The refractive index of the suspected glass can be known by using the backe- line method. This process takes place in a petrographic microscope. In this method an oil comparable to the refractive index of the glass is used. At a point when the refractive index of the glass becomes equal to that of the oil in which it is immersed the yellow line under the observation of the plane polarized light gets disappeared. This method can be used to identify the refractive index of the suspect glass and comparing it with the source of origin or sample glass.
