88 FUNDAMENTALS OF SUBMIC RO SCOPIC MORPHOLOGY I 



sense, and the definition of optically positive and negative is in com- 

 plete conformity with the terminology customary in mineralogy. In 

 those cases, however, where the object shows anisotropic behaviour 

 towards Hght incident along the reference axis, crystal optics use 

 other definitions to describe the optical character, and the customary 

 terminology in gels is no longer identical with that in crystal optics. 

 Whenever there exists a direction of isotropy, this should be chosen as 

 reference axis. 



Systematics of double refraction. In most cases the micellar texture itself 

 is birefringent, because the chain molecules constituting the strands 

 of the structure are themselves anisotropic. This kind of optical aniso- 

 tropy is called intrinsic double refraction. In this case the double refrac- 

 tion of the gel cannot be reduced to zero by changing the refractive 

 index n^ of the imbibition Hquid; there is a residual double refraction 

 in the minimum of the curve for form birefringence : the intrinsic 

 double refraction of the substance. In all cases examined so far, the 

 micellar strands behave like optically uniaxial systems, or at any rate at 

 a first approximation. They possess, therefore, two principal refractive 

 indices, designated by n^ (extraordinary index along the fibre axis) and 

 no (ordinary index perpendicular to the fibre axis). The intrinsic double 

 refraction is accordingly ng — no. As a rule it is positive, but sometimes 

 turns out to be negative. In those cases where the intrinsic double 

 refraction is different from zero, the refractive index n^ in Wiener's 

 formula is to be replaced by the average value \ (ng + no) or, better 

 still, by \ (ng + 2no). 



Both types of form birefringence (positive composite bodies 

 with rodlet texture and negative composite bodies with layer texture) 

 may be combined with the three possibilities, positive, negative and 

 zero intrinsic double refraction. On the whole one can, therefore, 

 distinguish between six types of double refraction (Frey, 1924). 



Both the form and the intrinsic birefringence can be attributed 

 to the structure of the object, but the intrinsic double refraction is 

 caused by the much finer structure of the crystal lattice, whereas the 

 form birefringence results from the coarser colloid structure. Hence 

 the latter is as a rule smaller than the former. 



The intrinsic and the form double refraction are both due to 

 morphological properties, in contrast to the phenomenon of incidental 

 double refraction^ which becomes apparent when solid objects are sub- 



