The classification of wool fibers into standard grades is based entirely on the average diameter of the fibers. The diffraction of light by a bundle of parallel fibers was employed by Thomas Young in 1824 in a simple ingenious instrument for the rapid direct measurement of average diameter, but no thorough investigation has ever been made of the practical possibilities of this method in the routine grading of wool.In the present paper a new construction of Young's instrument (the eriometer) is described, and a critical study is made of the accuracy and adaptibility of the instrument in the averaging of a wide range of diameters as distributed in a group of fibers. Sources of error and limitations of the method are discussed.It is found that the eriometer average is in excellent agreement with comparable data obtained with the microscope. The method affords considerable opportunity for the further development of instruments to include additional features desirable in the study of wool or other textile fibers.