63 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



the light passes through them lengthwise, and in passing is divided into two rays 

 of plane polarized light. The one of these rays obeying most nearly the ordinary 

 law of refraction is called the ordinary ray, the one departing farthest from the 

 law is called the extra-ordinary ray. These two rays are not only polarized, but 

 polarized in planes almost exactly at right angles to each other. The Nicol prism 

 totally reflects the ordinary ray at the cemented surface so that only the extra- 

 ordinary ray is transmitted. 



§ 151. Polarizer and Analyzer. — The polarizer is one of the Nicol prisms. It is 

 placed beneath the object and in this way the object is illuminated with polarized 

 light. The analyzer is the other Nicol and is placed at some level above the ob- 

 ject, very conveniently above the ocular. 



When the corresponding faces of the polarizer and analyzer are parallel, i. e., 

 when the faces through which the oblique section passed are parallel, light passes 

 freely through the analyzer to the eye. If these corresponding faces are at right 

 angles, that is if the Nicols are crossed, then the light is entirely cut off and the 

 two transparent prisms become opaque to ordinary light. There are then, in the 

 complete revolution of the analyzer, two points, at o° and at 180°, where the corre- 

 sponding faces are parallel and where light freely traverses the analyzer. There 

 are also two crossing points of the Nicols, at 90 and 270 , where the light is ex- 

 tinguished. In the intermediate points there is a sort of twilight. 



\ 152. Putting the Polarizer and Analyzer in Position. — Swing the diaphragm 

 carrier of the Abbe illuminator out from under the illuminator, remove the disk 

 diaphragm or open widely the iris diaphragm and place the analyzer in the dia- 

 phragm carrier, then swing it back under the illuminator. Remove the ocular, 

 put the graduated ring on the top of the tube and then replace the ocular and put 

 the analyzer over the ocular and ring. Arrange the graduated ring so that the in- 

 dicator shall stand at o° when the field is lightest. This may be done by turning 

 the tube down so that the objective is near the illuminator, then shading the stage 

 so that none but polarized light shall enter the microscope. Rotate the analyzer 

 until the lightest possible point is found, then rotate the graduated ring till the 

 index stands at 0°. The ring may then be clamped to the tube by the side screw 

 for the purpose. 



\ 153. Adjustment of the Analyzer. — The analyzer should be capable of moving 

 up and down in its mounting, so that it can be adjusted to the eye-point of the 

 ocular with which it is used. If on looking into the analyzer with parallel Nicols 

 the edge of the field is not sharp, or if it is colored, the analyzer is not in a 

 proper position with reference to the eye-point and should be raised or lowered 

 till the edge of the field is perfectly sharp and as free from color as the ocular 

 with the analyzer removed. 



\ 154. Objectives to Use with the Polariscope. — Objectives of the lowest power 

 may be used and also all intermediate forms up to a 2 mm. homogeneous immer- 

 sion. Still higher Objectives may be used if desired. In general, however, the 

 lower powers are somewhat more satisfactory. A good rule to follow in this case 

 is the general rule in all microscopic work, " use the pozver that most clearly and 

 satisfactorily shows the object under investigation.^ 



\ 155. Lighting for Micro-Polariscopic Work. — Follow the general directions 

 given in Chapter I. It is especially necessary to shade the object so that 110 un- 

 polarized light can enter the objective, otherwise the field cannot be sufficiently 

 darkened. No diaphragm is used over the polarizer for most examinations. Direct 

 sunlight may be used to advantage with some objects, and as a rule the object 



