CH. VI] MICRO-SPECTROSCOPE AND POLAR ISCOPE. 137 



may be used, and also all intermediate forms up to a 2 mm. homogeneous immer- 

 sion. Still higher objectives may be used if desired. Iu 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 tinder investigation. 



\ 214. Lighting for Micro-Polariscope Work. — Follow the general directions 

 given in Chapter II. It is especially necessary to shade the object so that no un- 

 polarized light can enter the objective, otherwise the field cannot be sufficiently 

 darkened. No diaphragm is used over the polarizer for most examinations. Direct 

 sunlight may be used to advantage with some objects, and as a rule the object 

 would best be very transparent. 



§215. Mounting Objects for the Polariscope.— So far as possible objects should 

 be mounted in balsam to render them very transparent. In man)- cases objects 

 mounted in water do not give satisfactory polariscopic appearances. For example, 

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

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



\ 216. 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-refriugent, 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 produc- 

 tion of colors. 



For petrological and miueralogical investigations 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 polariscopic and spectroscope 

 (Spectro-Polariscope). 



MICRO-POLARISCOPE — EXPERIMENTS. 



§ 217. Arrange the polarizer and analyzer as directed above (§ 211) 

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



(A) Isotropic or Singly Refractive Objects.— L,ight the micro- 

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

 dark as possible (§ 210). 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. 



(Bj Anisotropic or Doubly Refracting Objects.— Make a fresh 



