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Catechism of Familiar Things; Their History, and the Events Which Led to Their Discovery, A

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<h2>CHAPTER XX.</h2> <h3>Light, Lime Light, Magnesium Light, Electric Light, Rainbow, Prism, Spectrum, Colors, Photography, Camera Obscura, Stereoscope, Kaleidoscope.</h3> Do you know something about the nature of Light? Light is a mere form of vibration like sound, and like sound it requires some source to set this vibration going, and some medium to carry this vibration as air carries sound. Is not the air this medium? No, it is supposed that there is an elastic fluid called "ether" which pervades all space and matter, and if the molecules of a body are in motion they have the power of setting this ether in motion. The movement thus produced will appear either as heat or light according to its velocity. What sources of light do you know? We are told that the principal source of light on earth is the sun, either directly with its own beams or indirectly by supplying us with combustibles to produce light; for oil, gas, candles, and most of the substances used for producing light and heat when burning are but sending forth in another form the rays of the sun which were stored up in nature's economy. Another source of light is the result of chemical action, such as the lime, magnesium, and electric light. A third source of light is phosphorescence, as we see it in the glow-worm and fireflies. What is the Drummond or Lime Light? It is one of the most brilliant of artificial lights. When a stream of oxygen and one of hydrogen under pressure are brought together and mixed within a few inches of the end [208]of a blowpipe, the mixture on lighting burns with a colorless flame possessing intense heat. If this flame be made to play upon a ball of carbonate of lime, the lime on becoming white hot gives off a powerful incandescence. <div class="blockquot">Incandescence, the glowing whiteness of a body caused by intense heat. </div> What is a Blowpipe? A tube, usually bent near the end, terminated with a finely-pointed nozzle, for blowing through the flame of a lamp or gas-jet, producing thereby a small conical flame possessing intense heat. It is used in soldering silver, brass, etc. A mixture of oxygen and hydrogen when ignited constitutes the hydrogen blowpipe, invented by Dr. Hare of Philadelphia. What is Magnesium Light? When the metal magnesium is rolled out into a fine ribbon and heated to red heat it burns with a dazzling light. Which is the most powerful artificial light? The so-called Electric light. This light, whether produced by a series of galvanic cells or by dynamic power, is the most brilliant and useful. What is a Rainbow? The rainbow is that beautiful semi-circular band or arc of different colors in the clouds during the occurrence of rain in sunshine. When the clouds opposite the sun are very dark and rain is falling from them, the rays of the sun are divided by the raindrops as they would be by a prism. There are often two rainbows at the same time, because the primary bow is again reflected to another layer of clouds. What is a Prism? A triangular solid piece of glass, on which if a ray of light be cast it will be distinctly divided into the seven colors we see in a rainbow. By this fact we see that white light is composed of different rays which have different reflective susceptibilities. What is a Spectrum? [209] It is this beautiful band of seven colors obtained by the refraction of a ray of light through the prism. Whence come the colors in the objects we see in nature? They all come from light; every object has a power to absorb certain rays and to reflect others. A red cloth, for example, absorbs all the other colored rays except red, and this it gives off, thus appearing red. Why are the leaves of plants green? Because a peculiar chemical substance called chlorophyl, formed within their cells, absorbs all other rays of light, reflecting only blue and yellow—which mixture produces the different green tints. What is Photography? The word means "light drawing." It is a mode of fixing on certain substances the lights and shades of any object by means of a lens inserted in a camera obscura. This process was first called Daguerreotype from the name of the inventor, Daguerre. A plate of copper thinly coated with silver is exposed to the vapor of iodine, then placed in a camera obscura, where an image of the object to be presented through a lens is cast upon it. Ambrotype is the same application to glass. There are now different variations of method in the use of the same agents. Now photography consists in taking the images on what is called a negative—that is, a glass coated with a silvered collodion (gun-cotton dissolved in alcohol and ether) film. From this plate another image is taken on silvered paper, which we call the positive image. There are also other chemicals used instead of silver. What is a Camera Obscura? A small box or dark room into which the light is admitted through a lens. What is a Stereoscope? It is an instrument exhibiting the effects and advantages of seeing with two eyes. The instrument is so constructed that [210]from a flat picture we may see the solid body in its reality in nature. What is a Kaleidoscope? An instrument invented by Sir David Brewster, consisting of a tube with slips of reflecting glass so arranged in the interior that small beads, bits of colored glass, and similar things are, by revolving the tube, thrown into an endless variety of beautiful shapes. <hr style="width: 65%;" /> <div style="break-after:column;"></div>

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