Rarely, meteors may appear at heights of 150 or more miles and fireballs may penetrate to within a few miles of the earth. The average meteors, however, appear and disappear within a well-defined, high-altitude zone in the atmosphere. Fortunately, this atmospheric zone serves us as an effective shield against the constant bombardment of the smaller and much more numerous particles from outer space. In earlier times, scientists thought that the particles becoming visible as meteors must be tiny dense masses of iron or stone like the material composing the recovered meteorites. Most modern investigators, however, believe that the typical meteor-forming particles may be small loosely bound-together “dust-balls”; that is, fluffy clusters of matter held together by frozen cosmic vapors, generally referred to simply as “ices.” In any event, these masses are usually very small, ranging perhaps from the size of a pinhead to that of a marble. Because we cannot collect the tiny masses that are seen only as meteors, it is impossible to determine their composition by ordinary laboratory methods. The best we can do is to observe and record carefully the light these masses give off when they become incandescent in their plunge through the atmosphere. We can examine this meteor light by using the spectroscope and spectrograph. Through these specially designed instruments we can make the meteor light reveal the chemical elements present in the incandescent masses. Each such element sends out light rays as characteristic of its nature as fingerprints are of the individual who made them. Photographs taken of these characteristic light rays are called _spectrograms_, and what might be termed the “fingerprints of light” recorded on these spectrograms are known as _spectra_—which is the plural of the word _spectrum_. If the source of light is a meteor, the photograph shows a meteor spectrum. From a study of a considerable number of good quality meteor spectra, scientists have found that the principal elements in the masses responsible for meteors are iron, calcium, manganese, magnesium, chromium, silicon, nickel, aluminum, and sodium. As we have already noted, the resistance encountered by meteor-forming particles as they dash through our atmosphere is so great that they become incandescent and vaporize. These small bodies must therefore be in very rapid motion.