Experiments in Micro-metallurgy. 



87 



direction of the arrows, yielding takes place by finite amounts of slips 

 occurring at a limited number of places in the manner shown at 

 a, b, c, d, e (fig. 3). This slip exposes short portions of inclined 

 surfaces, and when viewed under normally incident light, these surfaces 

 appear black because they return no light to the microscope. They are 

 consequently seen as dark lines or narrow bands, extending over the 

 polished surface in directions which depend on the intersection of the 

 polished surface with the surfaces of slip. 



We have proved the correctness of this view by examining these 

 bands under oblique light. When the light is incident at only a small 

 angle to the polished surface, the surface appears for the most part 

 dark; but here and there a system of the parallel bands shines out 

 brilliantly, in consequence of the short cleavage or gliding surfaces which 

 constitute the bands having the proper inclination for reflecting the 

 light into the microscope. Fig. 4 is the photograph of a strained piece 

 of Swedish iron illuminated in this way. The magnification is 280 

 diameters. The- groups of parallel bright bands which appear in the 

 photograph may readily be observed under the microscope to be 

 exactly coincident with the black bands seen under vertical illumina- 

 tion ; and by changing the angle of incidence of the oblique light, the 

 same bands may be made to appear dark on a faintly luminous ground. 

 Eotation of the stage to which the strained specimen is fixed makes 

 the bands on one or another of the grains flash out successively, with 

 kaleidoscopic effect. In what follows we shall speak of these lines as 

 slip bands. Fig. 1, through a mixed illumination, shows some of the 

 slip bands bright and some dark. 



Incidentally fig. 4 illustrates the fact that oblique lighting picks out 

 the boundaries of the crystalline grains, showing that these boundaries 

 are marked by inclined surfaces connecting grains whose faces are at 

 different levels. This is observed also in the etched surface of the 

 metal before straining. The boundaries, which appear dark under 

 vertical light, are bright on one side of each crystalline grain, when 

 the light falls with grazing incidence from one side. But the sloping 

 surfaces which mark the boundaries between the grains have by no 

 means the sharply definite inclination which characterises the surfaces 

 which form the slip bands. One or more groups of slip bands will 

 shine out very brightly when the light has a particular angle of inci- 

 dence, and will vanish when the incidence is slightly changed. The 

 boundaries are not in general so bright, but they remain fairly bright 

 while the incidence is changed through wide limits. 



When the metal is much strained a second system of bands appears 

 on some of the grains, crossing the first system at an angle, and in 

 some cases showing little steps where the lines Cross. These bands are 

 clearly due to slips occurring in a second set of cleavage or gliding 

 surfaces. An example of the crossed systems of bands will be seen in 



