64 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



would best be very transparent. That is, tissues, fibers, etc., should be mounted 

 in balsam (Suffolk). 



§ 156. 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, mouo-refringent or isotropic, that is opti- 

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

 Whether the object is doubly refractive, birefringent or anisotropic, uniaxial or bi- 

 axial ; (C) F'leochromism ; (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 determination of very minute quantities of crystallizable substances ; (G) For 

 the production of colors. 



For petrological and mineralogical investigations the stage of the microscope 

 should possess a graduated rotating stage so that the object can be rotated and the 

 exact angle of rotation determined. It is also found of advantage in investigating 

 objects with polarized light where colors appear, to combine a polariscope and 

 spectroscope (Spectro-Polariscope). 



MICRO-POIvARISCOPE — EXPERIMENTS. 



§ 157. Arrange the polarizer and analyzer as directed above (§ 152) 

 and use an 18 mm. objective except when otherwise directed. 



(A) Isotropic or Singly Refractive Objects. — Light the micro- 

 scope well and cross the Nicols, shade the stage and make the field as 

 dark as possible (§ 151). As an isotropic substance, put an ordinary- 

 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 Na CI.), 

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

 scope, remove the analyzer, focus the crystals and' then replace the an- 

 alyzer and cross the Nicols- The field and the crystals will remain 

 dark. 



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

 preparation of carbonate of lime crystals like that described for pedesis 

 (§ 95)> 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 an- 

 alyzer and focus the crystals, then replace the analyzer. 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 crys- 

 tals will alternately flash and disappear. By observing carefully, some 

 of the larger crystals will be found to remain dark with crossed Nicols, oth- 

 ers will shine continuously. This shows that the crystals are uniaxial. If 

 the crystals are in such a position that the light passes through them par- 

 allel with the principal axis, the crystals are isotropic like the salt crys- 

 tal and remain dark. If, however, the light traverses them in any 

 other direction the ray from the polarizer is divided into two constitu- 

 ents vibrating in planes at right angles to each other, and one of these 



