Scientifically we never see the real light, but only its manifestations and reflections on various objects and surfaces. Light is invisible. We become aware of it only through its effects. Scientifically the eyes reveal only a part of the world in which we live; like all sense organs they are limited in function to a certain range and we cannot register beyond it. Science has had to invent and make many instruments to supplement this imperfect working of the senses. Detectors of X-rays and infra-red rays are cases in point. A German scientist once calculated that even the dense metal platinum would be reduced to a thousand-millionth part of its original volume if its molecules could be packed together so closely that they could not move. In other words, even the densest matter is mostly empty space! The eyes, however, see nothing of this truth and continue to testify to a platinum which exists more in appearance than in reality.
-- Notebooks Category 21: Mentalism > Chapter 1 : The Sensed World > # 55
