The experiment described gives a novel and simple method of measuring therefractiveindex of trans- parent platesusing a Michelsoninterferometer. The experimentwould be suitable in theundergraduate physics laboratory. Since it is necessary to use a computerprogram when determiningthe refractive index, studentscould be given the opportunity of writing their own programs.The main teaching advantage of the present tech- nique over the method of ‘white-light’ fringes is that students observe the movement of the fringes as the effective thickness of the glass in the beam increases, whereas in the ‘white light’ method the plate is either in orout of the beam andno fringe movement resulting fromtheplate is observedT. he present technique is also more suitablefor thick plates. Thus, in the presentexperiment it was sufficient tocount 200 fringes for a glassplate of thickness d = 1 mm. Thenumber of fringes counted in the ‘white light’ methodasthemirror is movedwould be approxi- mately 2000(m= 2d (n - l)/;., see Whittleand Yarwood 1973).As pointeodut by a referee, aandditional advantage of this method over the ‘white light’ method is thatstudents find it difficult to find the white light fringes, especially with a plate in one arm of theinterferometerE. xperimentallyt,he present technique is much morestraightforward even than the modification of the ‘white light’method described by Monk (1937).