178 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946: 
as well as one of the most valuable weapons for medical care. We shall 
look back and thank the pioneers in physics, electricity, radiology, and 
general medical pedagogy who made equipment practicable and who 
encouraged physicians to specialize in the field of X-ray diagnosis and 
treatment. 
GENERAL CONSIDERATIONS 
To appreciate the importance of X-rays it is desirable that certain 
basic facts be kept in mind. These rays are penetrating radiations 
corresponding to light rays but having much shorter wave lengths. 
They may be used in a manner similar to light rays for the inspection 
of some materials and, by virtue of their peculiar properties, for the 
analysis of others. Gross inspection methods include simple roent- 
genography (or X-ray “photography”) and roentgenoscopy (or X-ray 
fluoroscopy) ; detailed inspection methods include various other means 
such as microradiography, Roentgen spectroscopy, Roentgen diffrac- 
tion, and so forth. In practice, X-rays usually are generated by allow- 
ing a stream of high-speed electrons to impinge upon a metal target. 
They have wave lengths of from 10.0 to 0.01 angstroms, and effect a 
sensitized film in a manner similar to that of ight rays. They also 
cause certain substances to fluoresce. The materials through which 
they pass are ionized and give rise to scattered X-rays. Because of this 
ionization, the rays are of use in the treatment of certain medical con- 
ditions. X-rays can be reflected, refracted, and polarized by special 
means (including the use of crystals). 
The principal use of X-rays lies in their ability to penetrate opaque 
objects. Such objects arrest the rays approximately in direct propor- 
tion to their densities. Substances of low atomic weight such as cotton, 
gauze, and aluminum, are traversed readily by the rays, while other 
substances such as bone and heavy metals are opaque, and cast a dense 
shadow on a sensitized film. Some materials are transparent to ight 
rays but opaque to X-rays (for example, a plate of lead glass). 
In the radiographic examination of materials (both animate and 
inanimate) it is important to remember that the delineation of an 
object depends on its differing in density from its surroundings. 
Unless such difference exists, the outline of the object cannot be shown 
on ordinary roentgenograms. A simple example of this is the demon- 
strability of the heart in X-rays of the chest. Surrounded as it is by 
air-filled lungs, the heart is readily visible in chest films. However, if 
nature had placed it in the middle of the liver, no distinct shadow 
would be cast as the liver is of approximately the same density. 
Under ordinary circumstances, X-rays are not directly visible and 
require a specially prepared surface for their detection (a fluoro- 
scopic screen or sensitized film). We use the adjective “ordinary” 
because you actually can see X-rays with a little training and practice. 
