274 Transactions of the Society. 



glass slide, the specimen placed thereon, and pressed into a perfectly 

 horizontal position by a specially-made mounting apparatus, and 

 is then ready for examination. 



Etching. 



After the examination of the plain polished surface i& 

 completed the crystal structure is developed by etching. Many 

 reagents are used for etching, but perhaps the mention of one or 

 two will suffice. Probably the simplest is to dip the specimen into 

 concentrated HNO3 ; it then assumes the passive state, but on 

 rinsing-off under a jet of water, the metal is sharply attacked 

 momentarily. It is then immersed in alcohol, and dried in the hot- 

 air blast. A dilute solution of HNOg in alcohol, or a saturated 

 solution of picric acid, can be used. These etching solutions darken 

 the pearlite, but leave the ferrite in its original state. 



Illumination. 



Illumination is accomplished eitlier by oblique or vertical 

 lighting, and, although several illuminants can be used, the electric 

 arc is undoubtedly superior to any other form of light source. The 

 lens system employed in my laboratory consists of a 4^-in. 

 Meniscus and two plano-convex lenses of the same dimensions, 

 suitably enclosed ; the first parallelizing the light-rays received 

 from the illuminant, and the third projecting a converging beam to 

 the plain glass vertical illuminator which, at an angle of 45°, is 

 situated immediately above the objective ; from thence it is 

 reflected down through the objective and so to the specimen on the 

 stage. 



For oblique lighting the vertical ilhiminator is either removed 

 altogether, or turned into a horizontal position parallel to the 

 surface of the specimen. 



