If a specimen is a perfect crystal, X-ray beam divergent from a point is reflected only at a region which is nearly a straight line approximately parallel to the edge at which the plane of the crystal face and that of the photographic plate would meet. A slight change in orient-ation of the crystal causes a marked shift of reflection pattern. This can be applied to study sub-structure orientation. If the X-ray source is of finite dimension, its breadth perpendicular to the said edge broadens the region of reflection and lowers the sensibility for detecting orientation difference while its length parallel to the edge intensifies the pattern but lowers the resoloving power. Dimensions of the X-ray source should therefore be chosen in due consideration of these facts.