136 MICRO-SPECTROSCOPE AND POLAR/SCOPE. [CH. VI. 



MICRO-POLARISCOPE. 



\ 209. Ttie micro-polariscope, or polarizer, is a polariscope used in connection 

 with a microscope. 



The most common and typical form consists of two Nicol prisms, that is, two 

 somewhat elongated rhombs of Iceland spar cut diagonally and cemented together 

 with Canada balsam. These Nicol prisms are then mounted in such a way that 

 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 as it meets that surface at 

 an angle greater than the critical angle, and only the extraordinary or less refracted 

 ray is transmitted. 



\ 210. 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 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 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. I11 the intermediate points there is a sort of twilight. 



\ 211. 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 011 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 anal) zer 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. 



§212. 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 ocu- 

 lar 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 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 with the analyzer 

 removed. 



\ 213. Objectives to Use with the Polariscope. — Objectives of the lowest power 



