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.^ 
^ 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. 
