CHAPTER II. 
REFRACTIVE INDICES. 
Many methods have been suggested for measuring directly the refractive 
indices of minerals, but few of these have proved applicable to fine-grained 
minerals or minerals in the covered thin section.* For the accurate meas- 
urement of refractive indices it is, at present, necessary that at least one 
surface of the mineral plate be uncovered. In the covered thin section the 
petrologist is able, in general, to determine only the relative refractive in- 
dices of adjacent mineral plates and not their absolute values. With prac- 
tice he becomes able to estimate the approximate refractive index of a 
mineral plate from its relations to adjacent plates or the surrounding 
medium (usually Canada balsam), and this is ordinarily sufficient for actual 
determinative work. As the methods for ascertaining the relative re- 
fractive indices between mineral plates are primarily microscopical methods 
they will receive first consideration in the following paragraphs, while the 
methods for the direct measurement of the refractive indices of an isolated 
plate or prism will be referred to only briefly because they are described at 
length in the standard text-books. 
Underlying all methods which have been suggested for determining the 
relative refractive indices of mineral grains or plates in the thin section is 
the effect of different kinds of illumination of the object on the development 
of the image. This is a fundamental factor and will be considered in detail 
before attempting the descriptions of the different methods. 
If a wide cone of rays be incident on the object each element of the object 
receives light from all sides with the result that the different details appear 
about equally bright and can not be readily distinguished unless they differ 
in color or in the amount of light they absorb. This kind of illumination 
is, therefore, best suited for work with fine-grained particles embedded in a 
medium of about the same refractive index, as stained biological and bac- 
teriological specimens. With a wide cone of light the field is often "flooded 
with light" and the phenomena of refraction and reflection due to differences 
in refractive indices between adjacent elements in the object are in conse-__ 
quence veiled and lost to view. To determine the relative refractive indices 
of two adjoining sections, the aperture of the condenser should be decreased, 
either by lowering it or by closing the substage iris diafram or by both. 
Beams of parallel or weakly convergent polarized light are thus obtained 
and differences in the refractive indices are clearly marked. These dif- 
ferences are based on the phenomena of refraction, of dispersion, and of 
reflection. 
In Fig. 51, a, let a mineral grain, more or less round or lenticular in shape, 
be immersed in a liquid of higher refractive index and illuminated by a 
narrow cone of light from the condenser. The paths of the rays impinging 
*The method of the Due de Chaulnes (Mem. Acad. R. 423, 1767,) and its many modifications which have 
since been suggested, do not furnish accurate results and are rarely used by petrologists. In describing a new 
method for measuring the refractive index of a prism under the microscope C. Miculesco (Bull. d. Soc. 
Sci.. Bucarest, 14, 280-288, 1905; 16, 8-14. 1906) discusses the Due de Chaulnes method briefly. 
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