Dr Brewster on the Phosphorescence qf Fluar-sjpar. 171 
tlier at right angles, then, whether the surfaces are parallel or 
not, the ray DG which proceeds from the ordinary refraction^ 
suffers only the extraordinary refraciion in the loxcer crystal^ 
and the ray CE which proceeds from the extraordinary re- 
fraction^ siffers only the ordinary refraction. But in all 
the other positions, infinite in number beside those which I have 
determined, the rays DG, CE divided themselves into two by the 
refraction of the lower crystal, so that from the single pencil AB 
there are formed four pencils, sometimes of equal brightness, 
and sometimes of different brightnesses, according to the diflPe- 
rent positions of the crystals, but which do not seem to have 
more light, when taken all together, than the single ray AB. 
‘‘ When we consider (the rays CE, DG remaining the same,) 
that it depends on the position of the lower crystal whether these 
rays are divided or not, while the ray AB is always divided ; 
we are obliged to conclude that the waves of light, in order to 
have passed through the first crystal, have acquired a certain 
form or arrangement, in virtue of which, by meeting the texture 
of the second crystal in one position^ they are capable of moving 
the two different matters which serve for the two kinds of re- 
fraction ; while by meeting the second crystal in another posi- 
tion^ they have not the power of moving any of these matters. 
But in what manner this happens, I have not been able to form 
any satisfactory conjecture.” 
( To be continued.) 
AaT. XXXI. — On a singular Developement of Crystalline 
Structure by Phosphorescence. 
In prosecuting the experiments on the phosphorescence of 
minerals, published in our last Number, we observed a very re- 
markable phenomenon in a specimen of blue fluor-spar. When 
a thin slice was cut from this specimen, so as to be transparent, 
it resembled a leaf with veins inclined to the ridge or central 
line which divided it into two parts. The central line, and 
several of the veins were colourless ; while some of the veins 
were of a deep amethyst colour, and others of a pale amethyst 
colour. 
