Below, are two images that together produce an optical illusion. What you have to do is stare at the white dot in the centre of the first image for about 30-60 seconds, trying to keep your head as still as possible. Exactly how long is required depends on how good your eyes are, so if you have good eyes, you may need to look for a bit longer.

The longer you stare at the image, the more pronounced the effect will be. If your vision starts to go funny whilst you are staring, don't stop, that is supposed to happen. In fact, the more distorted your vision gets, the better the results (don't worry, you aren't causing any damage).

Once you decide you can't take it any longer, scroll down to the next image, and stare at the black dot in the centre for a few seconds. With any luck, you should see something appear from nowhere.

A detailed explanation of how it works can be found at the bottom of that page for anybody who is interested.

How it works

The back of the eye is called the retina. The retina is made up of a vast number of tiny, light-sensitive cells called rods and cones. The rods are good at detecting movement, but can only distinguish between black, white and varying shades of grey. The cones are are not so good at detecting movement, but are able to determine the colour of the light, and in a lot of detail.

All colours that we are capable of recognising are made up of three primary colours, red, green, and blue. By mixing these colours in varying amounts, we are able to produce any colour imaginable. The top colour chart on the right shows how you can mix pairs of the primary colours in equal amounts, to get the three secondary colours, yellow, magenta, and cyan. Mixing all three gives white. This is how TVs and computer monitors work, in fact if you look closely at your TV, you will see three little stripes of red green and blue on each pixel. You could try this on your monitor, but monitor pixels are usually much smaller, so you would need a magnifying glass.

You can also mix pairs of the secondary colours in equal amounts to get the primary colours back. Mixing all three secondary colours will give black. This is how printers work.

By mixing the correct colours in the right amounts, it is possible to produce any other colour.

Ok, back to the eyes. When light shines on your eyes, the rods and cones give off chemicals that describe what the light looks like to the brain. A slightly over-simplified way of looking at it is that your eyes contain red green and blue chemicals. When you look at the sky on a clear day, blue light enters your eyes and hits various parts of your retina. The parts that the light hits start producing this blue chemical, and your brain knows that you are looking at something blue.

If you stare at one colour for long enough, the cones that are affected will start to run out of the chemicals that represent that light, and the colour starts to distort. When you stop looking at the colour for a while, the eye has a chance to produce some more of the chemical, and things go back to normal after a few seconds.

Now lets introduce some colour maths. One of the colours in the picture above was yellow, and as you can see from the charts, yellow is made up of red and green.

yellow = red + green

So, after looking at the image for a while, certain parts of your eyes run out of red and green chemicals. Next you look at the second image, which is a plain white area (apart from the dot in the centre which just gives you somewhere to focus). The charts also show that

white = red + green + blue

But, we have run out of red and green chemicals, so when we look at the white image, the only chemical the eye has left is blue, so you see a blue ghost of the yellow image. The same principal applies to the other colours.

You can use this trick on any image. Open the image in a program such as Paint or GIMP, and then invert the colours (on Paint, this is under the Image menu). Stare at the picture for as long as you can (remember, the more distorted your vision, the less chemicals you have left, so the longer it takes the eye to recover, and the better the effect), then look at something white. You must keep looking at the same spot on the image (hence the dot in the centre of the one above) otherwise, some areas of your eye have a chance to recover, and you won't see the effect properly.

The reason that the above image works well is that it uses pure colours. By this I mean that the yellow is made up of maximum red, maximum green, and no blue. This causes maximum drain of the red and green reserves, leaving the blue untouched. If you were to use a photo, the colours would likely be a lot of two, and a little of the other, so you don't fully drain two reserves, and you also partially drain the third. Staring for long enough should still produce a ghost, but the results will never be as good.