Leica DM5000B, DM4000M Determining the optical character, Uniaxial crystals a, Biaxial crystals b

9. Contrast methods for Leica DM4000 B/DM4500 P/DM5000 B

•Insert the Bertrand lens (Fig. 70B) and focus it by rotating the operating button until the inter- ference image or the circular diffuse edge of the eyepoint is in focus.

If needed, center the Bertrand lens: Insert the hexagonal keys into the centering holes in se- quence. If necessary, align the right eyepiec- es so that the cross-hairs correspond approx- imately to the direction of movement during the centering process.

•Adjust the collector to its optimum setting; use the diffuser if necessary.

Determining the optical character

Uniaxial crystals (Fig. 71a)

When observing uniaxial crystals in the cono- scopic (diverging) beam path, a dark cross ap- pears whose center point indicates the optical axis. The cross is surrounded by colored inter- ference bands*. When a variable compensator (quartz wedge or tilt compensator) is used, the rings move toward the center point or outside two opposite quadrants in the cross. The optical character results from the direction of the movement of the rings according to Fig. 71.

Cross sections in which the crystal optical axis is sloped toward the direction of the viewer are suitable for determining the optical character. In addition, an optical character can be deter- mined even if the center point of the cross is lo- cated outside of the field of view. Fig. 71 shows that fixed compensators can be used in place of variable compensators for determining the opti- cal character.

Even if only one of the optical axes is within the observer’s direction of sight, the optical charac- ter can usually be identified. The brightness for specimens oriented in this way changes in the orthoscopic beam path very little or not at all when the objective is rotated. Consequently, only one of the two isogyres are visible in the conoscopic beam path.

Biaxial crystals (Fig. 71b)

The cross sections in which the bisectors of the optical axes run parallel to the direction of sight are especially suitable for determining the opti- cal character (the section is perpendicular to the acute bisectrix).

A dark cross can be identified in the divergent beam path. This cross splits into two hyperbolic lines, also called isogyres, when the specimen stage is turned. The cross and or hyperbolic

isogyres are surrounded by colored interference bands. After the compensator has been activated, the optical character can be determined from the direction of movement of these bands according to Fig. 71 or the following rule: The screw axis symmetry of the isogyres must run perpendicular to the γ−direction of the compensator:

Fig. 70 Functions of the HC P Pol tube optics

Control element	1x Orthoscopy	1.6x Orthoscopy	Conoscopy
Tube lens	1x	1.6x	1.6x
Iris diaphragm	user-defined,	adapted for	> Specimen
	as it is not	the field of vision
Bertrand lens	in the beam path	off	on
Polarization	on or off	on or off	crossed
		(not for Dichroism/
		Pleochroism)

O/B		O		B
1x	I	1.6x	I	1.6x	I

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