FLUORESCENCE. 55 



vessel. When that is done, you see the whole path of the 

 beam marked by this blood-red light This is a -very curious 

 phenomenon : what is the cause of it ? Sir David Brewster 

 seemed to imagine that the ultimate particles of the 

 substance reflected red light somewhat in the manner of 

 finely suspended vermilion. Suppose in fact you had a 

 fluid which was green by transmitted light, and you could 

 manage to form in that an excessively fine mud of ver- 

 milion, then it is conceivable that you might get a pheno- 

 menon of this kind ; I do not say that is the true cause, for 

 it is not. Brewster examined a number of substances, both 

 solutions and solid bodies, in a similar manner, and I may 

 mention one which is described by him in a later paper read 

 before the British Association in 1$38, a certain variety of 

 fluorspar. One of the varieties he mentions is a green kind' 

 as seen by transmitted light, found at Alston Moor in Cum- 

 berland, and I may mention that there is another variety, 

 which usually is purplish by transmitted light, which 

 abounds in Mr. Beaumont's lead mines at Allenhead, which 

 shows the phenomenon even better. This kind of fluorspar 

 shows a deep blue light in certain aspects. You see that to 

 perfection, if you plunge the spar into water, because then 

 you get rid in a great measure of the light reflected from 

 the surface. When a condensed beam of sunlight is ad- 

 mitted into the crystal, the path of it is marked by a 

 blue light. It is not, however, continuous, like the red light 

 in the green fluid, but it occurs in strata parallel to the 

 nearest faces of the cube. Evidently it depends upon some- 

 thing which took place during the growth of the crystal. 

 Possibly it may have crystallized thousands of years ago, 

 we know not how long, out of a solution, the nature of 

 which gradually changed as the crystal grew, and some sub- 

 stance probably was taken up by the crystal, to which this 

 effect is due. 



Some years later Sir John Herschel published a paper in 

 the Philos<>t>liical, Transactions, " On a case of superficial colour 

 in a colourless liquid," which was shortly afterwards followed 

 by a paper " On the epipolic dispersion of light. 1 ' Quinine as 

 you know is very much used in medicine and when a solu- 

 tion of quinine is formed, tolerably dilute, in water acidulated 



1 Philosophical Transactions, Jan. 1845, pp. 143, 147. 



