196 
ELEMENTARY CHEMICAL MICROSCOPY 
Make careful measurements of the openings and ascertain 
especially whether the openings are square or irregular. Deter¬ 
mining the diameter of the openings between wires requires 
great care in focusing the microscope since it is necessary to 
measure from wire to wire; the wires being bent, a sharp focus 
is difficult. A number of observations should always be made 
on each opening selected and the results recorded and averaged. 
Illuminate the fabric simultaneoulsy both by reflected light 
and transmitted light; in this way the wires are distinct and the 
openings sharply defined.^ 
Thickness of Protective Coatings. —Accurate results can rarely 
be obtained unless the coats can be studied and measured in 
cross-section. Platings, glazes, enamels, varnishes, paints, etc., 
can all be cut normal to their thickness if care be used. The 
sections can then be examined by means of a microscope and the 
number and character of the coats ascertained and their thick¬ 
nesses measured. Space forbids a consideration of individual 
methods for each type of coating. The simplest of all cases, 
that of a painted board, will be used as a type. Obtain a small 
block of the painted wood of such size as to be conveniently 
placed upon the stage of the microscope or if a block is not 
practicable a large sliver with the paint attached. With a sharp 
knife (a “ pattern ” knife will be found most convenient) cut 
away the wood until there remains next to the paint not more 
than three or four millimeters of wood. Now make several 
careful cuts, crosswise the coats of paint but lengthwise the 
grain of the wood, thus exposing cut surfaces of the paint layers 
almost at right angles to the painted surface. Now with a very 
sharp knife cut thin shavings from the prepared edge drawing 
the knife from outside inwards with a sliding cut. If the cut is 
made outwards, the paint fihn is usually torn loose. Reject the 
shavings cut off. Draw a finger tip very gently and slowly 
across the cut section to remove fragments and clean the surfaces 
of the paint coats exposed. Place the preparation, prepared 
side up, upon the stage of the microscope, illuminate strongly 
1 For information as to standard sieves, etc., consult Catalogue 36, Testing 
Sieves; The W. S. Tyler Company, Cleveland, Ohio. 
