If you live in an apartment block, chances are you'll have one of these contraptions plonked somewhere in the middle of your front door, logically around eye level, allowing for   the "peeping" their name implies. Through the magic of physics, the tiny opening's fish eye lens lets you peer out to the big bad world and see if that mysterious stranger ringing the doorbell is a delivery guy with a truckload of cash, or a proselytising Mormon (former = good; latter = not so much). DP

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