ANGLE REFRACTOMETRY 



the following phenomena: (a) the displace- 

 ment of the Becke lines; (b) the Christian- 

 sen effect. 



The Becke Lines. The displacement of 

 the Becke lines when a crystal is immersed 

 in a liquid is explained as follows. When a 

 crystal is seen through the microscope, the 

 image of the edge will move away from the 

 crystal when the objective is moved upward, 

 and its index is smaller than that of the im- 

 mersion fluid. It will not move at all when 

 the refractive indices are equal. 



The method applies also to crystals other 

 than those of the cubic system. In this case, 

 several Becke lines are visible, but, by add- 

 ing a polarization attachment to the micro- 

 scope, all but one of these lines at a time 

 can be immobilized and the corresponding 

 refractive index thus determined. With posi- 

 tive uniaxial crj^stals one index, n,^, , the 

 lowest, is the same in all directions of the 

 crystal. The other index varies between the 

 value above and a maximum value Ue . The 

 inverse relationship holds for negative uni- 

 axial crystals. 



With biaxial crystals, both indices n^, and 

 Ue , respectively, vary with the direction, 

 but a maximimi and a minimum value also 

 can be recorded when a mixture of crystals 

 of various sizes is observed. However, in this 

 case the final interpretation requires another 

 set of observations with conoscopical light 

 (convergent polarized light). 



A very complete discussion, including all 

 operational details and bibliography, will be 

 found in Chamot and Mason (58). 



The Christiansen Effect. The Christian- 

 sen effect takes place when two contacting 

 media (a crystal immersed in a liquid) have 

 the same refractive index for a given wave- 

 length, but a different spectral dispersion; 

 the Becke lines then have the aspect of small 

 bright spectra. The image of the object is 

 surrounded by colored fringes moving in one 

 direction or the other with the movements 

 of the microscopic objective. 



Observation of the Christiansen effect 



leads to very valuable information concern- 

 ing the spectral dispersion of the material 

 under investigation, if that of the immersion 

 liquid is known. 



Extensive lists of immersion liquids which 

 have proved their worth exist in the litera- 

 ture (46, 56 58). Let us mention only the 

 tetrasodium dioxypentathiostannate of Jel- 

 ley, which, being soluble in water, is valu- 

 able for the study of water-insoluble organic 

 materials. 



The methods available for adjusting the 

 refractive index of the immersion liquid to 

 equal that of the immersed specimen are : (a) 

 the dilution method, utilizing known mix- 

 tures of liquids (tedious, but very accurate); 

 (b) the Merwin (46) method. Use is made of 

 the different spectral dispersions exhibited 

 by the liquid and by the sepcimen. A mono- 

 chromator is used to change progressively 

 the wavelength until the Becke lines seen in 

 the microscope disappear; (c) the Emmons 

 method, using the different relationship exist- 

 ing for the immersion liquid and the speci- 

 men, between the refractive index and the 

 temperature: the microscope must be pro- 

 vided with a hot-stage attachment; the 

 method can also be combined with method 

 (b); (d) the Leitz-Emmons hemispherical 

 refractometer, which fits over any standard 

 microscope. The specimen and immersion 

 liquid are enclosed in a small hemispherical 

 cell together with a concentric hemispherical 

 hollow rotatable from outside, and having a 

 different diameter. When the indices of the 

 liquid and specimen, respectively, have been 

 matched by adjusting the temperature of the 

 hot stage, the refractive index of the liquid 

 is immediately determined by rotating the 

 hemispherical hollow until the position of 

 total reflection of light is found ; the appara- 

 tus serves as its own refractometer. 



The crystallographic immersion methods 

 described here are applicable to metallo- 

 graphic and metallurgical specimens. Most 

 of them can be carried out on rough speci- 



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