308 DOUBLE REFRACTION OF BLUE VITRIOL.- 



XI. 



Letter from Afr. Prieur to il/r. Guyton-Morveau, on 

 the double Refraction of Crystals of Sulphate of Cop- 

 per *. 



Blue vitriol J^ Have long suspected, my dear kinsman, that sulpliate 

 it's crystalliza- °^ copper, from the form of its crystallization, must exhi- 

 tion to have a bit the phenomenon of a double refraction ; but hitherto I 



double lefrac 

 tioo. 



have not been able to verify this conjecture, on account of 

 the general want of transparency of its crystals. At length 

 however I have succeeded in obtaining crj-^tals sufficiently. 

 This verified diaphanous for this observation, and found the double re-. 



fraction very manifest. 

 Modeofexa- I proceed in the known method, either viewing at some 

 minatioii. distance a pin between the crystal and the light, or looking 



at a luminous point produced by making a small hole in a 

 card. But sometimes I render the effects much move evi- 

 dent, by looking at an object more or less distant through 

 an opera-glass, between which and my eye I place the blue 

 crystal : or, which appears to me in general the best method, 

 I stand with my back to the window, and look at a very nar- 

 row slip of white paper, or a bright wire placed on a black 

 ground. 



A line joining the two images of a point observed with 

 the crystal of blue vitriol is always in a direction nearly 

 perpendicular to the edges of the prismatic surface of the 

 crystal ; and consequently it is requisite to hold these edges' 

 nearly parallel to the length of the pin or line we would see 

 double, in order to obtain the greatest eftect. ' 



The ci'ystals must be used before their surface is tar- 

 nished with that slight efflorescence, which they always ac- 

 quire after being kept a certain time. 



' - * Annales de Chimie, vol. LXV, p. 188. 



XII. 



