Double refraction proper

Now as regards the nomenclature one has so far called the double refraction appearing after a particular action on a system accidental double refraction thereby thus making a distinction with the double refraction already present in a given system, which is called proper double refraction, except in particular cases (only occurring in colloid systems) to which we shall return presently and which were included in the concept shape double refraction. 

Tile nomenclature depicted, which is based on phenomenology, leads to various difiScuIries, if one takes account of the nature of the phenomena The double refraction of a cellulose membrane, obtained from nature, for example, would be called proper double refraction, that of an artificial cellulose membrane, which one has obtained by stretching an isotropic cellulose gel, would on the contrary have tP be reckoned as accidental double refraction, while in both cases one is dealing with the same phenomenon Furthermore the nomenclature is not entirely consequential even from a phenomenological point of view, since the shape double refraction in many cases arises through an external intervention and then would therefore have actually to be reckoned as accidental ... 

Fig. 28. Combination of double refraction of oriented rods with the proper double refraction of the particles.

If the particles themselves are anisotropic, then one obtains throughout the proper double refraction which the particles have in addition. The proper double refraction from the nature of things can be both positive and negative in Fig. 28... 

A further description of this phenomenon is to be found in chap III, 3 b, p 109 and in vol II, chap IV, 7, p 115 chap V, 7, p 142 chap XIV, 3, p 693, 6 p 719 Magnetic and electric doul le refraction in liquid systems is also almost always an orientation double refraction See chap III, 3 c, p 113 If it concerns an orientation of molecules, then one has only to do with the proper double refraction of these molecules with particles of colloidal dimensions there is in addition a second component the shape double refraction ... 

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