The Michelsointerferometer halos ng been a popular piece of equipment in the physics under- graduate laboratory. One possible application is the measurement of the refractiviendex of tahin parallel-side plate of transparent material of known thickness(.Alternatively, if the refractiveindex is known, thethickness of the plate maybe determined.) This is accomplished by using'whitelight' fringes superimposeodmn onochromatic fringes and is described in standardlaboratorymanuals(Whittle andYarwood 1973). In thisarticle anothermethod for measurintghe refractiviendex is described. Monochromatic fringes are used and plates ranging in thicknesfsrom a few micrometretso several millimetres may be measured.