Royal Society. 389 



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 j)articular 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 improbalile. He was 

 thus compelled to adopt the other alternative, namel}*, to sui)pose 

 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 probabilitj' 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 ex])eriments. It will be sufficient 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. 

 The posterior surface marked the distance to which the incident rays 

 were able to penetrate before they were absorbed. This distance 

 was at first considerable, greater than the diameter of the vessel, but 

 it decreased with great rapidity as the refrangibility of the incident 

 rays increased, so that from a little beyond the extreme violet to the 

 end the blue space was reduced to an excessively thin stratum adja- 

 cent to the surface by which the incident rays entered. It appears 

 therefore that this fluid, which is so transparent with respect to 

 nearly the whole of the Aisible rays, is of an inky blackness with 

 respect to the invisible rays more refrangible than the extreme violet. 

 The fixed lines belonging to the violet and the invisible region beyond 

 were beautifully rej)rcsentcd by dark planes interrupting the blue 

 space. When the eye was properly placed, these planes were of 

 course projected into lines. The author has made a sketcii of these 

 fixed lines, which accompanies the paper. They may be readily 

 identified with the fixed lines rei^resented in M. Becquerel's map of 

 the fixed lines of the chemical spectrum. Tlie last line seen in a 

 solution of sulphate of quinine appears to be the line next beyond 

 thelast represented in M. Becquerel's map. Under very favourable 



