from p. 8 of The Subtle Body: An Encyclopedia of Your Energetic Anatomy, authored by Cyndi Dale, published by Sounds True, Boulder, Colorado, 2009

Subtle Fields are inherently difficult to see. Why? Human senses operate within a narrow range on the electromagnetic spectrum, the measurable band of energy that produces various types of light. Our eyes can only detect radiation, the term for the noticeable energy emitted by substances, in the range of 380 to 780 nanometers. That is visible light. Infared light, which we cannot see, has a wavelength of 1,000 nanometers, and far-ultraviolet operates at 200 nanometers. We cannot see what we are not physically capable of seeing – not trained to see. If subtle energies actually do occupy a negative time-space continuum, move faster than the speed of light, and have no mass, we can determine that we do not currently have the equipment needed to meausre them. This does not mean that what is invisible does not exist.