656 POPULAR SCIENCE MONTHLY. 



these shadows are produced by straigiit rays from a small sur- 

 face, 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 ex- 

 perimenters. It is one of the important points that have been 

 determined, and even this was distinctly intimated by Prof. Ront- 

 gen 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 accumu- 

 lated 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 exceed- 

 ingly 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 fea- 

 ture of manipulation, or detail of action of the rays, so that more 

 has been learned of how to work with them than of their essen- 

 tial 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 cer- 

 tain 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 lumi- 

 nosity, called fluorescence, is also excited in many substances by 

 the ultra-violet or colorless portion of light, but the X rays are 



