tests of theories of "large"extra dimensions require a detailed knowledge of the Casimir force. The need for"real,pedagogically clean fundamental experiments"is clearly as great as ever. This is a longer version of the article "The double-slit experiment"that appeared in the print version of the September issue of Physics World,on page 15.Three letters that appeared in the May 2003 issue of the magazine have been added to the end of this version of the article. References General T Young 1802 On the theory of light and colours(The 1801 Bakerian Lecture)Philosophical Transactions of the Royal Society of London 92 12-48 T Young 1804 Experiments and calculations relative to physical optics (The 1803 Bakerian Lecture)Philosophical Transactions of the Royal Society of London 94 1-16 T Young 1807 A Course of Lectures on Natural Philosophy and the Mechanical Arts (J Johnson, London) G I Taylor 1909 Interference fringes with feeble light Proceedings of the Cambridge Philosophical Society 15 114-115 PA M Dirac 1958 The Principles of Quantum Mechanics(Oxford University Press)4th edn p9 R P Feynman,R B Leighton and M Sands 1963 The Feynman Lecture on Physics (Addison-Wesley)vol 3 ch 37(Quantum behaviour) A Howie and J E Fowcs Williams (eds)2002 Interference:200 years after Thomas Young's discoveries Philosophical Transactions of the Royal Society of London 360 803-1069 R P Crease 2002 The most beautiful experiment Physics World September pp19-20.This article contains the results of Crease's survey for Physics World;the first article about the survey appeared on page 17 of the May 2002 issue. Electron interference experiments Visit www.nobel.se/physics/laureates/1937/index.html for details of the Nobel prize awarded to Clinton Davisson and George Thomson L Marton 1952 Electron interferometer Physical Review 85 1057-1058 L Marton,J Arol Simpson and J A Suddeth 1953 Electron beam interferometer Physical Review 90490-491tests of theories of "large" extra dimensions require a detailed knowledge of the Casimir force. The need for "real, pedagogically clean fundamental experiments" is clearly as great as ever. This is a longer version of the article "The double-slit experiment" that appeared in the print version of the September issue of Physics World, on page 15. Three letters that appeared in the May 2003 issue of the magazine have been added to the end of this version of the article. References General T Young 1802 On the theory of light and colours (The 1801 Bakerian Lecture) Philosophical Transactions of the Royal Society of London 92 12-48 T Young 1804 Experiments and calculations relative to physical optics (The 1803 Bakerian Lecture) Philosophical Transactions of the Royal Society of London 94 1-16 T Young 1807 A Course of Lectures on Natural Philosophy and the Mechanical Arts (J Johnson, London) G I Taylor 1909 Interference fringes with feeble light Proceedings of the Cambridge Philosophical Society 15 114-115 P A M Dirac 1958 The Principles of Quantum Mechanics (Oxford University Press) 4th edn p9 R P Feynman, R B Leighton and M Sands 1963 The Feynman Lecture on Physics (Addison-Wesley) vol 3 ch 37 (Quantum behaviour) A Howie and J E Fowcs Williams (eds) 2002 Interference: 200 years after Thomas Young's discoveries Philosophical Transactions of the Royal Society of London 360 803-1069 R P Crease 2002 The most beautiful experiment Physics World September pp19-20. This article contains the results of Crease's survey for Physics World; the first article about the survey appeared on page 17 of the May 2002 issue. Electron interference experiments Visit www.nobel.se/physics/laureates/1937/index.html for details of the Nobel prize awarded to Clinton Davisson and George Thomson L Marton 1952 Electron interferometer Physical Review 85 1057-1058 L Marton, J Arol Simpson and J A Suddeth 1953 Electron beam interferometer Physical Review 90 490-491