Probably what most aroused the interest of the public was the fact that when a structure, like the hand, was interposed in the path of these rays, the bones would cast a deep shadow, while the shadow cast by the flesh was very faint. It was thus possible to photograph the bones of the living body, and, of course, to show the presence of foreign substances or abnormal growths.
What has excited most surprise, perhaps, is the fact that these rays pass through bodies that are generally considered opaque, for it seems to those not familiar with the facts and demonstrations of science a most surprising thing that any rays should go through wood planks or sheets of metal or living flesh or brick walls. But is it really any more wonderful than that rays of light should go through glass or quartz or diamond or water? We are familiar with this last fact, because we can "see through" these substances. We know that glass does not shut out light, because we can see the space beyond it illuminated. But we have no sense that tells us of the presence of the Röntgen rays. We must resort to the photographic plate or the fluorescent screen (to be described further on), to show their presence, and for all information as to their behavior. The photographic plate is affected, while our eyes are not, and we are obliged to let such plates take the place of our eyes, and receive impressions which we can afterwards interpret.
But what are "rays"? When we stand in front of a fire of glowing coals we feel the warmth, and our eyes tell us of the light. Light and heat are said to radiate from the glowing coals, and both light and heat proceed in straight lines. These straight paths followed by radiations we have called rays. These rays are quivering motions in a medium which we call the ether, and which we believe extends through all space and pervades all bodies. They are waves, having the character of waves on the water, which we can see, and sound waves in air, which we know exist.
These ether waves, those that affect the eye and those which do not, differ from each other, as all wave motions differ, in the distance from wave to wave, or what is the same, in the frequency of the vibratory motions. As an example, compare the long ocean swell that comes thundering upon the beach at intervals of several seconds with the frequent swash, swash, swash of the little ripples on the shore of a fish-pond, or the vibrations that can be felt as a tremor of the whole church when the deep bass pipes of the great organ are sounded with the sharp shrill tones of the high treble pipe.
There are means of measuring the distances from wave to wave of the different rays in the ether, and the result is astounding. The frequency is something of which it is impossible to form any conception. About 20 millions of millions per second is the lowest, and about 1000 millions of millions the highest frequency. Of these, those only which lie between 400 and 760 millions of millions per second excite vision. In other words, the ether waves breaking upon the optic nerve must come at the rate of at least 400 millions of millions per second before that nerve will carry any impression to the brain—before we can "see" them. Why rays of these frequencies only should affect the eye we do not know. We only know that the structure of the eye is such that the other rays are powerless to produce vision. Neither do we know why the low-frequency rays will go through hard rubber and will not go through glass. We only know it is a fact.
All these ether rays may produce heat. The high-frequency rays affect the photographic sensitive plate, and also produce another effect that is of especial interest in connection with the study of the Röntgen rays. They have the power of exciting a peculiar luminosity, or light, in certain substances, which are for that reason called fluorescent.
Electrical discharges in vacuum tubes have long been known as sources of radiations which produce heat and affect the eye. Every student of physics knows the experiment with the aurora tube, which, when exhausted by a good air-pump and connected to a Holtz machine or induction coil, is seen filled with a pale light having something the appearance of the streamers of the Aurora Borealis.
FIG. 1.—THE APPARATUS USED IN MAKING RÖNTGEN RADIOGRAPHS.