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particles might be expected to be polarized by reflexion. And 

 such was, in fact, the case with those beams which were plainly 

 due to nothing but particles held in suspension. From the general 

 identity of the circumstances attending the two phenomena. Sir 

 David Brewster was led to conclude that epipolic was merely a par- 

 ticular case of internal dispersion, peculiar only in this respect, that 

 the rays capable of dispersion were dispersed with unusual rapidity. 

 But what rays they were which were capable of affecting a solution 

 of sulphate of quinine, why the active rays were so quickly used up, 

 while the dispersed rays which they produced passed freely through 

 the fluid, why the transmitted light when subjected to prismatic 

 analysis showed no deficiencies in those regions to which, with 

 respect to refrangibility, the dispersed rays chiefly belonged, were 

 questions to which the answers appeared to be involved in as much 

 mystery as ever. 



After having repeated some of the experiments of Sir David 

 Brewster and Sir John Herschel, the author could not fail to take a 

 most lively interest in the phenomenon. The firm conviction which 

 he felt that two portions of light were not distinguishable as to their 

 nature otherwise than by refrangibility and state of polarization, left 

 him but few hypotheses to choose between, respecting the explana- 

 tion of the phenomenon. In fact, having regarded it at first as an 

 axiom that dispersed light of any particular refrangibility could only 

 have arisen from light of the same refrangibility contained in the 

 incident beam, he was led by necessity to adopt hypotheses of so 

 artificial a character as to render them wholly improbable. He was 

 thus compelled to adopt the other alternative, namely, to suppose 

 that in the process of internal dispersion the refrangibility of light 

 had been changed. Startling as such a supposition might appear at 

 first sight, the ease with which it accounted for the whole pheno- 

 menon was such as already to produce a strong probability of its 

 truth. Accordingly the author determined to put this hypothesis to 

 the test of experiment. 



The experiments soon placed the fact of a change of refrangibility 

 beyond all doubt. It would exceed the limits of an abstract like the 

 present to describe the various experiments. It will be suflEicient to 

 mention some of the more remarkable results. 



A pure spectrum from sunlight having been formed in air in the 

 usual manner, a glass vessel containing a weak solution of sulphate 

 of quinine was placed in it. The rays belonging to the greater 

 part of the visible spectrum passed freely through the fluid, just as 

 if it had been water, being merely reflected here and there from 

 motes. But from a point about half-way between the fixed lines G 

 and H to far beyond the extreme violet the incident rays gave rise to 

 light of a sky-blue colour, which emanated in all directions from the 

 portion of the fluid which was under the influence of the incident 

 rays. The anterior surface of the blue space coincided of course with 

 the inner surface of the vessel in which the fluid was contained. 

 'Jlie posterior surface marked the distance to which the incident rays 



