METALLOGRAPHY 195 



intention of facilitating " Metallography." Invaluable 

 evidence is thereby obtained of the mechanical or physical 

 treatment to which a metal has been subjected, and as to 

 its state of purity or the nature of its alloyed components, 

 and so forth. The details of the preparation of the surface 

 of the specimen by polishing and etching vary with the 

 nature of the metal. An outline of a useful general pro- 

 cess, practically Stead's, is given with Plate 29, others 

 will be found in works on metallography. To get the best 

 results, particularly with medium- and high-power objec- 

 tives, a beam of light must be projected vertically on to 

 the prepared surface which, to prevent avoidable reflections 

 causing a haze, is not covered with a cover-glass, and 

 consequently dry objectives should be corrected to work 

 under those conditions. 



The vertical illuminator in a form that can be adapted 

 to any ordinary microscope consists of a nose-piece which 

 screws into position above the objective, and carries a 

 circle of thin optical-glass or cover-glass adjustable about 



Fig. 62 



VERTICAL ILLUMINATOR 







a horizontal axis by a small milled head (Fig. 62). A 

 pencil of light is projected by means of a condenser 

 strictly at right angles to the microscope into the vertical 

 illuminator, and reflected thence through the objective, 

 which acts as a condenser and focusses the light on the 

 object. The image-forming rays from the object pass 

 through the reflector without hindrance. 



Another form of vertical illuminator contains a reflect- 

 ing prism in place of a cover-glass circle, and is shown as 



