Why do annealed brass grains show contrast in optical microscopy?

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temaire
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Here is an optical microstructure of annealed brass:

[PLAIN]http://img576.imageshack.us/img576/7955/annealed.png

Where does the contrast between grains come from?
 
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The molecular structure is oriented at different angles in each grain, and therefore reflects light differently.
 
It comes from your etchant attacking different parts of the surface of your material at different rates.