CH. VIII] THE POLARISCOPE IN MICROSCOPY 267 



426. Lighting for micro-polariscope work. Follow the general 

 directions given in Chapter II. It is especially necessary to shade the 

 object so that no unpolarized light can enter the objective, other- 

 wise 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 the object should be 

 as transparent as possible. 



427. 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 polariscope. For example, if starch is mounted 

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

 in balsam (Davis, p. 337). 



428. Purpose of a micro-polariscope. (i) To determine 

 whether a microscopic object is singly or doubly refractive, i.e., 

 isotropic or aniso tropic. (2) To determine whether or not a body 

 shows pleochroism. (3) To show whether an object rotates the 

 plane of polarization, as with sugar. (4) 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 investigating objects with polarized light where 

 colors appear, to combine a polariscope and spectroscope (spectro- 

 polariscope) . 



MICRO-POLARISCOPE EXPERIMENTS 



429. Arrange the polarizer and analyzer as directed above 

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



(i) Isotropic or singly refracting objects. Light the microscope 

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

 as possible. 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 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 micro- 

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



