32-4 F. E. Wright — Measurement of the Optic Axial AngU 



Part I. Theoretical. 



In this part of the paper, the attempt has been made to 

 describe briefly the methods, at present known to the writer, 

 for measuring the optic axial angle of mineral sections under 

 the microscope and to emphasize particularly their modus 

 operandi and the principles on which they are founded. In 

 the preparation of these descriptions constant use has been 

 made of the jVIikroskopiscbe Physiographie, I, 1, by Rosen- 

 busch and Wiilfing, and of the original papers in which the 

 methods were first mentioned, especially those by Becke and 

 Fedorow. Several of the text-figures below have, in fact, been 

 derived, with slight modifications, from these sources. 



Convergent Polarized Light. 



Optical phenomena in general can be observed and be meas- 

 ured by use of either convergent or plane polarized light. The 

 study of optic axial interference figures has, however, until 

 recently, been effected solely in convergent polarized light, 

 and the methods applicable thereto are better known and more 

 fully developed than those for plane polarized light. We shall, 

 therefore, begin with the methods for measuring the optic 

 axial angle of minerals in convergent polarized light. 



There are several different lens combinations which can be 

 used to advantage for obtaining and observing interference 

 figures under the microscope in convergent polarized light; 

 and of these, the one suggested by Amici* in 1830 has been 

 found to be the best suited for optical measurements. With 

 this method, the primary interference image, which is formed 

 directly above the high power objective, is magnified and 

 reproduced as a secondaiw image in the upper part of the 

 microscope tube, where it can be observed either with magni- 

 fying glass or ocular with cross hair and micrometer scale. 

 The small Aniici-Bertrand lens by means of which this change 

 of microscope to conoscope is effected must be inserted at such 

 a point between ocular and objective that the secondary inter- 

 ference image observed through the ocular is as sharp and 

 clear as possible. 



Both theory and observation show, however, that all parts of 

 the interference figure thus formed are not in perfect focusf 

 at one and the same time. Fig. 5 illustrates the path of a light 

 beam from condenser lens to eye of observer. From this 

 figure, it is evident that the surface of focus for light waves 

 entering in all possible directions is not a plane but roughly 



*Ann. Chim. Pliys. (3), xii, 114, 1844. 



f Kosenbusch-Wiilfing, Mikroskop. Pkys., I, 1, 215, 1904. 



