A method for direct.jneas.ure.inent pf per.iodic length of self-ifceasing media is put forward, with which inhomogeneity can also be observed directly and the rejeyant distri-bution constant is thus obtained. In’addition, it can also lie’used to judge the ion-exchange depth, the optical axis of self-focustktig rod knd the axial distribution plate.The inethod is alsoused to measure n0 simultamouly.