358 On Crystallography. 



There are various ways of observing the double refrac- 

 tion. One of the most simple consists in taking a pin by 

 the point, and presenting it against the window at a cer- 

 tain distance from the eye, against which we shall keep 

 At the sanie time the mineral applied bv one of its faces. 

 By making the pin assume various positions, we shall fin^ 

 that there is one in which we see two distinct images 

 of this pin parallel to each other, and generally prismatic 

 (?m%5*). Then, if we gently turn the pin until it is per- 

 pendicular to its first position, we shall see the two images 

 approach by degrees, until they fall upon one and the same 

 line, in such a manner, however, that one of the two heads 

 will frequently exceed the other. We may also m^ke use 

 of a card on which we have traced a line with ink of a 

 good tint. 



The separation between the images is more sensible, the 

 distance between the object and the eye and all other cir- 

 jcumstances being alike, when the diaphanous body used in 

 the experiment is of a greater thickness. And if we sup- 

 pose this thickness, in its turn, to be constant, and the ob- 

 ject removed from the eye, the two iniages will be more 

 and more removed from each other, at the same time that 

 they will be diminished in distinctness. 



The following is a third advantageous process for short- 

 sighted people. Place a lighted candle at a certain distance 

 in a dark room. Having afterwards made a hole in a card 

 with the point of a pin, apply it to one of the faces of 

 the stone, so as to make the hole correspond to a point of 

 this face ; then having approached with the eye the ppposite 

 face, seek the position proper for enabling you to perceive 

 the flame of the candle. You wilj then have the two 

 images distinct and well defined, because the effect of the 

 hole made with the pin is to dismiss the kind of irradiation 

 which dazzles them, when we employ the, stone by it- 

 self. : 



It would be difficult to find a character more prominent 

 than that which is drawn from the double refraction, since 

 it belongs to the very essence of the minerals in which it 

 exists. But ue cannoi always observe it on taking these 

 bodies in the natural state. Several require to be prepare4 



• When the double refraction is not ccnsiderahle, it may happen that 

 the two images touch each other. B«it, upon attentively exan\iniug the head 

 of the pin, we can distinguish at this place as it were two small circles 

 which intersect each other : and be^des, we shall observe that the same co- 

 four which edges on one side the prismatic band reappears on the line of 

 the middle part, where the same series reconmicuces. 



for 



