While every gem stone refracts light which enters it from the air, _each stone has its own definite ability to do this, and each differs from every other in the amount of bending which it can bring about under given conditions_. The accurate determination of the amount of bending in a given case requires very finely constructed optical instruments and also a knowledge of how to apply a certain amount of mathematics. However, all this part of the work has already been done by competent scientists, and tables have been prepared by them, in which the values for each material are put down. THE HERBERT-SMITH REFRACTOMETER. There is on the market an instrument called the Herbert-Smith refractometer, by means of which anyone with a little practice can read at once on the scale within the instrument the _refractive index_, as it is called, of any precious stone that is not too highly refractive. (Its upper limit is 1.80. This would exclude very few stones of importance, _i. e._, zircon, diamond, sphene, and demantoid garnet.) Those readers who wish to make a more intensive study of the construction and use of the refractometer will find a very full and complete account of the subject in _Gem-Stones and their Distinctive Characters_, by G. F. Herbert-Smith, New York; James Pott & Co., 1912. Chapter IV., pp. 21-36. The Herbert-Smith refractometer is there described fully, its principle is explained and directions for using it are given. The price of the refractometer is necessarily so high (duty included) that its purchase might not be justified in the case of the smaller retailer. Every large dealer in colored stones, whether importer, wholesaler, or retailer, should have one, as by its use very rapid and very accurate determinations of stones may be made, and its use is not confined to unmounted stones, for any stone whose table facet can be applied to the surface of the lens in the instrument can be determined. LESSON III DOUBLE REFRACTION EXPLANATION OF DOUBLE REFRACTION. In Lesson II. we learned what is meant by _refraction_ of light. While glass and a small number of precious stones (diamond, garnet, and spinel) bend light as was illustrated in Fig. 1, practically all the other stones cause a beam of light on entering them to separate, and the path of the light in the stone becomes double, as shown in Fig. 2.