148 ANNUAL OF SCIENTIFIC DISCOVERY. 



for the light emitted from the latter cause is red, besides which it is 

 totally different in other respects from regularly reflected light. In 

 conclusion, the author observed that the general fact of the reflection 

 of colored polarized pencils had been discovered by Sir David Brews- 

 ter in the case of chrysammate of potash ; and in a subsequent com- 

 munication he had noticed in the case of other crystals the difference 

 of effect depending upon the azimuth of the plane of incidence. 

 Accordingly, the object of the present communication was merely to 

 point out the intimate connection which exists (at least in the case of 

 the salt of quinine,) between the colored reflection, the double absorp- 

 tion, and the metallic properties of the medium. 



Specimens of Sensitive Media were exhibited by Prof. Stokes. 

 These were : - - A crystal of green fluor spar, which, by the develop- 

 ment of blue light within it, changed its color ; the solution of the 

 common bi-sulphate of quinine in acidulated water, which, by its 

 action on the invisible rays developed blue light ; and the solution of 

 the green coloring matter of leaves in alcohol, which by a similar 

 action became blood red. 



PROF. STOKES' RESEARCHES ON LIGHT. 



THE researches of Prof. Stokes took their origin from an unex- 

 plained phenomenon discovered by Sir John Herschel, and communi- 

 cated by him to the Royal Society in 1845. A solution of sulphate 

 of quinine examined by transmitted light, and held between the eye 

 and the light, or between the eye and a white object, appears almost 

 as transparent and colorless as water ; but when viewed in certain 

 aspects and under certain incidences of light, exhibits an extremely 

 vivid and beautiful celestial blue color. This color was shown by Sir 

 John Herschel to result from the action of the strata which the light 

 first penetrates on entering the liquid ; and the dispersion of light 

 producing it was named by him epipolic dispersion, from the circum- 

 stance that it takes place near the surface by which the light enters. 

 A beam of light having passed through the solution was to all appear- 

 ance the same as before its entrance ; nevertheless, it was found to 

 have undergone some mysterious modification, for an epipolized 

 beam of light meaning thereby a beam which had once been trans- 

 mitted through a quiniferous solution, and had experienced its disper- 

 sive action is incapable of further epipolic dispersion. In speculating 

 on the possible nature of epipolized light, Prof. Stokes was led to 

 conclude that it could only be light which had been deprived of certain 

 invisible rays which in the process of dispersion had changed their 

 refrangibility and had thereby become visible. The truth of this 

 supposition, novel and surprising as it at first appeared, has been con- 

 firmed by a series of simple and perfectly decisive experiments; 

 showing that it is in fact the chemical rays of the spectrum, more 

 refrangible than the violet, and invisible in themselves, which produce 

 the blue superficial light in the quiniferous solution. Prof. Stokes has 

 traced this principle through a great range of analogous phenomena, 



