In the measurement of length or motion a most refined instrument is the interferometer, devised by Professor A. A. Michelson, of the University of Chicago. It enables an observer to detect a movement through one five-millionth of an inch. The principle involved is illustrated in a simple experiment. If by dropping a pebble at each of two centres, say a yard apart, in a still pond, we send out two systems of waves, each system will ripple out in a series of concentric circles. If, when the waves meet, the crests from one set of waves coincide with the depressions from the other set, the water in that particular spot becomes smooth because one set of waves destroys the other. In this case we may say that the waves interfere. If, on the other hand, the crests of waves from two sources should coincide, they would rise to twice their original height. Light-waves sent out in a similar mode from two points may in like manner either interfere, and produce darkness, or unite to produce light of double brilliancy. These alternate dark and bright bands are called interference fringes. When one of the two sources of light is moved through a very small space, the interference fringes at a distance move through a space so much larger as to be easily observed and measured, enabling an observer to compute the short path through which a light-source has moved. In the simplest form of interferometer, light from any chosen source, S, is rendered approximately parallel in its rays by a double convex lens at L. The light falling upon the glass plate A is divided into two beams, one of which passes to the mirror M, while the other is reflected to M¹. The rays reflected from M¹, which pass through A, and those returned from M reflected at d, are reunited, and may be observed at E. In order to produce optical symmetry of the two luminous paths, a plate C exactly like A is introduced between A and M. When the distance from d to M and to M¹ are the same the observer sees with white light a central black spot surrounded with colored rings. When the mirror M¹ is moved parallel to itself either further from or nearer to A, the fringes of interference move across the field of view at E. A displacement of one fringe corresponds to a movement of half a wave-length of light by the mirror M¹. By counting the number of fringes corresponding to a motion of M¹ we are able to express the displacement in terms of a wave-length of light. Where by other means this distance is measurable, the length of the light-wave may be deduced. With intense light from a mercury tube 790,000 fringes have been counted, amounting to a difference in path of about one-fourth of a metre. [Illustration: Light-wave distorted in passing through heated air.]