470 HISTORY OF SCIENCE. 



directions. Now, the fact of the (plane) polarization of light is re- 

 garded by the theory as consisting simply of an actual resolution of all 

 the transversal vibrations into two perpendicular planes. It is easy to 

 rudely represent by models and diagrams how such effects occur in 

 polarization ; and, as we are about to place before the reader a repre- 

 sentation of a model intended to illustrate the experiment of the tour- 

 malines, we must guard him against supposing that in ordinary light the 

 vibrations are confined to any particular planes. On the contrary, 

 it is precisely in this circumstance that polarization consists. Fig. 212 

 therefore, as a model of a ray of light, would properly represent a 

 polarized ray ; but, if we can suppose that by a kind of anticipation 

 the vibrations in the ordinary ray are already referred to the two per- 

 pendicular planes into which the polarization really separates them, we 

 may illustrate by models some of the effects of polarization. Fig. 212 



B 



FIG. 212. 



represents a model of the effects of the tourmaline plates. A is the 

 first plate upon which falls the beam of ordinary light B A. Only the 

 vibrations executed in the vertical plane pass on ; those in the hori- 

 zontal plane are destroyed that is, their effect is in some way quenched 

 or absorbed by the tourmaline. When the ray which has passed the 

 first plate falls upon the second plate, it passes through that also if the 

 axes of the crystalline plates are parallel ; if they are perpendicular to 

 each other, as represented by the model, the ray does not pass. This 

 effect is indicated in the model by representing the plates as gratings, 

 which, according as they are placed in a vertical or in a horizontal posi- 

 tion, would allow or prevent one plane or the other to pass through. 



The discovery of double refraction in Iceland spar has already been 

 mentioned, and the theory of that effect explained (page 461). When 

 the two emergent rays into which a beam of ordinary light is divided 

 by Iceland spar are examined by a slice of tourmaline, it is at once 

 found that these two rays are polarized in perpendicular directions. 

 When the tourmaline is in a certain position the ordinary ray will not 

 pass through it, while the extraordinary ray is transmitted with maxi- 

 mum brightness. When the tourmaline is turned 90 from this position 



