INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 55 



of vibration of the light issuing from the polarizer. Observation of the 

 Becke line will then show whether the refractive index of the medium is 

 too high or too low. If it is too high, for instance, then a liquid of a 

 lower index can be tried until the refractive index of the fibre is "brack- 

 eted"; when by progressively narrowing the upper and lower limits, an 

 increasingly close approximation to the refractive index of the fibre can 

 be made. With care and under appropriate conditions, the refractive 

 index can thus be determined to the third place of decimals. To obtain 

 the necessary gradation in refractive indices it is necessary to use a 

 mixture of liquids and a number of liquids are available (12, 24). A very 

 useful combination is a-monobromnaphthalene and liquid paraffin 

 which fulfils the requirement that neither component shall swell the 

 fibre (see below) and each shall have about equal volatility so that the 

 refractive index of any mixture shall not change appreciably during the 

 observation. 



The interpretation of refractive indices 



Although, however, the technique of refractive index determination 

 does not present any great difficulties, the precise interpretation is not 

 always very easy. This arises in virtue of the heterogeneity which has 

 been shown to be a feature of the organization of cellulose (Fig. 25). 

 Since cellulose is composed partly of chains arranged quite regularly in 

 crystalline fashion and partly of chains with a varying degree of ran- 

 domness, then the refractive indices of the "micelles" will differ from 

 those of the non-crystalline material between, and therefore from the 

 refractive indices of the fibre as a whole. Most often, and particularly 

 in botanical research, it is the refractive indices of the micelles which 

 would seem to be wanted, since these give some idea of the regularity of 

 arrangement of the micelles with respect to each other; and so long as 

 the older ideas of discrete micelles remained feasible (Fig. 24) then it 

 seemed possible, by choosing non-swelling media which would neverthe- 

 less penetrate the intermicellar spaces, actually to determine micellar 

 refractive indices. Now we realize that this was an over-simplification 

 then it is obviously practicable only to measure the refractive indices of 

 the fibre as a whole, and this is the procedure always adopted. It is then 

 possible under certain feasible assumptions, to calculate the micellar 

 refractive indices, i.e. the indices which the fibre would have if it were 

 composed entirely of cellulose chains arranged in crystalline fashion. 



Theoretically there is another effect which must be taken into account, 

 an effect which bulked largely in the development of the micellar 

 hypothesis of cellulose structure but which is, however, of little practical 



