EXPERIMENTS IN CHEMISTRY 25 



ing through a second prism held at right angles to 

 the first one, although it can pass through the sec- 

 ond prism when it is held parallel to the first. The 

 ray has acquired the peculiar property of polariza- 

 tion in passing through the first prism. This is ex- 

 plained as due to the fact that the molecular struc- 

 ture of the crystal has reduced the undula- — 

 tions of the ray of light to one plane, thus: EEEEE; 

 The polarized beam can therefore pass — 

 through a second prism held in the same position 

 as the first, but when it is placed at right angles 

 to the first the light is quenched. 



Some crystals have the property of twisting or 

 rotating the plane of polarization of light which is 

 passed through them. If I have two prisms of 

 crystalline substance so arranged that the one cuts 

 off the light transmitted by the other one, and if 

 I place between them some substance which rotates 

 the plane of polarized light passing through the first 

 prism, it will be found that some of the light will now 

 pass through the second. The plane of polarization 

 has been changed and hence some light is able to get 

 through the second prism. The amount of rotation 

 can be determined by finding through how many 

 degrees the second prism must be turned in order 

 completely to cut off the light. 



