how to show part of big image on mouse rollover

I want when user roll over on my image it shows a part of big image of the same picture i known its possible in flash but how i don't Known ?? here is the link of the live example the开发者_开发技巧 site use the same technique in flash click here

Thank you in advance

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the big image is masked with the magnifier sprite and it's moved according to the position of magnifier (to show the correct part )