these shadows are produced by straight rays from a small surface, they are usually as large as the object itself, or larger. Many experiments were made to determine the source from which the rays proceed before it was learned definitely that they emanate from the surface upon which the cathode rays first impinge a fact that was announced almost simultaneously by several experimenters. It is one of the important points that have been determined, and even this was distinctly intimated by Prof. Röntgen in the twelfth section of his original paper.
In intensity they vary inversely as the square of the distance from their source.
They electrify some bodies positively and some negatively, and whatever charge a body may already have they reduce or change it to the charge which they would independently give to the body. Their penetrating power depends upon the length of time they act.
Thus, gradually, these and many additional isolated facts have been established, and no doubt enough data will be accumulated eventually to permit generalization into laws; but that stage has not yet been reached.
Four theories have been suggested:
1. "They are ether waves, like ordinary light, but of exceedingly brief period, therefore ultra ultra-violet."
2. "They are streams of material particles."
3. "They are vortices of the intermolecular ether, forced from the cathode when the gas pressure is sufficiently low. Rectilinear propagation, absence of reflection, etc., follow from the properties of vortices."
4. "They are variations of stress in the dielectric surrounding the vacuum tubes."
Each of these theories is entitled to the Scotch verdict "Not proven," though the preponderance of opinion is on the side of the first. Still, it can not yet be said to be more than opinion.
Of the hundreds of papers that have been written during the year, the greater number have had reference to some special feature of manipulation, or detail of action of the rays, so that more has been learned of how to work with them than of their essential character. This has led naturally to improved apparatus.
It is well to keep in mind that the X rays do not make objects visible by their direct action, as light does. They do make certain substances self-luminous, causing them to emit a soft light of a grayish-blue or yellow or green color, depending on the nature of the substance, but this color is ordinary light, and not, at least to any considerable extent, the X rays. This luminosity, called fluorescence, is also excited in many substances by the ultra-violet or colorless portion of light, but the X rays are
