LABORATORY STUDY OF TEST PANELS 165 



powder required to show the disruption of the coating is recorded 

 and reported as the measure of the " abrasion resist." The 

 panel requiring the greatest weight of emery to cause abrasion 

 is evidently the most resistant to abrasion. Paint is often sub- 

 jected to serious abrasion, through the blowing of sand, especially 

 at the seashore, and to withstand such action should contain 

 a proportion of pigments especially resistant to abrasion, such 

 as silica, zinc oxide, asbestine, and barytes. 



Making Photomicrographs. The photomicrographs which 

 are herewith shown were made in the following manner: A 



Formula No. 47, A. C. 



part of a panel was placed upon the stage of the microscope and 

 held firmly in place with clips. By varying the adjustment 

 and carefully running over the field the condition of the surface 

 was readily given, using the same eye-piece and objective through- 

 out the tests, and obtaining a magnification of thirty-three. 

 Great care was exercised to secure an average field showing the 

 general and typical appearance of every panel. Little difficulty 

 was experienced in so doing, as the laboratory panels gave very 

 representative surfaces of the large panels on the fence. The 

 instrument was then inclined horizontally and the eye-piece 

 was fitted into the camera nose. In the back of the bellows of 

 the camera was placed the ground glass for focusing. To secure 



