FLUORESCENCE. 57 



the quantity of light removed from the spectrum may not be 

 sufficient to show any dark bands of absorption in the blue 

 region, but if you repeated the process on the light, making 

 it pass through different vessels in succession giving out 

 this blue stratum at the surface of each, perhaps then you 

 would have sufficiently weakened the blue of the transmitted 

 spectrum to show what particular rays were taken out by 

 the fluid. Sir John Herschel however observed that when the 

 light had passed through a thin stratum of the fluid in the 

 first instance, though it resembled ordinary light when it 

 came out, it had lost its power, for some reason or other, of 

 producing this phenomenon. What the power was did not 

 at the time further appear. Sir John Herschel called the 

 phenomenon ejripnlic dispersion from a Greek word signify- 

 ing surface, and he called the light which having passed 

 through a moderate thickness of solution of quinine had 

 been shorn of the power of producing that effect, epipolized. 

 In one of his experiments he had occasion to throw 

 sunlight vertically downwards on the fluid, and in that case, 

 the light being pretty strong, he observed the blue colour 

 extending to a depth of half an inch or more into the solution. 

 It was much stronger at the surface, but extended a con- 

 siderable way down. 



After the appearance of Sir John Herschel's paper, Sir 

 David Brewster took up the subject and examined this 

 particular fluid, the solution of quinine, as he had done 

 the solution of the green colouring matter of leaves and 

 fluorspar, and various solutions, 1 and he found that when 

 a beam of sunlight, concentrated by a lens, was admitted into 

 a solution of the quinine in dilute sulphuric acid, the whole 

 of the path of the beam was marked by this blue light. At 

 the same time, if you repeat the experiment, you will see at 

 once that the blue colour is decidedly more copious in the 

 immediate neighbourhood of the first surface. He further 

 examined the beam as to its polarisation, by viewing it 

 through a rhomb of calcareous spar, and stated that a con- 

 siderable portion of it, consisting chiefly of the less refran- 

 gible of its rays, was polarised in the plane of reflection, 

 while the greater part, constituting an intensely blue beam, 

 was found to be unpolarised. It is almost impossible to get 



1 See the paper already referred to, Edinburgh Transactions, xvi., or 

 Phil. Mao. June 1848. 



