CH. VI] MICRO-SPECTROSCOPE AND POLAR/SCOPE 173 



MICRO-POLARISCOPE 



<S 240. The micro-polariscope, or polarizer, is a polariscope used in con- 

 nection 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 the 

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

 the law is called the extra-ordinary ray. These two rays are polarized in 

 planes 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 less refracted extraordinary ray 

 is transmitted. 



\ 241. 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 en- 

 tirely 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 corresponding 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 extinguished. In the intermediate points 

 there is a sort of twilight. 



§242. Putting the Polarizer and Analyzer in' Position. — Swing the dia- 

 phragm 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 diaphragm 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 indicator 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. Or, more easily, 

 one may set the index at 0°, clamp the ring to the microscope, then rotate the 

 draw-tube of the microscope till the field is lightest. 



\ 243. Adjustment of the Analyzer. — The analyzer should be capable of 

 moving up and down on its mounting, so that it can be adjusted to the eye- 



