When you hear things and see visions and there is no stimulus to trigger that, it is a hallucination. When you have a stimulus and perceive it wrongly due to mental conditions, it is an illusion. When you have a stimulus and perceive it wrongly and others do the same thing, it is due to our sensory limitations as humans. These are called perceptual errors but can be called illusions as well.
Now somebody decided to make some money exploiting the perceptual errors humans make. Mathmos has created the Mathmos Revolution, using light and heat energy to create optical illusions of a moving, haunting vision which scares the hell out of you.Revolution uses the heat of two candles to spin angled blades which are attached to cut outs of skeletal horsemen which start to move without being touched. The piece weighs 1.2 Kilos and is priced 39.99 Pounds. It looks pretty cool when kept in a dark corner during a party.
Via: Drinkstuff