For most cases in observations with white light the “optics axes” may be regarded as approximately fixed in position. Footnote 15: Often spoken of as: a, the axis of maximum; b, the axis of intermediate, and c, the axis of minimum elasticity. In the more recent American text-books on Optical Mineralogy by Iddings, Winchell and Phillips a is denoted by _X_, b by _Y_ and c by _Z_. Footnote 16: For a short historical sketch of the use of the microscope in connection with Petrology, see G. H. William’s pamphlet, _Modern Petrography (Monographs of Education)_, Boston, 1886. For detail description and adjustments, see _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, pp. 11, 61. Footnote 17: For description of the ordinary microscope, eye-pieces, objectives, magnification, etc., see _Manipulation of the Microscope_, by Ed. Bausch. Footnote 18: Text Book of Mineralogy, by E. S. Dana, 1898 Ed., p. 176. Footnote 19: It is convenient to assume that the vibrations of the polarized light are taking place in this plane, called the “plane of vibration,” but all the phenomena caused by polarized light could be also explained on the assumption that the vibrations were taking place at right angles to this plane. Footnote 20: This condensing lens must be removed when very low power objectives are used. Footnote 21: Some microscopes are provided with adjusting screws bearing on the frame holding the objective, which can then be accurately centered to the axis of rotation of the stage. Footnote 22: In the Seibert microscope use objective No. 00 for the first general study of a rock section, No. II for general use and No. V for observations with convergent light. In the Fuess microscope use objective No. 4 for general use and No. 7 for convergent light tests. In the case of an English microscope a 1″ to ¾″ objective is used for general purposes and a ¼″ to ⅕″ for observations with convergent light. Footnote 23: See pp. 33 and 34. Footnote 24: In some microscopes the analyzer is in the form of a “cap” nicol, arranged to be fitted over the top of the eye-piece, and not introduced in the microscope tube as shown here. This form is not so convenient, as the “cap” nicol must be set by hand every time it is desired to make observations with crossed nicols. But at the same time it avoids any possible refocusing which may be necessary when the other type of analyzer is introduced in the tube. Footnote 25: When accurate adjustments are possible of the vibration planes of the nicols and the cross-wires parallel to these vibration planes, reference should be made to _Methods of Petrographic-Microscopic Research_, p. 61, 1911, Fred. E. Wright. Footnote 26: