LIGHT. CHAPTER I. INTRODUCTION. ALL about us are men busy with their various trades and professions: sailing ships, digging in mines, making all manner of useful tools and machinery, planting seeds and reaping harvests, and doing many other works and labors according to certain fixed rules that they found printed in books, or that they learned of others, or that they discovered for themselves. Each one has to do with the physical phenomena around him. The more he knows about these phenomenathe more he knows about things, their relation to each other, and their action one upon another—the better he can work. A knowledge of the phenomena of Nature is the most important knowledge one can have who wishes to succeed in life. More than this, the observation of facts in Nature gives readiness of perception, and study and reading upon the causes of these facts stimulate the mind to healthful and pleasurable action. The laws that govern the physical phenomena about us were not told to us by ancient gods, or divinely-instructed men. They were discovered by experiment or observation. Men asked questions of Nature; they watched her phenomena, till they felt sure they saw a reason for their action. Sometimes they did not understand all that happened, and made strange guesses at the laws that governed the happenings. Other men repeated the experiments and got new answers; and thus, in time, the truth about things became known. Many of these facts in Nature, and the laws that govern their action, are now known of all men. Others are still obscure, or dimly known, and are being investigated every day in the hope that they may be better understood. The farmer, the sailor, the mechanic, and artisan, most familiar with these facts and laws of Nature, is, other things being equal, the most likely to be successful in his work. You hope to have a share in the world's work, and you wish to study Nature and her phenomena. You can read about these things in books. A better way is to make experiments-to ask Nature yourself to examine the phenomena of light, heat, sound, electricity, etc., to study these phenomena, and find their causes for yourself. To try an experiment means to put certain things in certain relations with other things, for the purpose of finding out how they affect each other. Experimenting is thus a finding out. It is the design of this book to tell you something about experiments in the phenomena of light-to show how these experiments illustrate the action of light, and to explain briefly some of the elementary laws that govern this action. All of these experiments may be performed with the cheapest and most common materials that can be found. They are all easy and simple, and they are, at the same time, interesting and entertaining. Some of the things here described are capable of affording amusement for a large number of people, and many of the exhibitions and displays that may be made with them are wonderfully attractive and beautiful. CHAPTER II. THE SOURCES OF LIGHT. WHEN the sun rises in the morning, the darkness of the night seems to fade away, and, wherever we look, without or within, all the air and space about us appears to be full of light. When evening comes again, the daylight disappears, and the moon and the stars give us another light. In the house we start the lamps, and they give us another light. Out-ofdoors, in the dusky meadows, we see the fire-flies darting about, and giving out pale sparkles of yellow light as they fly. We look to the north in the night and see the aurora, or we watch the lightnings flash from cloud to cloud, and again we see more light. This light from sun and moon, the stars, the fire, the clouds, and sky, is well worth studying. It will give us a number of the most beautiful and interesting experiments, and, by the aid of a lamp, or the light of the sun, we can learn much that is both strange and curious, and perhaps exhibit to our friends a number of charming pictures, groups of colors, magical reflections, spectres, and shadows. All light comes from bodies on the earth or in the air, or from bodies outside of the atmosphere; and these bodies we call the sources of light. Light from sources outside of the atmosphere we call celestial light, and the sources of this light are stars, comets, and nebulæ. The nebulæ appear like flakes and clouds of light in the sky, and the comets appear only at rare intervals, as wandering stars that shine for a little while in the sky and then disappear. The stars are scattered widely apart through the vast spaces of the universe, and they give out their light both day and night. The brightest of these stars is the sun. When it shines upon us, the other stars appear to be lost in the brighter light of this greater star, and we cannot see them. At night, when the sun is hid, these other stars appear. We look up into the sky and see thousands of them, fixed points of light, each a sun, but so far away that they seem mere spots and points of light. Besides these stars are others, called the planets, that move round the sun. These give no light of their own, and we can only see them by the reflected light of the great star in the centre of our solar system. Among these stars are the Moon, Venus, Mars, Jupiter, and many others. We might call celestial light starlight; but the light from the great star, the sun, is so much brighter than the light of the others, that we call the light it |