266 THE POLARISCOPE IN MICROSCOPY [Ch. VIII 



plete revolution of the analyzer, two points 180 apart where the 

 corresponding faces are parallel and where light freely traverses the 

 analyzer. There are also two crossing points of the Nicols, midway 

 between the parallel positions, where the light is extinguished. In 

 the intermediate positions there is a sort of twilight. 



§ 423. Putting the polarizer and analyzer in position. — Swing 

 the diaphragm carrier of the condenser out from under the condenser, 

 open widely the iris diaphragm, and place the polarizer in the dia- 

 phragm carrier; then swing it back under the condenser. 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 indicator shall stand at o° 

 when the field is lightest, or darkest. This may be done by turning 

 the tube down so that the objective is near the condenser, then 

 shading the stage so that none but polarized light shall enter the micro- 

 scope. Rotate the analyzer until the lightest possible point is found, 

 then rotate the graduated ring until the index stands at o°. The 

 ring may then be clamped to the tube by the side screw for the pur- 

 pose. 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, or if the darkest point is made zero, rotate the 

 draw-tube until the field is darkest. 



§ 424. Adjustment of the analyzer. — The analyzer should be 

 capable of moving up and dowm on 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 the proper posi- 

 tion 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 itself is when the analyzer is removed. 



§ 425. Objectives to use with the polariscope. — Objectives of 

 all powers may be used, including the homogeneous immersion. 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. 



