You may not have understood before that color as shown under the metallographic microscope depends not so much upon actual color of the material itself as upon its ability to reflect light. For metallographic observations it is necessary to have very strong illumination. Usually the powerful beam from an electric arc is concentrated by means of condensing lenses upon a thin disc of glass called an oblique reflector which directs the beam upon the polished and etched specimen beneath the objective of the microscope. Often a prism is used. The rays of light returning from this highly illuminated “field” under observation return up through the tube and eye piece of the microscope and can be focused upon a small screen convenient for observation or upon the ground glass of the attached camera by means of which the pictures are taken. Unless the surface of the specimen being examined is perfectly plain and level, not all of the vertical rays thrown down upon it will be reflected back up through the tube and eye piece. Those portions of the field which are absolutely at right angles to the vertical rays appear at the eye piece or upon the screen as white or light-colored portions, while those which, during the polishing or etching have been dug or eaten away reflect the light imperfectly or in directions other than up the tube of the microscope, wherefore such portions show as darker or black sections. The pearlite, then, is made up of little plates of soft ferrite alternating with others of a very much harder constituent. The harder plates are much less affected during polishing and etching than are those of the softer ferrite, hence they stand out in relief and reflect abundant rays of light, whereas the “dug-out” ferrite plates reflect the light imperfectly or not at all and therefore appear as dark lines. These white, hard plates of the pearlite contain all of the carbon of the low carbon alloys. They are this other constituent, “cementite,” so named because it was first discovered in steel made by the “cementation” process. It is a very hard and brittle substance, hard enough to scratch glass. It is the chemical compound (Fe_{3}C), unvarying in composition as chemical compounds always are. It consists of just three atoms of iron (93.4% by weight) and one of carbon (6.6%). Pearlite, therefore, is a sort of mechanical mixture of two separate constituents, ferrite or pure iron, and this chemical compound, carbide of iron, which is called cementite. Pearlite is common to all unhardened steels whether of low, medium or high carbon content and may be considered characteristic. That we may understand clearly the structures of the annealed steels, let us start with pure iron and gradually change it into higher and higher carbon steels by gradual addition of carbon. Pages 328 and 329 show such a series.