X-Rays, Apparatus and Methods Page 16

from the metal of the stand. They carry little spring clips at their extremities, by which the wires are caught without tying or bending.* The whole stand is of bicycle tubing and very rigid.

The Subject. — The patient should, of course, be placed in as easy a position as possible. For body pictures, hip and knee joints, etc., a recumbent position may be assumed. The plate should lie beneath and upon a stiff board backing to avoid risk of breakage. Work upon the shoulder, neck, head, etc., may be done with the patient straddling a straight-backed chair, facing the back, and leaning the body against the back for support.† The plate may be bound fast to the body by bandages.

Laws for Exposure.— Since the X-rays proceed in straight lines from (approximately) a point source, the intensity of their action varies inversely as the square of the distance from the source. Hence, the time of exposure should vary directly as the thickness of the intervening substance and the square of the distance from the plate. To

* This is also a great convenience in changing the position of the stand after everything is connected up — as must often be done. The wires simply slip out of the clips, thus relieving the tube of all strain. 
† For this suggestion we are indebted to the practice of Dr. Max Stern, of the Philadelphia Polyclinic.