10 EwiNG, The Structure of Metals. 



which we are supposed to be looking. AC, CB are the 

 surfaces of the two grains and the boundary between 

 them comes down from C into the interior of the metal. 

 We suppose that the piece is stretched by applying a 

 pull which acts in the direction AB. This causes a slip 

 in each grain, but it is not a homogeneous slip occurring 

 throughout the whole grain. It is a disconnected slip which 



V 



B 



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occurs at a series of places lying a little way apart and it 

 has the effect of producing steps by lowering one part of 

 the surface relatively to another, as the lower figure {Text- 

 fig. 2) illustrates. Imagine what happens when you look 

 down through the microscope on this surface. These little 

 sloping steps at a, b, c, d, and e are dark, because the light 

 coming down upon them is not reflected back into the 

 microscope, but is thrown away to the side ; consequently 

 everyone of these steps forms a dark line. 



As soon as Mr. Rosenhain and I realised that these 

 lines were little steps, we also realised that by choosing 

 a suitable direction for the light to fall upon the specimen, 

 namely, from the side instead of the top, it might be 

 possible to show them up as bright lines when all the rest 

 of the specimen was dark. Think of the light (in Text-fig. 2) 

 as coming in from the side, striking one of those sloping 



