In this note, the isogyres width of the interfuence figure in a convergent polarizing microscope is considered. Due to the occurence of this width, the angle through which the isogyres is brought from the black cross to the position just beyond the view field of a polarizing microscope would be increased. By taking into account this effect, the pricision of optical. Orientating trick crystal with large optical axis angle and high refraction index can be enhanced substantially.