ON THE CRYSTALLINE STRUCTURE OF MF.TALS. 363 



readily deserved under the microscope to be exactly coincident uitli the Muck bands 

 seen under vertical illumination. Figs. 16 and 17 are photographs of the same 

 strained crystalline grains of iron under vertical lijdit and under oblique light. 



Rotation of the stage upon which the strained specimen is fixed makes the bands 

 OH one or another of the grains Hash out successively with kaleidoscopic effect. 

 When the specimen is rotated through 180 from the position in which the lines 

 show brightly on one particular grain, the lines on that grain do not shine out 

 again, though they may be visible as black lines on a very faintly luminous back- 

 ground ; this is important as proving that the lines are not due to either furrows or 

 ridges, but to steps in the surface. In this respect there is a striking contrast 

 between the slip-hands and any accidental furrows or scratches which may have been 

 left on the specimen through imperfect polishing. 



Incidentally, fig. 17 illustrates the fact that oblique light picks out the boundaries 

 of the crystalline grains, showing that these Ijoundaries are marked by inclined 

 surfaces connecting grains whose faces are at different levels. This is observed 

 also in the etched sui-face 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 sur- 

 faces which mark the boundaries tetween the grains have not the sharply-defined 

 inclination of the slip-surfaces. The lines due to Blip-hands on one or more grains 

 will shine out brightly when the light has a particular angle of incidence, and will 

 vanish when the incidence is slightly changed. The boundaries are generally not so 

 bright, but remain visible under a considerable range of incidence. 



Figs. 18, 19, and 20 present a striking example of the kaleidoscopic effect under 

 oblique light just referred to. Here the metal is lead, the surface has been obtained 

 by allowing molten lead to solidify on a smooth glass plate, and the metal has l>een 

 strained by landing. The figures show the appearance under vertical light and 

 also under two different incidences of oblique light. Figs. 21 and 22 show another 

 example of strained lead under vertical and oblique light. 



In their preliminary notice* of some of the present results (communicated to the 

 Royal Society on March 16, 1899) the authors have applied the name " slip-bands " 

 to the lines developed on metallic surfaces by plastic strain, and in what follows they 

 will be referred to under that name.t 



So far as the writers' observations go it appears that slip-bands occur in all metals 



* "Experiment* in Micro-metallurgy: Effect* of Strain," 'Proc. Roy. Soc.,' vol. 65, p. 85. Sin.-c 

 writing tint jwper the authors' attention has been directed to the following remark l>y Mr. STKAD: "It 

 would appear that there is a tendency for iron to etch out into thin platen, and when such etched 

 specimens are distorted or pulled out for these etched plates to slide over one another " (" On Brittleness 

 Prodiurd in Soft Steel by Annealing," 'Journal of the Iron and Steel Institute,' 1898). It will, however, 

 be shown presently that the development of slip-bands is independent of any previous etching. 



t "Experiment* in Micro-metallurgy: Effect* of Strain," Ewixo and ROSEXIIAIN, 'Roy. Soc. 

 Proe.,' 1899, vol. 65, p. 85. 



3 A 2 



