Since physiological contrast has great influence on interpretation of radiographs, the authors made an experiment on the changes in border line between visible part and inbisble under various conditions, using the Phantom devised by G.C.E. Buger. The following results were obtained. 1. The thinner and wider an object is, the more visible is the image. 2. In high valtage radiography, the visible extent is wide but with Lysholm's diagram, it is narrow ; the irregular image make an image invisible of which size is the same as that of irregularity.
