Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: 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. I, 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. This behavior is called double refraction. It may be used to distinguish those stones which are doubly refracting from those which are not. For example, in the case of a stone which is doubly refracting to a strong degree, such as a peridot (the lighter yellowish green chrysolite is the same material and behaves similarlytoward light), the separation of the light is so marked that the edges of the rear facets, as seen through the table, appear double when viewed through a lens. A zircon will also similarly separate light and its rear facets also appear double lined as seen with a lens from the table of the stone. The rarer stones, no. 2. sphene and epidote, likewise exhibit this property markedly. Some colorless zircons, when well cut, so closely, resemble diamonds that even an expert might be deceived, if .caught off his guard, but this simple test of looking forthe doubled lines at the back of the stone would alone serve to distinguish the two stones. A Simple but very Valuable Test for the Kind of Refraction of a Cut Stone. In the case of most of the other doubly refracting stones the degree of separation is much less than in peridot and zircon, and it takes a well- trained and careful eye to detect the doubling of the lines. Here a very simple device will serve to assist the eye in determining whether a cut stone is singly or doubly refracting. Expose the s...