INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 57 



length, and it has been seen that the refractive indices depend on the 

 direction of vibration of the electric vector. In fact, there are only 

 two refractive indices for light passing in this direction; the light vibra- 

 tion is resolved in the fibre into two vibrations, one parallel and one 

 perpendicular to the length of the cellulose chains, with vibrations in no 

 other directions. This will be illustrated experimentally later on. The 

 question now arises, however, as to the effect on the refractive indices 

 of a change in the direction of propagation of light through the fibre. 

 This is a very practical problem, for we are very unlikely always to meet 

 cells so beautifully organized that their cellulose chains lie strictly 

 parallel to the fibre length, as we have supposed up to now in the 

 theoretical examples considered. In addition, in any case, we often do 

 need to observe fibres in more than one direction. It becomes therefore 

 essential to oudine the variation of refractive indices with direction of 

 propagation. 



It has been seen that the refractive index for light vibrating along the 

 length of the cellulose chain is much greater than in any direction 

 perpendicular to it, and that this is understandable in terms of the 

 structure of cellulose. Similarly it should be noted that since the 

 molecular chains of cellulose consist of a series of rings which, although 

 somewhat puckered, are nevertheless rather flat, then for light pro- 

 pagated parallel to the length of the chains, the refractive index for 

 light VIBRATING parallel to the planes of the rings should be different 

 from that perpendicular to this plane. In other words, cellulose should 

 have a large, a medium and a small refractive index. So far as the writer 

 is aware, although there are vague statements in the literature that this 

 is actually true, there is no real evidence for the existence in cellulose of 

 more than two refractive indices in this sense; in fact it is difficult to 

 see how such evidence could be obtained. Cellulose can therefore be 

 considered, and is usually considered, to have only two principal 

 refractive indices. This is one example of the class of crystals called 

 uniaxial. 



If, in such crystals, the refractive indices are measured for some 

 direction of propagation other than those so far considered, then it is 

 found that all these refractive indices can be correlated in the following 

 way. Suppose an ellipsoid is constructed (Fig. 27) with half the major 

 axis numerically equal to n^, the largest refractive index, and lying 

 parallel to the direction of the cellulose chains, and with half the minor 

 axis numerically equal to «„, the smallest refractive index. Then, 

 remembering that the refractive index depends on the direction of 

 vibration of the light, the refractive indices for a ray of light running in 



