210 PROGRESS IN MICROSCOPY 



within the sphere. At the temperature obtaining when the exposure 

 was made, the indices of both eutectic and sphere were not equal. 

 The eutectic index is educed from the sphere's. 



9. MEASURING BIREFRINGENCE THROUGH A POLARIZING INTERFERENCE 



MICROSCOPE 



Polarizing interference microscopes are suitable for measuring bi- 

 refringence of anisotropic substances. This requires the direction of 

 the vibration, passing through the object, to be adequately defined. 

 Interference microscopes in which the light is linearly polarized, just 

 before impinging on the object, are suitable, i.e. the microscopes 

 shown in Figs. 3.19 and 3,24. The instrument, shown in Fig. 3.28, 

 is also adequate, provided the compensating system W^ be substituted 

 for a sht. Measurements are also feasible when the polarizer is placed 

 after the object since natural light can be considered as consisting 

 of two incoherent vibrations at right angles. The polarizer only lets 

 through one of the vibrations and phenomena occur as if defini- 

 tely-directed vibrations were passing through the object. 



In a given orientation of the incident wave, an anisotropic object 

 exhibits two rectangular directions in which vibrations may propagate 

 without change. To both these directions corespond two indices //' and 

 n". In an anisotropic uniaxial medium only, one of these two indices 

 is constant: the ordinary index, iiq. The other one fluctuates between 

 the ordinary and the extraordinary index, «„ and //^, respectively. 

 Birefringence of the medium is the rto—n^ difference. Let us take 

 a textile fibre as example: it acts as a uniaxial medium whose optical 

 axis is parallel to the fibre's. Let us assume it were possible to immerse 

 the fibre in a known-index liquid. The procedure to follow is de- 

 scribed in § 3. The fibre index is measured when the fibre is parallel 

 to the polarizer-originated incident vibration, this educing n^. The 

 measurement is resumed after rotating the fibre 90' about its axis so 

 that it is at right angles to the incident vibration, thus educing n^, 

 whence the birefringence n^) — Hg. 



10. CONCLUSION 



In the foregoing methods, the measurements shown referred to 

 simple-shape, diagrammatic objects. It follows that such measurements 

 are also applicable to objects of any shape as the measurement may 

 be applied to only a portion of the object. For instance, in the flat- 



