»fi 



NfeW PROPERTY OE REFLECTED LIGHT* 



parallel to the incident ray, because each ray undergoes ths 

 same kind of refraction at the two opposite faces. 

 Thefworays If "ow we receive on a second rhomboid, the principal 

 received oii section of which is parallel to that of the first, the two rays 

 tal are not di- ^^^^ have already pissed throngh this, they will rto longer 

 vidt^d, when be diiided into two pencils, as rays of direct light would. 

 iSi'ois ar?"*' The pencil from the ordinary refraction of the first crystal 

 parallel. will be refracted by the second according to the law of the 



ordinary refraction, as if this crystal had lost the faculty ©f 

 doubling images. In like manner the pencil from the ex- 

 traordinary refraction of the first crystal will be refracted 

 by the second according to the law of the extraordinary 

 refraction. 

 But by alter- If, the first crystal rernaining fixed, we turn the second 

 ingthepof!- so, that the face of incidence shall reraain p&rallel with 

 of the crynals itself, each of the two rays arising from the refraction of 

 they are di the first crystal begins to divide itself into two pencils; so 

 ' that one portion of the ray from the ordinary refraction, 



for example, begins to be refracted extraordinarily, which 

 andapainre- produces fouV images. Finally, after a quarter of a revo- 

 duced to two. lution, the pencil from the ordinary refraction of the first 

 crystal is entirely refracted extraordinarily by the second j 

 and, vice versa, the pencil, from the extraordinary refrac- 

 tion of the first crystal is wholly refracted according to 

 the ordinary law by the second ; which again reduces the 

 number of images to two. This phenomenon is independent 

 of the angles of incidei^ce, since during the movement of 

 tlie second crystal the refractive faces of the two rhomboids 

 preserve the same inclination toward each otiier. - 

 Distinction Thus the character that distinguishes direct light from 



' andTe'iracted I'K^'t that has been subjected to the action of a crystal 

 light. is, that ihe one constantly possesses the faculty of being di- 



vided into two pencils, while ir) the other this faculty de- 

 pends on the^ angle comprised between the plane of inci- 

 dence and that of the principal section. 

 Light affected This faculty of altering the cliaracter of light, and of im- 

 in the same . ', ' ^ i- i -^ • •.»•.• 



WHv by ;ill pvessmg on It a new property, which it carries with it, is 

 double refract not peculiar to the Iceland spar: I have found it in all 

 jBg^u ^^^"f*^'" ]j„own substances that double images; and, what la re- 

 markable in this phenomenon, it is not necessary for its 



production, 



