THE METALLOGRlAPIiy OF METEORIC IRON 3 



is little changed, if at all, to the eye — and examined under a micro- 

 scope with a vertical illmiiinator. 



The picture thus seen is strikingly revealing. There is none of 

 the roughness that obscures the field with ordinary etching; it ap- 

 pears like a finely drawn and delicately shaded map, upon which 

 minute details can be photographed at magnifications up to 3,000 

 diameters or more. As compared with the same field viewed with 

 oblique illumination, the lights and shades of structure are reversed. 

 The strong contrast between taenite lamellae and the darker kamacite 

 bands, as seen in oblique light, disappears. The sheen of the kama- 

 cite also disappears, and it forms a bright clear background upon 

 which such details as grain boundaries and minute inclusions are 

 conspicuous. Strange and beautiful eutectoid structures appear, 

 which are wholly invisible with the ordinary microscope. 



The reversal of lights and shades is explained by the fact that 

 under oblique illumination areas not attacked by the etchant rem_ain 

 smooth and reflect the light away from the objective, while areas 

 roughened by etching reflect light upward 'through the objective, 

 thus making the former appear dark and the latter light. With 

 central illumination the imattacked areas reflect all the light upward 

 and therefore appear bright, while roughened surfaces reflect it 

 partly away from the axis of the objective and thus appear darker. 



The revelation of a structure by etching and microcxamination is 

 extended by varying the degree of etching, or the reagent used, so 

 as to bring out certain structures or to show differences in solubility 

 or composition, and by selective etching with reagents that affect 

 only certain components. 



Whether etchmg leaves a given detail depressed or in relief can 

 often be determined by focusing upon adjacent structures un- 

 der high magnification. Giving the axial beam of light a slight 

 obliquity (e.g., 15°) may produce effects of shading that help to 

 determine details of structure. 



The use of polarized light, spectroscopic analysis, and X-ray 

 analysis is discussed in Chapter XVII. 



Despite the advantages of central illumination, ordinary oblique 

 light still has important uses. It can be employed at much lower 

 magnification — such as 5 or 10 diameters — which is not practicable 

 with central illumination and therefore is useful in a general survey 

 of the surface to be studied. The brilliant oriented sheen in certain 

 irons is seen only in oblique light, and the colors of certain inclu- 

 sions are more distinct. Taenite lamellae and particles sometimes 

 produce striking effects that can be observed only with macroetching 

 and oblique illumination. 



Chapter XVII outlines the technical methods employed in the 

 m.etallographic study of meteoric irons. 



