396 Dynamic Theory. 



the optic axis, one gyrating to the right and the other to the left, one 

 answering to the ordinary and the other to the extraordinary ray of re- 

 fraction. When these two rays are passed through a nicol prism used 

 as an analyzer, one of them is refracted out of sight and the other trans- 

 mitted. When the Iceland spar is used the rays are separated, but not 

 so greatly but that both are transmitted. If the apparatus be arranged 

 so that the images of the two rays be superposed, one on the other, the 

 resultant effect is pure white, which it should be if the colors of the sep- 

 arated rays are complementary. Another thing remarkable in conse- 

 quence of its bearing on organic problems, is the fact that many liquids 

 can produce rotary polarization. < ' The effect was first observed in oil 

 of turpentine, but has since been found in most essential oils, in solutions 

 of sugar, dextrine, the vegetable alkaloids, camphoric and tartaric acid, 

 and the tartrates. " Some are laevogyre and some dextrogyre. Narco- 

 tine in alcohol and ether, sulphate of quinine in water with sulphuric 

 acid, and uncrystallizable cane sugar or molasses are laevogyre. The 

 solution of crystallizable cane sugar, and the solution of grape sugar 

 when prepared from the juice and before solidification are dextrogyre. 

 But if dry grape sugar be redissolved it is laevogyre. Crystallizable 

 cane sugar is made uncrystallizable by heat, and when this is done its 

 rotary polarization is altered from right to left. The rotary power of 

 many solutions is reversed by the addition of an acid. If to cane 

 sugar one-tenth of its volume of hydrochloric acid be added, at 150 tem- 

 perature, the rotary polarization is reversed. ( Refer to page 218 for 

 rotary polarization in glucose, &c. ) 



"Magnetization of Light. " This is the title given to the curious effects 

 of magnetism discovered by Faraday in 1845. "If any homogeneous 

 transparent body be placed under the influence of a powerful electro- 

 magnet, it will possess the property, while the magnetism is maintained, 

 of turning the plane of a ray of polarized light, traversing it in the di- 

 rection of a line joining the magnetic poles, in the same manner as such 

 a ray is turned by quartz or by liquids possessing the power of rotary 

 polarization." In this experiment a pair of nicol prisms are used, one 

 for a polarizer and the other an analyzer, with an electric lamp behind 

 the polarizer, and between the nicols is placed a piece of homogeneous 

 glass. The two ends of this glass are connected respectively with the 

 two poles of an electro-magnet. As long as the planes of the two 

 nicols are parallel, the light, polarized in the first will pass on through 

 the glass and the second nicol. But if the second nicol be turned over 

 90 it will intercept the light. If now a magnetic current be passed 

 through the glass the polarized plane of light is twisted over in the glass 

 so it can pass on through the second nicol. Thus the glass is tempor- 

 arily rendered either dextrogyre or laevogyre according to the direction 



