STRUCTURE AND LIGHT OF THE SKY. 255 



the case of an ordinary beam of light, the vibrations of the 

 ether-particles are executed in every direction perpendicular 

 to it ; but let the beam impinge obliquely upon a plane- 

 glass surface, as in the case of Malus, the portion reflected 

 will no longer have its particles vibrating in all directions 

 round it. By the act of reflection, if it occur at the proper 

 angle, the vibrations are all confined to a single plane, and 

 light thus circumstanced is called plane polarized light. 



A beam of light passing through ordinary glass executes 

 its vibrations within the substance exactly as it would do 

 in air, or in ether-filled space. Not so when it passes 

 through many transparent crystals. For these have also 

 their two-sidedness, the arrangement of their molecules 

 being such as to tolerate vibrations only in certain definite 

 directions. There is the well-known crystal tourmaline, 

 which shows a marked hostility to all vibrations executed 

 at right angles to the axis of the crystal. It speedily ex- 

 tinguishes such vibrations, while those executed parallel to 

 the axis are freely propagated. The consequence is, that a 

 beam of light, after it has passed through any thickness of 

 this crystal, emerges from it polarized. So also as regards 

 the beautiful crystal known as Iceland spar, or as double- 

 refracting spar. In one direction, but in one only, it acts 

 like a piece of glass ; in all other directions it splits the 

 beam of light passing through it into two distinct halves, 

 both of which are perfectly polarized, their vibrations being 

 executed in two planes, at right angles to each other. 



It is possible by a suitable contrivance to get rid of one 

 of the two polarized beams into which Iceland spar divides 

 an ordinary beam of light. This was done so ingeniously 

 and effectively by a man named Nicol, that the Iceland spar, 

 cut in his fashion, is now universally known as Nicol's prism. 

 Such a prism can polarize a beam of light, and if the beam, 

 before it impinges on the prism, be already polarized, in 

 one position of the prism it is stopped, while in another 



