430 



POLARIZATION. 



FIG. 337. 



FTG. 338. 



668. Planes of Vibration in Sunbeam. If 



we imagine a sunbeam to be cut by a plane 

 perpendicular to the direction of the beam, 

 we may suppose the section to consist of 

 vibrations moving in every possible plane, as 

 represented by Fig. 337. It is not to be 

 supposed that all of these planes will inter- 

 sect at the same point. There will be many rays whose 

 planes of vibration are vertical, many whose planes of 

 vibration are horizontal, etc. 



669. Polarization by Absorp- 

 tion. If a sunbeam fall upon a substance 

 whose molecular structure allows vibrations 

 in only a particular plane, say vertical, the 

 substance may be compared to a frame with 

 vertical bars, as represented by Fig. 338. 



Such a frame or such a substance will absorb the rays 

 whose vibrations lie in a plane that is horizontal or nearly 

 so, convert them into absorbed heat, and transmit, as 

 polarized light, those rays whose vibrations lie in a plane 

 that is vertical or nearly so. A plate cut 

 in a certain way from a crystal of tour- 

 maline acts in such a way; it is called a 

 tourmaline analyzer. If the sunbeam fall 

 upon a substance that allows vibrations 

 in only a horizontal plane, the substance 

 may be compared to a frame with hori- 

 zontal bars, as represented in Fig. 339. Such a body will 

 quench all the rays whose vibrations lie in a plane that is 

 vertical or nearly so, and transmit, as polarized light, those 

 rays whose vibrations lie in a plane that is horizontal or 



FIG. 339. 



