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. 1 



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 film is usually torn loose. Reject the 

 shavings cut off. Draw a ringer 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. 



