Localization by means of roentgenography presents no difficulty in the case of a finger. Here there are distinct landmarks, and it is easy to take two pictures in planes at a right angle to each other. If we employ a ray vertical to the plate at the supposed position of the foreign body, the latter is so near the plate that no correction is required for the slight lateral displacement of its shadow if not exactly at the spot where the ray is normal. In many other situations, however, two pictures at a right angle are either impracticable or inadequate, owing to the thickness of the part, the absence of very accurate bony landmarks, and often the great distance of the foreign body from the surface. The exact depth at which the foreign body is located is the difficult problem.
The methods of localization now in use fall into two