212 



MINERALOGY 



The limiting ray to, striking the surface of the section at O, is 

 totally reflected in the direction Ot'. The angle t'Oe is the critical 

 angle. Any ray, as mO, within this angle will at the point O be re- 



FIG. 362. 



fracted and pass out in the direction Ot", and some will be reflected 

 within the angle t'Oe, and the field t'Oe will be semi-illuminated. 

 Any ray, as 8O, striking the plane surface s at an angle greater 



than the critical angle 

 eOt', will be totally 

 reflected in the di- 

 rection of O8' and 

 the field a'Ot' will 

 be fully illuminated. 

 At the boundary 

 between 'these two 

 fields, t'O will be 

 marked by a shadow. 

 The field of the tele- 

 scope, represented by 

 the circle F, is brought 

 with the cross hairs 

 to the shadow, and 

 a reading which gives the critical angle eOt' is taken. Then 

 n = N sin r, where N is the index of refraction of the glass hemi- 

 sphere and r is the critical angle. 



This method has the advantage of the possibility of determining 

 all three indices of refraction in one and the same section. As 



