PHYSICAL LABORATORY PAINT TESTS 89 



PAINT SECTION MEASUREMENTS UNDER MICROSCOPE 



Polar Micro-Examinations and Photomicrographs. By Polar 

 Micro-Examination is meant the examination of pigments under 

 polarized light. A polarizing apparatus, though not an essential 

 in the hands of the paint chemist, is nevertheless much to be 

 desired, for by its help deductions may be drawn as to the con- 

 tents of a paint, which by other means might not be possible. 

 The polarizing apparatus as marketed by most manufacturers 

 of the microscope is attached in the following manner: 



The diaphragm immediately under the sub-stage container 

 is swung out and opened to its widest limit, allowing the insertion 

 of the polarizer. This polarizer carries one of the pair of Nicols 

 prisms and is countersunk to allow of the introduction of gypsum 

 or selenite plates. The analyzer fits over the eyepiece on the 

 tube. 



The use of polarized light upon paint is valuable on account of 

 its action upon crystalline substances. The re-enforcing pig- 

 ments, such as Asbestine, China Clay, Gypsum, Silex, Barytes, 

 etc., are crystalline and consequently act upon the polarized 

 light. In most cases these pigments are used in ready-mixed 

 paints in small amounts, varying between 5 and 25%. When 

 a slide containing a small amount for example, less than 



