1. Optical Illusions

2. Try and count the black dots
Try and count the black dots.... If you keep your eyes on the black dots, they appear to form and vanish at the intersections of the horizontal and vertical lines. The effect is diminished if one is very close to the screen, further away or if one views at a 45 degree angle. This illusion is known as the Scintillating grid, and was discovered by E. Lingelbach in It is a modification of the Hermann grid illusion.

3. Focus on the dot in centre
Focus on the dot in centre... Focus your eyes on the dot in the centre of the circle and then move your head backwards and forwards. The outer wheels of the circler should begin to rotate in opposite directions.

4. Spinning wheels... If you focus on the picture, all the wheels start to spin (in both directions). However, if you then concentrate on a single wheel, that wheel will stop whilst the others keep turning.

5. Hidden details. An elderly couple share a loving embrace
Hidden details... An elderly couple share a loving embrace...but is that all? Keep looking and an entirely different image wil emerge.

6. White lines or grey spots. Look closely at the black squares
White lines or grey spots... Look closely at the black squares. You'll begin to see grey spots appearing at the intersections of the white lines amongst the black squares. This illusion was discovered by Ludimar Hermann in the 1870s and is known as the Hermann Grid Illusion.

7. Of course, optical illusions are now being put to commercial use by graphic designers.

8. White is black, or is it... At first glance, it appears to be a black and white chess board with a green cylinder casting a shadow diagonally across the board. However, the black and white squares are simply different shades of grey. The 'white' squares underneath the shadow (including 'B') are actually exactly the same shade of grey as the 'black' squares outside (including 'A'). This is known as the Adelson Checker Shadow Illusion.

9. This optical illusion, the Cafe Wall Illusion, was first discovered by Dr Richard Gregory who observed an anomaly in the way that the tiles of a wall in his local cafe in Bristol displayed. The horizontal lines appear to be bent, although they in fact parallel straight horizonal lines. The effect only works if the tiles have a border (in this case grey).

10. Named after Wilhelm von Bezold, the German professor of meteorology ( ), who discovered that a colour may appear different depending upon its relation to adjacent colours. In this case, the Bezold Effect is such that the red seems lighter when combined with the white and darker when combined with the black.

11. Look closely, the orange circles are the same size
Look closely, the orange circles are the same size... An illusion that plays on our perception of relative size. The first central circle appears smaller than the circle on the right yet they are identical in size. This is the Ebbinghaus Illusion.

12. First described by the British psychologist, James Fraser, in 1908 this is known as the Fraser Spiral Illusion. The overlapping black arcs appear to form a spiral; but, the arcs are simply a series of concentric circles.

13. The Impossible Cube Illusion is an illusion that draws on our natural desire to view the whole rather than individual elements of a drawing or picture. Each part of the drawing is ambiguous, where two lines cross it does not show which is behind and which is in front.

14. An example of the Zollner Illusion
An example of the Zollner Illusion. The long black lines are in fact parallel to each other. The illusion is created by the shorter lines being at an angle to the longer lines, this creates the impression that one end of the longer lines is nearer to us.

15. In this image, the two shapes are in fact identical in size
In this image, the two shapes are in fact identical in size. The Jastrow Illusion was discovered by Joseph Jastrow, the American psychologist, in 1889.

16. Perhaps the most commonly known optical illusion
Perhaps the most commonly known optical illusion. The Kanizsa Triangle was introduced by Gaetano Kanizsa, an Italian psychologist. One sees an inverted white equilateral triangle in front of the three black disks. However, the triangle is not drawn and the illusory triangle looks brighter than the background.

17. First discovered in 1860 by Johann Poggendorf, the Poggendorff Illusion, illustrates how the brain can be tricked by the interaction of diagonal and horizontal lines. At first viewing, one assumes that the blue line is a continuation of the black line. However, on closer viewing it is actually the red line.

18. Another example of an impossible object
Another example of an impossible object. Three circular prongs at one end become two rectangular prongs at the other. This is known as a Blivet.

19. Known as White's Illusion, is a counterintuitive illusion
Known as White's Illusion, is a counterintuitive illusion. When a grey rectangle is mainly surrounded by black it should look lighter. In this case, the grey rectangles are exactly the same shade of grey.

20. A Motion Illusion. The brain's reaction to the colour contrasts and position of the shapes is such that this static image appears to be moving.

21. A drawing of a woman strolling towards a bridge
A drawing of a woman strolling towards a bridge. Yet look closely (maybe move away form the screen) and a human face will appear.