This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
Scientists have made many efforts to define light and many more to determine what it is. So far no fully satisfactory definition has been formulated and no one would be so dogmatic as to claim he knows what it is. We shall devote no space to recounting any of the hypotheses that have been invented in an effort to explain light, but shall view its physical properties only.
Light is a composite entity, which may be broken up, by means of a prism, into the color band of the spectrum--red, orange, yellow, green, blue, indigo, violet. These different colors represent different rates of vibration of the light rays. From red to violet the wave lengths decrease, while the rate of vibration increases.
Sunlight contains, in addition to the color rays, a number of other rays, the "vibrations" of which are not perceptible to our ocular sense and are therefore invisible.
Today it is known that there is a continuity from one end to the other of the spectrum from waves of electricity miles long at one end to the recently discovered Milikan waves, at the other, which are shorter even than gamma rays. These long electric rays vibrate only a few times a second, while gamma rays, which are only three-hundred millionth of a millimeter in length, vibrate one hundred thousand billion times a second.
Expressing these smaller waves in centimeters or even in millimeters quickly brings us to exceedingly small fractions, so that smaller standards have come into use, such as millimicrons, equalling a millionth of a millimeter, or an Angstrom Unit (A.U.), equalling a ten millionth of a millimeter. An Angstrom unit measures approximately 1,250,000,000th of an inch. This is to say, it requires 10,000,000 of them to equal the diameter of a human hair of average thickness.
The visible or color spectrum is composed of rays that vibrate from 8100 A.U. (red) to 3900 A.U. (violet). These are the rays of light visible to the eye and give us sensations of light, color and heat. They possess chemical and heating power. Their heating power is greatest at the red end of the spectrum where they blend with the infra-red rays, while their chemical activity is greatest at the violet end, where they blend with the ultra-violet rays. To the right of the violet band is a group of very short rays called ultra-violet rays, ranging in sunlight from 2900 A.U. to 3900 A.U. To the left of the red band are invisible rays, of longer wave-lengths than the visible red--the infra-red rays.
Waves between the length of 3900 A.U. and 2900 A.U. are classed as waves of middle ultra-violet. These rays are highly destructive and do not reach us from the sun; being, fortunately for us, filtered out by the atmosphere. No rays shorter than 2900 A.U. reach us from the sun. Shorter rays must be produced by artificial radiation. The infra-red rays run as big as 600,000 A.U., but there is no extension of the ultra-violet end of the sun's spectrum beyond 2900 A.U. even on the tops of high mountains.
The invisible rays of the sun seem to be the most beneficial to our bodies--the infra-red, below the lower (red) end of the spectrum, and the ultra-violet, above its upper (violet) end, are the rays to which the greatest importance is attached. As will be seen later, however, the complete solar-spectrum, with all its colors and shades so blended and proportioned as to produce white light, is needed for ideal growth and development.
It is estimated that the amount of ultra-violet in the sun's total radiation is, upon entering earth's atmosphere, 5%; visible (light) rays, 52%; infra-red rays, 43%. Due to atmospheric conditions, the amount of ultra-violet energy reaching the earth's surface is only about 1% with the light constituting 40%, and the infra-red 59%. The amount varies with locality, season, altitude, cloudiness, etc., of the atmosphere. Enough sunlight passes through clouds and fog to vitalize plant and animal life.
Infra-red rays are absorbed by carbon dioxide, and water vapor in the atmosphere. While water vapor is transparent to ultraviolet rays, smoke absorbs both these and the visible rays, particularly violet, blue and green. Glass is opaque to rays of shorter wavelengths than 3000 A.U.