220 THE ELEMENTS OF VEGETABLE HISTOLOGY 



This phenomenon is termed polarisation, and the 

 light rays emerging from these materials constitute 

 polarized light. Calcite or Iceland 'spar is the material 

 used in the construction of polarizing apparatus. 

 The calcite is cut into long rhombic prisms (nicol 

 prisms), which are then cut obliquely and cemented 

 together with Canada balsam. These prisms trans- 

 mit certain light rays with but slight refraction, so 

 that they pass through both parts of the nicol and are 

 visible. Other light rays are refracted to such an 

 extent that, upon striking the cemented surface 

 between the prisms, they are totally reflected and 

 are not visible at the upper surface of the nicol. The 

 totally refracted light waves are termed ordinary rays, 

 while the waves which are so slightly refracted that 

 they pass through both prisms are termed extraordi- 

 nary rays. 



A complete polarizing apparatus consists of two 

 nicol prisms, the analyser, which is placed below the 

 object under observation, and the polarizer, which is 

 placed above it. The mountings of both polarizer 

 and analyser may be graduated, and either or both 

 may be so arranged as to rotate easily. A graduated 

 rotating stage provided with centering devices is 

 necessary in all but the most superficial observations. 

 Upon rotating the polarizer one observes that the 

 field becomes alternately light and dark. It will be 

 further noted that, starting with a field of maximum 

 darkness, rotation through an angle of 90 degrees 

 will be required to secure a field showing maximum 

 brightness. If the prisms are so placed that the light 

 rays passing through them are parallel, a light field 

 results, and the nicols are said to be in the uncrossed 



