CHAPTER II.

THE PETROGRAPHICAL MICROSCOPE.[16]

The _petrographical microscope_ is essentially an ordinary microscope[17] with the following important additional equipment. It must be provided with: 1° a _polarizer_ (a piece of apparatus for giving polarized light) placed below the stage; 2° an _analyzer_ (a piece of apparatus for analyzing the rays of light after they have passed through the polarizer and transparent section) placed between the objective and the eye; 3° a _stage rotating_ about an axis which is the line of sight of the microscope. A convenient type of microscope is that made by Seibert of Wetzlar (No. 11 _a_), Fig. 2.

The reflector _a_ is usually fitted with a plane mirror on one side, and a parabolic mirror on the other. The plane mirror should be used with sunlight, and the parabolic mirror with artificial light, in order to make the rays of light as nearly parallel as possible.

The _polarizer_ is in most cases a nicol prism, set in a suitable frame _b_, and made as follows:

For the _nicol prism_ “a cleavage rhombohedron of calcite (the variety Iceland spar is universally used in consequence of its transparency) is obtained, having four large and two small rhombohedral faces opposite each other. In place of the latter planes, two new surfaces are cut, making angles of 68° (instead of 71°) with the obtuse vertical edges; these then form the terminal faces of the prism. In addition to this, the prism is cut through in the direction HH′, Fig. 3, the parts then polished and cemented together again with Canada balsam. A ray of light, _ab_, entering the prism, is divided into two rays polarized at right angles to each other. One of these, _bc_, on meeting the layer of balsam (whose refractive index is less than that of the ray _bc_), suffers total reflection, and is deflected against the blackened sides of the prism and extinguished. The other, _bd_, passes through and emerges at _e_, a completely polarized ray of light, that is, a ray with vibrations in one direction only, and that the direction of the shorter diagonal of the prism.”[18] The vertical plane through the shorter diagonal may be called the _plane_ of _vibration_[19] of the nicol.

[Illustration:

FIG. 2. ]

[Illustration:

FIG. 3. ]

The polarizer must be below the stage _c_, and is generally adjusted so as to have its plane of vibration parallel to the N. and S. cross-wire in the eye-piece _o_. It is important to know the direction of the plane of vibration of the polarizer or lower nicol, as we can then determine, when absorption of light occurs in a mineral, the direction in this mineral parallel to which the absorbed rays are vibrating. The polarizer slides in an outer shell or frame, and, by means of a lever _d_, can easily be raised or lowered.

A convenient test for the location of this plane of vibration of the polarizer is as follows: Make use of a section of biotite, cut at right angles to the basal plane, hence showing the basal cleavage cracks. Biotite has the property of absorbing to a marked extent the light vibrating parallel to these cleavage cracks. Rotate such a section on the stage of the microscope until the position of maximum darkness is reached and when such is the case the plane of vibration of the polarizer must be parallel to these cleavage cracks.

On the top of the nicol is placed the _condensing lens_ for getting _convergent_ light, and the adjustments are so arranged that when the nicol is up as far as it will go, the condensing lens[20] is brought almost in contact with the lower surface of the transparent section resting on the stage.

The _rotating_ circular _stage e_ is supported on a suitable frame _c_, and arranged so that its axis of rotation coincides with the line of sight of the microscope. The stage is graduated, and, by means of an index fixed to the frame, the angular rotation can always be obtained. It is also provided with two adjusting screws[21] _f_, by means of which the axis of rotation can be accurately centered.

The method of centering is as follows: Bring some prominent mark in the section exactly in coincidence with the intersection of the cross-wires in the eye-piece. Rotate the stage 180°, and correct one half the error by means of the centering screws, and the other half by moving the section on the stage. Check the result by rotating the stage 180° again, and if necessary repeat the corrections in the same way until the adjustment is satisfactory.

The _objective_[22] _g_ screws into the collar _h_, which has a slot _k_, in the upper portion, for the introduction of a sensitive color plate, a one quarter undulation mica plate or a quartz wedge.[23]

The slot _k_ is so arranged that, when the sensitive color plates are introduced, the vibration directions of these plates will make an angle of 45° with the planes of vibration of the crossed nicols, and the interference color will thus be at its maximum intensity.

A revolving _nose-piece_ is sometimes used which can be attached to the collar _h_ and arranged to carry two or three objectives, which can thus be very quickly brought into position for use. This is convenient in passing rapidly from observations with parallel light to observations with convergent light, which must be made with a high power objective. The difficulty is that recentering is generally required. The modern microscopes are provided with a clip for holding the objectives, instead of the screw-thread collar _h_.

The _analyzer_[24] or upper nicol is contained in the frame _l_, which is arranged so as to slide in and out of the tube of the microscope.

The plane of vibration of the analyzer is fixed by the instrument maker so as to be at right angles to the plane of vibration of the polarizer, hence in the Seibert microscope parallel to the E. and W. cross-wire in the eye-piece. Consequently when the frame _l_ is pushed into the tube, the analyzer is introduced in the line of sight between the objective and the observer’s eye, with its plane of vibration at right angles to the plane of vibration of the polarizer; that is the nicols are crossed. When the nicols are crossed, if they are properly adjusted, no light can pass through to the eye and the field of view should be dark.[25]

The _eye-piece_[26] _o_ fits into the top of the tube, and, by means of a little projecting piece fitting into a slot in the frame, can always be adjusted so as to have its cross-wires parallel to the planes of vibration of the two nicols.[25]

Some instruments are provided with an additional slot in the tube between the analyzer and the eye-piece for the introduction of a Bertrand lens, which is used to magnify the interference figures produced by convergent light.

The first approximate focusing is made by the screw _m_, and the fine adjustment by the screw _n_.

In focusing always start with the objective very near the section, and move it away until the right focus is obtained. Never move down towards the section in obtaining the focus, as there is danger then of striking the objective against the section.

For cleaning the microscope xylol can be used, as it will not injure the lacquer. To lubricate any of the parts use a small quantity of soft tallow, good clock oil or paraffin oil.