60 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



must remain constant, then the wavelength A must be different since 

 vX=c, the velocity of light. Hence, as will be clear from Fig. 29{b), the 

 two vibrations which pass into the wall in step, will emerge out of step 

 and will not reconstitute the same vibration PP as went in. The light 

 passing from the wall will therefore have a component along AA and 

 the wall will appear bright. If, however, the specimen is turned so that 

 the direction BC is parallel to PP, Fig. 29(c), then the vibration PP 



P 



A 



-4— 



a 



c 



A 



© 



P 





ic) 



Fig. 29 

 For explanation, see text. 



LM represents the wall thickness through which light is passing from left to 

 right. For convenience in drawing, both vibrations in the wall are made to lie 

 in the plane of the paper although they are actually at right angles to each other. 

 The two vibrations enter the wall at L in step, but leave at M out of step. 

 For explanation, see text. 



obviously passes unchanged and is therefore extinguished by the analy- 

 ser; the specimen is black. Similarly, when PP is made in turn parallel 

 to ED, CB and DE. These positions of darkness are referred to as the 

 extinction positions. Two of them, corresponding to the larger of the two 

 refractive indices, correspond to the major extinction position (referred 



