tial high enough to give a spark of several inches in air, and a vacuum tube in which the spark was to be discharged, seemed to be requisites, and wherever these were obtainable the experiments were attempted. Poor facilities led to efforts to dispense with good ones, the prevailing meagerness of equipment becoming thereby a means all the earlier of deciding the limitations for successful operation. Of course, the excitement quieted when the novelty wore off, but investigations in this new field must continue for a long time.
The pure physics of the subject was, naturally, the side which most appealed to scientific professors. How was the strange agent to be set to work, and how did it work? Was it light, or was it electricity? Was it material, or ethereal? Was it due to the cathode or to the anode terminal of the vacuum tube? The year's work upon these questions leaves them answered only partially and unsatisfactorily. Very little indeed has been added to the facts brought out by Dr. Röntgen in the first instance. The Physical Society of London, in its abstracts of physical papers from foreign sources, classes all work with the Röntgen rays under the head of "Light," but upon very scant grounds. Numerous experiments have been made to test the character of these mysterious activities by the accepted criteria of light—namely, reflection, refraction, interference, and polarization—all results being negative, or so slight and uncertain as to leave them still open to question, and to make the name "X rays" not only the most common one by which they are mentioned, but the one best suited to express our knowledge—or ignorance—of their nature. Several attempts have been made to determine a length for them, supposing them to be waves, resulting in a supposed upper limit of length not greater than one hundredth that of violet light, and probably not greater than one three-hundredth. In the first few months of the furore of experimentation and discussion scarcely a result was announced by one observer that was not controverted by another; yet out of this very contradictoriness came a rational conclusion that at all events the rays are not homogeneous, but differ among themselves in their properties, as do the constituent rays of ordinary heterogeneous light. This would account for their noninterference. Of refraction there is as yet no evidence, nor, so far as known, is there any possibility of bringing the rays to a focus and thus producing an image of any object by means of them. All that can be done in that way as yet, as at first, is to obtain a shadow of varying intensity by reason of the various penetrability of different objects or portions of one object; and so the pictures thus produced are called by various names, as skiagraphs, radiographs, X-ray pictures, etc.—all chosen to avoid the idea that they are real light-pictures or photographs. Since
