ON LIGHT. 239 



reflected is the only portion which can be rendered sen- 

 sible to sight or otherwise traced. But if transparent, a 

 very remarkable phenomenon occurs. The incident ray 

 is, as it were, split or subdivided at the point where it 

 meets the surface of the body ; one portion pursuing its 

 subsequent course outside of it, as a reflected ray, in the 

 manner above described ; the other within it, undergo- 

 ing what is called " refraction" being bent aside from its 

 former direction at its point of entry, after which it pur- 

 sues a straight course within the substance or " medium." 

 (23.) If the "refracting medium" be a liquid, a glass, 

 a jelly, or any substance in which no indications of 

 inequality of internal texture can be discovered no 

 signs of lamination or " grain" shown by a greater ten- 

 dency to split or " cleave" in one direction more than 

 another, this intromitted portion is single. The whole of 

 the refracted light pursues its course from the point of 

 its entry as one ray. The same is also the case when 

 the refracting medium belongs to the class of bodies 

 called " crystallized," or which present a definite " cleav- 

 age ;" provided the " primitive form" of their crystals 

 be either a cube, a regular octohedron, or a rhomboidal 

 dodecahedron, such as rock-salt, alum, or garnet. In 

 all other transparent crystals the intromitted portion of 

 the light divides itself from the moment of its entry into 

 two distinct rays, pursuing different courses, and present- 

 ing the phaenomenon known under the name of " double 

 refraction," such substances being called ** doubly- 

 refractive media," of which the substance called Iceland 

 Spar, or crystallized carbonate of lime, offers a beautiful 



