386         ON THE CHANGE  OF  REFRANGIBILITY  OF   LIGHT.
most striking law relating to internal dispersion is not obeyed in the case of phosphorescence.
In the same paper Professor Draper remarks, "Some time ago I determined the refrangibility of the rays of an electric spark which excite phosphorescence in sulphuret of lime ; they are found at the violet extremity of the spectrum," In what way Professor Draper determined the refrangibility of rays with respect to which glass is so opaque, he does not give the least hint. Being perfectly in the dark as to the evidence on which the conclusion is based, I cannot accept it in contradiction to my own experiments. Perhaps, however, "at the violet extremity" may mean nothing more than somewhere in the highly refracted region beyond the visible rays. If so, Professor Draper's statement is in accordance with my own conclusions.
222.    When one part of a phosphorus has been excited, the phosphorescence is found gradually to extend itself to the neighbouring parts*. In this respect a substance which exhibits internal dispersion presents a striking contrast.    The finest fixed lines of the spectrum are seen sharply defined, whether in a solution, or in a clear solid, or on a washed paper.
223.    Of course, theoretically, there ought, to a certain extent, to be a communication of illumination from one part of a sensitive fluid to another, on account of the  light which  is twice, three
as it had such an important bearing on the subject, and he immediately tried it. We obtained, however, only a negative reHult, as the Canton"h phosphorus which had been acted on by bright light which had been panned through a Holutiou of bichromate of potash did not give out in the dark any sensible light. Long afterwards, as I was engaged with Home experiments on Balmaiu's luminous paint, it occurred to me that possibly Draper's result might have boon due to a latent effect of a previous exposure to light; and I wrote to him to enquire whether thin might have been possible. He replied that he generally heated his phosphor! before proceeding to an experiment, in order to guard against the possible existence of a latent effect of previous exposure ; that at that distance of time he could not be certain whether that had been done in the particular experiment referred to; that if it had not, the result he had mentioned might have been brought about in that way.]
* [M. Becquerel afterwards explained this apparent result by attributing it to the increased sensitiveness of the eye arising from continuance in the dark, which enabled the observer to see outlying portions of a phosphorescing patch which were not seen at first, thereby giving the impression that the phosphorescence was extending.]