a parallel bundle of filaments in a direction perpendicular to the filament axis and the diffraction pattern is recorded on photographic film sodium d rays wavelength 589 millimicrons are used as a light source and the filaments are disposed in a diagonal position the birefringence u03b4n of the specimen is computed from the following equation equ2 when n is the interference fringe due to the degree of orientation of the polymer molecular chain r is the retardation obtained by measuring the orientation not developing into the interference fringe by means of a berek 39 s compensator u03b1 is the diameter of the filament and u03bb is the wavelength of the sodium d rays the crystal size l is a value obtained in accordance with the following p scherrer 39 s equation which represents the size of a crystal in a direction approximately at right angles to the fiber axis equ3 wherein b is a 010 diffraction peak width in radian unit when the diffraction intensity is it iam 2 in which it is a diffraction intensity at 010 peak position and iam is a meridional x ray diffraction intensity at a bragg 39 s reflection angle of 2 u03b8 17 7 u00b0 the term 34 shrinkage of the fibers in boiling water 34 is defined as 34 percent decrease in length of material when exposed to elevated temperatures for a period of time and under 0 05 g p d tension 34 in the present invention the percent thermal shrinkage is measured in a boiling water bath of 100 u00b0 c.