If a low tension current of electricity passes through a wire, it cannot be sensed directly by man; but if a delicately adjusted magnetic needle be placed above and parallel to such wire, the current will turn the needle to one side and keep it there. The magnetic needle then makes known the presence of a current of electricity which has no appreciable effect upon any of man's five senses. Similarly, the magnetic currents passing over the earth are not felt by man in such a way as to be recognized, but a magnetic needle, properly adjusted, will immediately assume an approximately north and south direction, in obedience to the pull of the magnetic currents. In this manner the magnetic needle, again, reveals to man the existence and presence of forces that he cannot sense directly. A piece of glass into which has been incorporated a small amout of the element uranium, is an instrument which reveals many wonders of the unsensed world. If the uranium glass be brought near the violet end of the spectrum of sunlight, it immediately glows, because it has the power of changing the invisible chemical rays into ordinary, white light rays. With such an instrument, darkness can be literally changed into light. Similarly, many of the class of rays to which belong the X-rays, and which are dark to the eye, and do not directly affect any of the other senses, are converted by uranium glass into visible rays. This glass, then, becomes another means whereby the world which does not directly affect our senses, may be made known. The X- or Roentgen rays have been mentioned several times. It is generally known that they have the power of passing through the body and various other opaque bodies. The rays themselves are invisible, both before entering and after leaving the body; moreover, they do not affect any of the other senses of man. Were it not that the power is possessed of changing these rays to light rays, man could know nothing of the Roentgen rays. In fact, a screen, covered with powdered crystals of a chemical compound known as barium platinocyanide, is held behind the object through which the rays are passing, and the moment they touch this substance they are changed to light rays, and the screen glows. Or, instead, a photographic plate may be used, for the Roentgen rays affect the materials from which these plates are made. The screen of barium platinocyanide is, therefore, another means for revealing the unknown world. [Sidenote: "Tuning" to establish sympathetic vibrations is a form of the aids for explaining the unknown.]