Roentgen examination of the chest often fails to satisfy the demands of the clinician. In part, this is due to unavoidable limitations inherent in the method, the roentgenogram being a composite picture of the densities encountered by the roentgen ray in its passage through the chest. Since the differences in the densities of normal and pathologic tissue may be slight and furthermore may neutralize each other, serious and intrinsic difficulties arise which even improvements in technic cannot completely circumvent.
Two difficulties resulting from the physical character of the roentgenogram—the lack of perspective vision and the inability to define a structure which is in the confines of an area of greater density—may be combated by maneuvers which, though not unknown, have not hitherto attained the popularity which they deserve. Stereoscopic roentgenograms have been studied by many workers. There are, however, too few systematic studies of roentgenograms taken in other than the