In this lecture we shall look at two triumphs of modern technology which enable us to measure and to control mechanisms with a precision of better than one ten thousandth of a centimetre. The first is familiar enough - the video disc - which gives us an instant replay of a football goal, or records a whole television programme. And the second is the microchip which involves the manufacture and positioning of minute stencils to etch electronic circuits - parts of which may be less than one thousandth of a centimetre across - and to reproduce them precisely by the million. Without such accuracy the microchip revolution could never have occurred.