Fluorescence 395 



conception, although it turned out to be incorrect and will be con- 

 sidered later. 



In the meantime, David Brewster (1781-1868) , a Scottish preacher 

 of many talents, editor of the Ediriburgh Encyclopedia, and an 

 experimentalist in the field of optics (polarization by reflection 

 and double refraction) , became interested in colors. He described 

 (1833) the red color of a beam of white light passed through an alco- 

 holic tincture of leaves (chlorophyll) when observed from the side, 

 and compared it with the blue light coming from a light beam 

 passing through the interior of a crystal of fluorspar. Brewster had 

 been previously interested in the phosphorescence of minerals, espe- 

 cially on heating (1819, 1821) and had discovered (1820) a par- 

 ticular specimen of fluorspar with material arranged in layers or 

 strata, which gave different colored luminescences on heating. He 

 noted exactly the same colors when this specimen was traversed 

 by a beam of white light, but at that time did not realize that the 

 effect was an actual light emission. He spoke of the blue color of 

 ordinary fluorspar as a diffusion or scattering of light which " must 

 be produced by extraneous matter of a different refractive power 

 from the spar, introduced between the molecules of the crystal 

 during its formation." At that time (1838) the red light of chloro- 

 phyll and the green light of a Stramonium seed extract were also 

 spoken of by Brewster as dispersion. 



In the meantime, Herschel had continued his studies of fluorspar 

 and also of quinine and lignum nephriticum solution in a glass 

 vessel exposed to a beam of white light. In two consecutive papers 

 in 1845, he expressed the view that the bright blue color at the 

 surface, especially noticeable with quinine and lignum nephriticum 

 solutions, was " a case of superficial colour presented by a homo- 

 geneous liquid, internally colourless " and called the phenomenon 

 " epipolic dispersion," from the Greek, epipole, a surface. Had 

 Herschel used a more intense beam or a less concentrated quinine 

 solution, he would have seen that the blue color actually did appear 

 in the path of the beam in the interior of the liquid. With con- 

 centrated solutions, absorption of light is so marked where the beam 

 enters that all the fluorescence appears to be at the surface. 



Using a prism, Herschel also determined that only the blue end 

 of the spectrum, not the red end, gave rise to the " epipolic disper- 

 sion," whose spectral analysis showed blue, gieen, and a slight 

 amount of yellow, without dark or bright lines. No polarization 

 could be detected. 



Herschel's papers stimulated Brewster (1846, 1848) to emphasize 

 anew the fact that a concentrated beam of white light is blue within 



