J^ NEW PROPERTY OF REFLECTED LIGHT. 



second surface will be modified in the same manner. If the 

 incident ray be such, that all its particles escape the partial 

 reflection and pass through the face of fentrance, they will 

 equally escape by traversing the face of exit. This new 

 property of light affords the means of measuring with pre- 

 cision the cjuantity of rays absorbed at the surface of diapha- 

 nous bodies, a problem, which the partial reflection tendered 

 almost impofesible to be solved. 

 Light reflected When a body, that produces a double refraction, reflects 

 troiii ihebur. the light at its first bOrface', it cdmpOrts itself like a cdm- 

 dcrbl^e- mon transparent substance. Tbfe light reflected underacer- 

 fraciiiig body, tain angle of incidence acquiffes the property I have de- 

 scribed ; arid this angle is independant of the position of 

 the principal section, which influences Only the double re- 

 fraction, or the reflections that take place in the interior of 

 the crystal. 

 Kavs reflected In- fact^ thfe rays that dre reflected interiorly at the second 

 interiorly ex- surface* exhibit peculiar phenomena^ which depend both on 

 phepdmena" ^he refractive power, and the properties of reflected light 

 that I have already described. 



When a pencil of light has been divided iilto two rdys 

 at the first surface of a rhomboid of calcareous spar, these 

 two rays issue out by the' second face in two pencils parallel 

 to the incident ray, because each of them experiences at that 

 face the same kind Of refraction as at the first face. It ist 

 not the same with reflected light. Though the ray re- 

 fracted ordinarily at the first surface is refracted ordinarily 

 ■ at the isecond, it is nevertheless reflected at this surfac6 int 

 (wo pencils, one ordinary, the other extraordinary. In like 

 manner the ray refracted extraordinarily is reflected in two 

 others; so that there are four reflected rays, while there are 

 but two emergent. These four rays, in returning to the 

 first face of the crystal, issue out in four parallel pehciUj 

 which make with this face the same angle as the incident 

 ray, but in a contrary direction, and are parallel to the 

 plane of incidence. To connect this kind of reflection with 

 that of double refraction, we must conceive at the two 

 points of emergence of the second face two incident rays, 

 making with this face the same angle as the emergent rays/ 

 bu't in the opposite direction. These two rays, by their re- 

 • ■ fraction 



