174 MICRO-SPECTROSCOPE AND POLARISCOPE [ Cff. V[ 



point of the ocular with which it is used. If on looking into theanalyzer with 

 parallel Nicols the edge of the field is not sharp, or if it is colored, the analyz- 

 er is not in proper position 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 itself is when the analyzer is removed. 



\ 244. Objectives to Use with the Polariscope. Objectives of all powers 

 may be used, including the homogenous immersion. In 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 clear- 

 ly and satisfactorily sJiows the object under investigation. 



$ 245. Lighting for Micro-Polariscope Work. Follow the general direc- 

 tions given in Chapter II. It is especially necessary to shade the object so 

 that no unpolarized light can enter the objective, otherwise the field cannot be 

 sufficiently darkened. No diaphragm is used over the polarizer for most exam- 

 inations. Direct sunlight may be used to advantage with some objects, and 

 the object should be as transperent as possible. 



\ 246. Mounting Objects for the Polariscope. So far as possible objects 

 should be mounted in balsam to render them transparent. In many cases 

 objects mounted in water do not give satisfactory appearances with the polar- 

 iscope. For example, if starch is mounted dry in water, the appearances are 

 not so striking as if mounted in balsam (Davis, p. 337 ; Suffolk). 



247. Purpose of a Micro-Polasiscope. (A) To determine whether a 

 microscopic object is singlv or doubly refractive, i. e. isotropic or anisotropic. 

 (B) To determine whether or not a body shows pleochroism. (C) To show 

 whether an object rotates the plane of polarization, as with sugar. (D) To 

 give beautiful 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. It is also found of advantage in investi- 

 gating objects with polarized light where colors appear, to combine a polar- 

 iscope and spectroscope (Spectro-Polariscope). 



MICRO-POLARISCOPE EXPERIMENTS 



248-. Arrange the polarizer and anlyzer as directed above 

 ( 242) and use a 16 mm. objective except when otherwise directed. 



(A)" Isotropic or Singly Refracting Objects. Light the 

 microscope well and cross the Nicols, shade the stage and make the 

 field as dark as possible ( 241). For an isotropic substance, put 

 an ordinary glass slide under the microscope. The field will remain 

 dark. As an example of crystals belonging to the cubical system 

 and hence isotropic, make a strong solution of common salt (sodium 

 chlorid) put a drop on a slide and allow it to crystallize, 



