Manchester Memoirs, Vol. li. (1907), No. VS. 11 



steps, and being thrown into the microscope. That step, 

 and the other steps on the same grain parallel to it, would 

 then seem bright instead of dark, while the rest of the sur- 

 face would, of course, seem dark, because it throws none 

 of the light up into the lens. By changing the direction 

 of the incident light you can get one or other strained 

 crystal to show its lines as bright lines ; and always when 

 one line in a crystal becomes bright the other lines 

 forming a parallel system with it flash into brightness at 

 the same time, thereby giving a most satisfactory proof 

 that the lines are in fact due to the presence of strictly 

 parallel steps, which are explained by slips that have 

 occurred in the process of straining. It is to these slips 

 that plasticity is due. In Fig. 10, Plate V, you have under 

 high magnification an example of the slips in a crystal of 

 strained lead, which has altered its form by slips taking 

 place in three independent sets of planes, and the 

 photograph shews well the regularity and the independent 

 character of the successive slips. 



Mr. Rosenhain* has continued this investigation, and 

 has devised a very ingenious further proof, if any were 

 needed, of the character of these lines. He took a piece 

 of iron which was severely strained, and he deposited copper 

 electrolitically upon its polished surface. According to 

 the theory I have just given you the surface of a strained 

 piece of iron is really not a plane surface any more, but is 

 composed of a series of steps. He deposited copper in 

 order to demonstrate the existence of these steps. Then 

 he cut the piece across at right angles to the surface, and 

 polished both the iron and the copper, and the polished 

 section clearly shows the little steps as serrations in the line 

 dividing iron from copper. 



Connected with this subject is a matter of very great 



* four. Iron and Steel Inst., 1906, No. II., p. 189. 



