INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 53 



(Fig. 26). When the light is vibrating parallel to the line joining the two 

 atoms (Fig. 26(a)) then the polarizing effect of the light on one atom is 

 enhanced by the induction of the other (polarized) atom: dipole A 

 increases the polarization of dipole B and dipole B enhances the 

 polarization of dipole A. The polarization is therefore high. When, 

 however, the molecule is turned through 90° (Fig. 26(^)) each dipole 

 opposes the polarization of the other and the polarization is lower. In 

 the first position, therefore, the refractive index is high and in the second 



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Fig. 26. Structural asymmetry in a diatomic molecule and the refractive indices. 

 The direction of vibration of the light is parallel to the arrow. 



(a) When the line joining the atoms is parallel to the direction of vibration, the 

 polarization of each atom increases that of the other (since "unlike poles 

 attract"). 



(b) When this line is at right angles to the direction of vibration the polarization 

 of each atom reduces that of the other. 



one lower. If a crystal could be imagined composed of such diatomic 

 molecules, arranged strictly parallel to each other, i.e. in crystal form, 

 then the direction of the line joining them could be determined by a 

 study of the refractive indices of the crystal. In a fibre in which the chain 

 molecules lie parallel to each other and to the length of the fibre, it 

 would therefore be expected that the refractive index for light vibrating 

 parallel to the length of the fibre would be very much greater than for 

 light vibrating at right angles to fibre length; and this expectation is 

 fulfilled since the refractive index of such fibres parallel to their length 

 is of the order of 1-60 (n^) and at right angles to it 1-53 {nj. Irregu- 

 larities of structure and other effects can modify the refractive indices 

 considerably, but these will be considered later as occasion arises: it 

 should be noted here, however, that such irregularities will have the 

 effect of reducing the value of n^ and increasing the value of «„. By 

 observing the values of these two refractive indices, and their direction 

 in the cell wall, a good deal of information concerning structure can be 

 derived. Under certain circumstances, the direction of the cellulose 



