66 CHEMICAL PHYSICS. 



through a crystal from one obtuse angle to the other. The two 



parts, after their surfaces have 

 been well polished, are joined 

 by a transparent cement of 

 Canada balsam. Fig. 27 rep- 

 resents a section of this prism. 

 DB indicates the line where the 

 crystal has been cut and ce- 

 mented. A ray of light passing 



Nicol's prism. fr m the b J ect tO th f side AD 



is doubly refracted in such a 



manner that the ordinary ray on striking the balsam is totally re- 

 flected to the side AB, and there refracted out of the crystal, while 

 the extraordinary ray passes on and emerges at the side BC as a 

 polarized ray. If this ray is now passed into a second NicoPs prism 

 parallel to the first, as if it were a continuation of the latter, it will 

 pass through unchanged. If the second prism be turned through an 

 angle of 90 degrees that is, if the two prisms be crossed the ray of 

 light will be cut off entirely. The ray in the second prism becomes 

 an ordinary one, and is totally reflected at the layer of balsam 

 through the side of the prism and is lost, as in the case of the first 

 prism. In intermediate positions between the crossed and the parallel 

 ones the extraordinary ray from the first prism is decomposed in the 

 second one partly into an ordinary and partly into an extraordinary 

 ray. The former is reflected out of the prism, while the latter goes 

 through, so that more and more light will pass through as the second 

 prism is turned so as to approach parallelism to the first. Thus it is 

 easily seen that a Nicol prism may serve not only to produce polar- 

 ized light, but also to detect such light. 



Polarized light is extensively used in the examination of minerals 

 and salts, as thin slices of crystals belonging to different crystallo- 

 graphic systems when brought between two NicoPs prisms show 

 rings or bands of colors characteristic of the respective systems. 



Many organic liquids and solids in solution have a peculiar action 

 on polarized light. Such substances, for example, are sugars, tar- 

 taric acid, alkaloids, essential oils, etc. If two NicoPs prisms are 

 crossed so that no light is emitted, and a solution of sugar, for ex- 

 ample, is placed in a glass tube between the prisms, light will pass. 

 The sugar turns the direction of vibrations of the light as it comes 

 from the first prism, and the effect is the same as if the second prism 

 had been turned with respect to the first one as described above, 



