Wright — Two Microscopic- Petrographical Methods. 387 



lower iris diaphragm found on some microscopes back and 

 forth, or by using a stop diaphragm. The finger, however, 

 furnishes the simplest means and after some practice is used 

 almost unconsciously. Having thus covered half the field in 

 shadow, observe the mineral section on the border of the 

 shadow. If the mineral projects from the shadow into the 

 light field, and appears still lighter and brighter, its refractive 

 index is lower than that of the adjacent mineral. If the min- 

 eral lies within the light field and its edge protrudes slightly into 

 the darkened half, the phenomena are reversed. The four dia- 

 grams (3, 4, 5, and 6) show the possible cases which may result. 



Index of Kefraction. 

 Lower Higher 



4 5 



Edge 



brighter 



darker 



darker 



brighter. 



than that of adjacent mineral. 



If the refractive indices of the two are about the same, the 

 colored bands, described above, can often be seen. (Quartz 

 and Canada balsam.) 



The phenomena described above are distinct and easy recog- 

 nizable. By placing the finger before the reflector and lower- 

 ing the condenser lens slightly, the relative index of refraction 

 of two adjacent minerals can be determined accurately.* 



In the diagnosis of minute crystals (as chromite crystals 

 within plates of olivine, etc.) the method has been used to 

 advantage. 



2. On the Use of the Optic Normal in the Microscopic Deter- 

 mination of Minerals. 



Method for Determining the Optical Character of an Aniso- 

 tropic Mineral on a Section Perpendicular to the Optic Nor- 

 mal. (B ellipsoidal axis.) 



In the thin section the only plates of an anisotropic mineral 

 which can be recognized readily by their optical characteristics 



* Mr. Geo. W. Corey of the Michigan College of Mines has recently called 

 the attention of the writer to the fact that the relative refractive index of 

 adjacent minerals in the thin section can be ascertained by this method with- 

 out the aid of the microscope. Observe the thin section with a pocket lens, 

 holding the slide up toward the sky ; by placing the finger in front of the 

 thin section and thus casting a shadow over part of the field, the relative 

 index of refraction of two adjacent minerals can be determined by the rules 

 given above; the phenomena observed are identical with those already 

 described. 



