From Genuine Mediumship OR The Invisible Powers by Swami Bhakta Vishita (Hindoo Master)

Super-Sensible Vibrations

As every text book on science informs us, there are sounds too low as well as those too high for the human ear to register, but which are registered by delicate instruments. Again, there are colors beyond the place of red, at one end of the visible spectrum; and others beyond the place of violet at the other end of that spectrum, which the human eye is unable to register and detect, but which our apparatus in the laboratory plainly register. The ray of light which registers on the photographic plate, and which causes sunburn on our skin, is too high a rate of vibration for our eyes to perceive. Likewise the X-Rays, and many other of the finer rays of light known to science are imperceptible to the unaided human vision—they are actually "dark rays" so far as the human eye is concerned, though man has devised instruments by means of which they may be caught and registered.