Note 78 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.

For methods of measuring the axial angle, see Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 147.

For convenience in many cases only 2E is recorded, as then an indication is given as to whether the axial angle is visible with an ordinary microscope (arranged for observation with convergent light for interference figures). If 2E is very large the axial angle can only be observed by covering the section with some transparent, strongly refracting fluid. For the Seibert microscope with objective V the limit for good results is about 2E = 90°–100°.