Harper's Round Table, April 14, 1896

by Various

FIG. 4.—WING OF A PIGEON, SHOWING THE SHADING EFFECT IN BONES.

Fig. 4 shows the wing of a pigeon, which is interesting, because while the outline of the flesh is distinctly marked the feathers have practically disappeared. The bones are not only clear, however, but the thinner parts are lighter than the thicker. Fig. 5 shows the leg and head of the pigeon. Around the head it is just possible to make out the outline of the feathers, the flesh is clearly marked, and all the bones of the neck are visible. In like manner the leg is interesting.

The transparency of the flesh makes it possible to show the presence and location in the body of foreign substances. Bullets, needles, and bits of glass have already been located by means of Röntgen ray photographs, and afterwards removed by a surgical operation.

FIG. 5.—HEAD AND LEG OF PIGEON.

It is curious that the part of the eye which is transparent to the light, and through which light passes to reach and affect the optic nerve, is nearly opaque to the Röntgen rays. Vision by means of these rays would therefore be impossible, even if the optic nerve were sensitive to them.

But suppose these rays could excite vision. What should we see? Holding a purse between the eye and a Röntgen ray source, we should see the coins within it. If a person stepped in the path of the ray we should see his bony skeleton. We might see something of his internal organs; perhaps we could see his heart beat. A broken bone could be seen, and the operation of setting it could be watched. Diseased bones or enlarged joints could be examined. Tubercles in the lungs would be visible. But these things would be visible only when they came between the eye and the source of the rays, much as on a dark night objects might be visible between you and a camp fire.

In daylight objects become visible by means of the light which falls upon them and is reflected to the eye. This brings out the detail of the visible surface. But the Röntgen rays are scarcely at all reflected, and even if they produced vision, objects would become visible only as they intercepted the rays. They would not be illuminated as they are illuminated by rays of light, and only outlines, therefore, would be seen. Even fluorescent bodies which appear light under the action of the Röntgen rays are not really illuminated, but are rendered luminous—that is, are made to shine by their own light. When bodies opaque to the rays are placed before the fluorescent screen, merely a shadow is seen on it. So the photographs or "radiographs" obtained are only shadows, but they are not the flat featureless shadows of the "shadow pictures" often introduced as an entertainment at social gatherings, when the identity of the person casting the shadow is often impossible to make out. Few substances are entirely opaque to the Röntgen rays, hence the shadows of thicker portions of an object will be deeper than of the thinner portions, and the shadow becomes a shaded picture that may give details of the surfaces of the object. A Röntgen ray shadow of an aluminum medal may show the design stamped upon the surface. The shading effect is well shown in the bones of the pigeon.

But if there are few substances entirely opaque to the newly discovered rays, there appear to be none that are entirely transparent. Even in air the rays appear to be rapidly absorbed, so that an extremely powerful apparatus is required for producing effects at any distance. Air seems to behave toward the Röntgen rays much as fog behaves to light, and it seems unlikely that effects can be procured at any great distance, perhaps not more than one hundred feet from the source.