I began work on this electronic text decades ago when I thought computers could more precisely calculate and plot extinction angles that one could do with a calculator and pen and ink drafting equipment. The text and diagrams have been revised again and again, a forgotten number of times. Quantitative treatments of birefringence, again with a large number of revisions, have joined extinction angles as something to calculate. Through the years, extinction angles as tools to estimate crystal compositions have lost their importance and luster. However, if the crystal compositions are known, extinction angles combined with birefringence can be used to estimate crystal orientation and degree of order in individual crystals in thin section. This text is still incomplete and past history indicates it may forever remain in that state. Please, look at this text as a work in progress and if there is something in here that you can use, don't hesitate, use it, refine it, or develop it. If you do, I would be pleased to hear about it.
If you click on the button below, labeled Programs, you will be taken to a page where you can download computer programs that implement concepts described in the text.