ON LIGHT. 383 



ness, and extent, falling to the share of that set which 

 the position of the analyzer most favours. 



(161.) It is manifest that these colours originate in 

 the interference of two series of undulations propagated 

 with different velocities within the crystal, and which 

 therefore must necessarily belong the one to the 

 ordinary, the other to the extraordinary, pencils into 

 which the incident light is divided, which, as before 

 shown, travel with different velocities within its sub- 

 stance. These pencils, however, during their progress 

 through it, are proceeding in different directions (by 

 reason of the double refraction of the medium) and are 

 oppositely polarized so that, while within the crystal, 

 they cannot interfere. Their interference, then, must be 

 accomplished after their emergence, when their direc- 

 tions have been again reduced to parallelism, and they 

 have been (wholly or partially) brought to a common 

 plane of polarization by the action of the second tour- 

 maline. Let us, therefore, examine how this is brought 

 about. And first, along the vertical arm of the black 

 cross, the whole of the incident light being polarized in 

 the plane of a vertical section of the crystal containing 

 its axis, will pass into the ordinary pencil, and none into 

 the extraordinary so that there will be nothing to in- 

 terfere with it : and emerging wholly polarized in that 

 plane, will be wholly stopped by the analyzing tourma- 

 line the result being darkness. But if this tourmaline 

 be turned 90 round in its own plane, it will be wholly 

 transmitted, and the arm of the cross in question will be 

 white. As regards the horizontal arm of the cross, in 



