In the old ways of optical mineralogy, upon considering the appearance of minerals in thin section as observed in ordinary transmitted light of polarizing microscope, it was seen that some idea as to the refractive index of a mineral could be obtained by inspection of the surface and margin of a fragment. The comparison is being made between the refractive index of the mineral fragment and that of the mounting medium, the Canada balsam. A test of determining whether the mineral had a higher or lower refractive index than that of the Canada balsam was also given.

            The method is obviously inadequate as a means of determining the refractive index of a mineral. This is because, after having determined its position in the scale whether higher or lower with reference to Canada balsam, one has to rely on the degree of relief in order to approximate the real value. The inadequacy of the method is inevitable since Canada balsam is the only satisfactory mounting medium. If, however, isolated fragments of the mineral are available for examination, these may be immersed and examined successively in liquids differing from one another, and each having a known value of refractive index.

            The fragments of a given mineral may be examined in turn in a small pool of liquids such as water(1.34), alcohol(1.36),chloroform(1.45), olive oil(1.47), castor oil(1.48), xylol(1.49), oil of cedar wood(1.51), monochlorbenzol(1.53), oil of cloves(1.54), brom-toluene(1.55), dimethylaniline(1.56), orthotoluidine(1.57), aniline or cinnamon oil(1.59), cassia oil(1.60), iodobenzene(1.62), monobromonaphthalene(1.66), methylene iodide(1.74). The refractive index of the mineral being approximately equal to that of the mineral being approximately equal to that of the liquid in which its outline is least conspicuous.

            However, the method of procedure is not the best either as regards accuracy or speed. It is more satisfactory to have a means of comparing the refractive index of mineral and liquid. In the field of optical mineralogy, this comparison may in some cases be effected by using the Becke’s test. However, it is important to note that as a rule, the shape of the fragment is not such as to give the best results. A method that is more adapted during the old optical mineralogy world to the requirements of the case is that known as the shadow method.