thus yellow uranite instead of its usual appearance showed a well-defined image of a very ordinary looking yellow stone and a long drawn out gleam of green light due of course to the fading phonphorescence.]
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