CH. VI~\ MICRO-SPECTROSCOPE AND POLARISCOPE 151 



219. 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 object, 

 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 passes 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 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 extin- 

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



\ 220. 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 indi- 

 cator 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 o. The ring may then be clamped to the tube by the side screw for the 

 purpose. Or, more easily, one may set the index at o, clamp the ring to the 

 microscope, then rotate the draw-tube of the microscope till the field is lightest. 



I 221. 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. 



222. Objectives to Use with the Polariscope. Objectives of the lowest pow- 

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

 mersion. 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 power that most clearly 

 and satisfactorily shows the object under investigation. 



$ 223. Lighting for Micro- Polariscope Work. Follow the general directions 

 given in Chapter II. It is especially necessary to shade the object so that no 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 

 would best be very transparent. 



\ 224. Mounting Objects for the Polariscope. So far as possible objects 

 should be mounted in balsam to render them transparent. In many cases objects 

 mounted in water do not give satisfactory polariscope appearances. For example, 



