OPTICAL I'lJul'MJTii.s m < RYSTAlfl 211 



linage when monochromatic light is used. Having measured the 

 angle of lea>t deviation, (he index of refraction is obtained from the 

 foniiul:i above. In :ill isotropic sul)st:i.nces n will he of the same 

 value, whatever the relation of the edge of the prism to the crystal 

 may lie. This is not the ease in ani<ot ropir substances, where 

 the prism edge must l>e cut with a definite relation to the axes of the 

 indicatrix. 



In the tetragonal and hexagonal systems, the edge of the 

 prism containing the angle a should be cut parallel to the base, 

 and t he plane bisecting the angle parallel to the vertical axis, or the 

 prism edge containing a may be cut parallel to the vertical axis. In 

 either case, in measuring the angle of least deviation, two signals will 

 appear, one caused by the ordinary ray, the other by the extraordi- 

 nary ray. These two readings substituted in the formula will 

 yield two indices of refraction, one that of the ordinary ray o>, the 

 ot her that of the extraordinary ray . 



In biaxial crystals, where there are three indices of refraction 

 to lie determined, two prisms are necessary. One must be cut 

 with the edge containing the angle a parallel to an axis of the in- 

 dicatrix, and the plane of symmetry of the indicatrix containing this 

 edge of the prism must also bisect the angle a. The second prism 

 must be cut in the same relation to a second axis and plane of the 

 indicatrix. Each of these prisms will yield two signals as in the 

 uniaxial prism and therefore two indices of refraction. One prism 

 will yield a and p, the other |3 and y. The index repeated or 

 determined in both prisms will depend upon the axes of the in- 

 dicatrix to which the edges of the prisms are parallel. 



II. Method of total reflection. Fig. 362 is a diagrammatic 

 section of the Abbe Total Reflectometer, in which C is a hemisphere 

 of .Jena flint glass, having an index of refraction n = 1.8904, the 

 tipper surface of which, b, is polished to a true plane passing through 

 the center of the sphere and adjusted so as to pass through the 

 axis of the vertical graduated circle from which the readings are 

 taken and which is not represented in the diagram. A polished 

 section S is cut parallel to a plane of symmetry of the indicatrix. 

 Often a cleavage surface will fill this requirement. A small drop of 

 a highly refracting liquid, usually methylene iodide, n = 1.742, 

 ha.ving first been placed on the center of the plane b, then the pol- 

 ished face of the section S is placed gently on the hemisphere with 

 a thin film of the highly refracting liquid separating the two sur- 

 faces. M is a mirror which reflects light in the required direction. 



