As is well understood, however, even these apparently unknowable manifestations of nature may be known, if proper aids be secured. In every case the problem is this: To obtain some medium, be it natural or manufactured, which transforms the unknown force into a known force, that is capable of affecting the senses of man. The search for such media is one of the most important labors of science. For instance, sunlight has been known from the beginning of the human race, and its nature has been studied by almost every generation of thinkers. To the time of Newton, it was only white light--or little more. Newton discovered that if a ray of white light be allowed to fall upon a triangular prism or glass, it is dispersed or broken into a number of colored rays known as the spectrum. All sunlight, passed through a glass prism, produces this colored spectrum; and the colors are arranged invariably in the same order; namely from violet through the intermediate colors to red. By passing this spectrum through another prism, white light is produced. Sunlight was thus proved to consist of a number of kinds of colored light. The eye alone is incapable of resolving white light into its elements: the glass prism thus becomes an aid to the sense of sight, by which a new domain of science is laid open to view. Above the red end of the spectrum, obtained from white light, nothing is visible, yet if a delicate thermometer be placed there, the increase in temperature shows the presence of certain invisible heat rays, and by moving the thermometer, it may be shown that the invisible heat spectrum is longer than the light spectrum itself. This, again, makes known to man a world that the five senses can recognize only with difficulty; and in this case, the thermometer is the necessary aid. Even more interesting is the violet end of the spectrum. Like the red end, it is invisible. In fact, for centuries it was believed that the light spectrum represented the whole spectrum. During the last century it was found that if a photographic plate be placed below the violet end of the spectrum, it is affected by invisible light rays, which are popularly denominated chemical rays. By placing the photographic plate in various positions, it has been discovered that the chemical spectrum is as long as the visible part. Since the days of Newton, therefore, the known part of the spectrum of sunlight has been trebled in length, and there is no certainty that all is now known concerning the matter. In this particular, the photographic plate has become a means of revealing an unknown world to the senses.