384 ON LIGHT. 



like manner, the visual ray throughout its whole extent 

 is inclined to the axis in a plane at right angles to that 

 of the primitive polarization. The light, therefore, in- 

 cident in this plane through the first tourmaline will 

 pass wholly into the extraordinary pencil, and will there- 

 fore emerge polarized in a plane at right angles to the 

 (now) horizontal section of the crystal containing its axis 

 in which its direction lies, *>., again, in a vertical plane, 

 and will be stopped, for the same reason, by the second 

 tourmaline ; so that this arm of the cross also will be 

 black in the horizontal, and white in the vertical, posi- 

 tion of the analyzer. Let us now consider a ray inci- 

 dent in a plane 45 inclined to the vertical, or in a plane 

 intermediate between the arms of the cross (the axis of 

 the crystal being in all cases supposed held horizontally). 

 The incident ray then will fall on the crystal in a sec- 

 tion through its axis 45 inclined to that of its primitive 

 polarization, and will therefore be equally divided be- 

 tween the ordinary and extraordinary pencils. These 

 portions will emerge parallel, and of equal intensity, 

 though differing in phase by such a number of undula- 

 tions, and parts of an undulation, as the latter, by 

 reason of its greater velocity, has gained on the former. 

 In this state they are both incident on the second tour- 

 maline, having its axis 45 inclined to both their planes 

 of polarization, which therefore will subdivide each of 

 them into two equal portions oppositely polarized, sup- 

 pressing or absorbing one, and allowing the other to pass> 

 and the transmitted portions, being of equal intensity, 

 similarly polarized (viz., both in the plane of the axis of 



