152 MICRO-SPECTROSCOPE AND POLAR/SCOPE \CH. VI 



if starch is mounted dry or in water, the appearances are not so striking as in a 

 balsam mount (Davis, p. 337 ; Suffolk). 



\ 225. Purpose of a Micro-Polariscope. — The object of a micro-polariscope is 

 to determine, in microscopic masses, one or more of the following points : (A) 

 Whether the body is singly refractive, mono-refringent, or isotropic, that is, opti- 

 cally homogeneous, as are glass and crystals belonging to the cubical system ; (B) 

 Whether the object is doubly refractive or anisotropic, uniaxial or biaxial ; (C) 

 Pleochroism ; (D) The rotation of the plane of polarization, as with solutions of 

 sugar, etc. ; (E) To aid in petrology and mineralogy ; (F) To aid in the determi- 

 nation of very minute quantities of crystallizable substances; (G) For the pro- 

 duction of colors. 



For petrological and mineralogical investigations the microscope should possess 

 a graduated, rotating stage so that the object can be rotated, and the exact angle 

 of rotation determined. Fig. 126. It is also found of advantage in investigating 

 object swith polarized light where colors appear, to combine a polariscopic and 

 spectroscope ( Spectro-Polariscope ) . 



MICRO-POLARISCOPE — EXPERIMENTS 



§ 226. Arrange the polarizer and analyzer as directed above (§ 220) 

 and use a 16 mm. objective except when otherwise directed. 



(A) Isotropic or Singly Refracting Objects. — Light the mi- 

 croscope well and cross the Nicols, shade the stage and make the field 

 as dark as possible (§219). As an isotropic substance, put an ordin- 

 ary glass slide under the microscope. The field will remain dark. As 

 an example of a crystal belonging to the cubical system and hence iso- 

 tropic, make a strong solution of common salt (sodium chloride) 

 put a drop on a slide and allow it to crystallize, put it under the 

 microscope, remove the analyzer, focus the crystals and then replace 

 the analyzer and cross the Nicols. The field and the crystals will re- 

 main dark. 



(B) Anisotropic or Doubly Refracting Objects. — Make a fresh 

 preparation of carbonate of lime crystals like that described for pedesis 

 (§ 151), or use a preparation in which the crystals have dried to the 

 slide, use a 5 or 3 mm. objective, shade the object well, remove the 

 analyzer and focus the crystals, then replace the anal} T zer. Cross the 

 Nicols. In the dark field will be seen multitudes of shining crystals, 

 and if the preparation is a fresh one in water, part of the smaller 

 crystals will alternately flash and disappear. By observing carefully, 

 some of the larger crystals will be found to remain dark with crossed 

 Nicols, others will shine continuously. If the crystals are in such a 

 position that the light passes through them parallel with the optic 



