In the first place, what is the present state of our knowledge of the X rays? Have we more efficient methods of producing them, and can we see farther into the recesses of the human body? In regard to the first question, we can say that, although we may not be able to answer dogmatically that we know what these rays are, we have valuable hints in regard to their character, and our knowledge of their manifestations and their relation to light waves and magnetic waves has greatly increased during the four years which have elapsed since their discovery. They are now believed by the best authorities to be magnetic and electrical pulses, or waves of extremely short length. In the spectrum of sunlight formed by sending a beam through a prism of quartz the X-ray pulses or waves are to be found, according to this hypothesis, beyond the violet color of this spectrum--far into the dark region invisible to the eye, and only brought into view at present by the aid of photography. In this invisible region reside many singular manifestations of energy closely analogous to those of the X rays. The ultra-violet rays invisible to the eye have the property of refraction. They can be bent out of their course by prisms made of quartz. The X rays, however, can not be directed from a straight course. This is their greatest peculiarity, and many attempts, both mathematical and experimental, have been made to elucidate it. It is not a fatal objection to the X rays being classed with light waves, for, under certain conditions, even light waves can be made to lose the power of being bent aside.