MICROMETRY — MICROMETRIC MICROSCOPES 197 



with reflected light, using a microscope lamp with condenser 

 and " daylite " glass or a Silverman illuminator with " daylite ' 

 glass lamp. Count the number of coats, note their color, the 

 character of each paint represented and the uniformity with 

 which each coat has been applied. Measure the thickness of 

 the different coats. 



Knowing the thickness of a film of paint, it is possible to com- 

 pute the number of square feet of surface a gallon of this paint will 

 cover. It will be found that the values obtained in this way 

 closely approximate those met with in actual practice on the 

 same kind of surface. The actual shrinkage of paint films 

 ageing indoors appears to be less than one would ordinarily 

 suspect. In the author's laboratory on boards painted for 

 student work, certain paints after five years ageing yield the 

 same thickness of paint films as they did shortly after the paint 

 had become " dry. ' Other paints show a shrinkage of from 

 one-quarter to one-third their original thickness. 



Measurement of the thickness of wet films of paint can be 

 made by focusing upon the surface upon which the paint has 

 been spread and then focusing up with the fine adjustment until 

 the plane has been reached in which the upper surface of the 

 film lies. From the amount of displacement as indicated upon 

 the scale of the fine adjustment the thickness of the film is cal- 

 culated. The great difficulty with this method is that of placing 

 the surface of the painted objects so that it is exactly normal 

 to the optic axis of the microscope in the line of the two positions 

 which are focused. 1 



1 Valuable data relative to the thickness of paint films may be found in Cir. 71, 

 Paint Manufacturers Association of the U. S., Oct. 1919; Spreading Rates of 

 Prepared Paint Products, by H. A. Gardner. 



