84 FUNDAMENTALS OF S U BM I C ROSC OP I C MORPHOLOGY I 



sign2itc2.smoufitmg\iqmd, or better as imbibition liquid. Hence the double 

 refraction changes with varying refractive index n of the mounting 

 liquid. For this reason the double refraction of such composite bodies 



s 



Fig. 6i. Dichroism of bast fibres (SE vibration plane of polarizer) from Frey (1927b). 

 a) Stained with chlorozinc-iodine : black/colourless; b) stained with gold: green (marked 



with little lines)/claret (dotted). 



differs essentially from the double refraction of crystals, which re- 

 presents a constant characteristic of the crystal. 



Fig. 62 shows the changes in double refraction observed when 

 epidermal hairs of incinerated barley awns are mounted successively 

 in air (n = i.oo), water (n = 1.33), alcohol (n = 1.36), xylene 



(n = 1.49), benzene (n = 1.50), Canada 

 balsam (n = 1.54), mono bromo naph- 

 thalene (n = 1.66), potassium mercury 

 iodide (n = 1.72). The variation of the 

 birefringence with the refractive index 

 no of the imbibition liquid obeys a hyper- 

 bolic law. The double refraction is zero 

 when n^ = n^ (n^ = refractive index of 

 micellar component). Composite bodies 

 with rodlet texture are optically positive, 

 those with layer texture are negative. 

 Wiener's formula for the rodlet bire- 

 fringence runs : 



in A 



1200 



900 



600 



300 



10 



1.2 



14 



1.6 



1.B 02 



Fig. 62. Curve of rodlet birefrin- 

 gence of the epidermal hairs of in- 

 cinerated barley awns (from Frey, 

 1926b). Abscissa: refractive index 

 ng of the imbibition liquid. Or- 

 dinate: retardation yk in A units. 



n 



n 



■1 



(^ 



i)n| 



+ ^2^1^ 



Here n^^ represents the extraordinary 

 refractive index (parallel to the axis of the composite body) and n_^ 

 the ordinary index (perpendicular to the axis), n^ the refractive index 



