DOUBLE REFRACTION. 75 



the two refracted rays. These more general laws of double 

 refraction will be more fully considered hereafter. 



(95) We may now proceed to illustrate some of the more 

 remarkable effects of double refraction. 



If a rhomboid of Iceland spar, or any other double-re- 

 fracting crystal, be placed close to a small object as, for 

 example, a black spot on a sheet of paper it will be ob- 

 served that one of the images is sensibly nearer than the 

 other ; and that the difference of their apparent distances 

 changes with the thickness of the crystal, and with the ob- 

 liquity of the ray. 



This effect is easily accounted for. It is a well-known 

 principle of optics, that when an object is viewed through 

 a denser medium bounded by parallel planes as, for ex- 

 ample, a cube of glass the image is nearer to the surface 

 than the object ; the difference of their distances being to the 

 thickness of the medium, ag the difference of the sines of in- 

 cidence and refraction to the sine of incidence. This interval, 

 through which the image is made to approach, increases 

 therefore with the refractive power of the medium ; thus in 

 water it is one-fourth of the thickness, in glass one-third, 

 and so for other media. Now as double-refracting crystals 

 have two refractive indices, of different magnitudes, there 

 will be two images, at different distances from the surface. 

 In Iceland spar, the ordinary index is greater than the ex- 

 traordinary, and therefore the ordinary image is nearer than 

 the other. The reverse is the case in positive crystals, such 

 as quartz, in which the extraordinary index is the greater. 



(96) The refractions being equal at the two parallel sur- 

 faces of the rhomb, whether the refraction be ordinary or ex- 

 traordinary, the two rays will emerge parallel to the incident 

 ray, and therefore parallel to one another ; and the distance 



