70 THE UNIVERSE A VAST SYSTEM OF PARTS a "collimating lens." The other tube also contains a lens to carry the image of the spectrum to the eye. As one views the light which is a mixture of all visible light rays through a spectroscope, the resulting spectrum appears as a series of beautiful colors like a cross section of a rainbow. A continuous spectrum (that produced by light from an incandescent solid or liquid surface) is found to vary in intensity at different positions. This variation in the intensity of different wave lengths is connected with the temperature so that the spectrometer enables one to determine the temperature of an incandescent body by analyzing the light which it emits. The spectrum obtained from a luminous gas is called a bright-line spectrum because it is not continuous but contains certain narrow lines or bands of color. Inasmuch as light has its origin within the smallest molecular and atomic electrical systems of matter, it is not surprising to learn that the analysis of light from any given source reveals an enormous amount of information concerning that source. There is a definite position for each of the bright lines, which is always the same for the same element of matter. Not only do the elements,^ such as hydrogen, nitrogen, and helium, show characteristic spectra, but so likewise do many compounds of these elements. By the use of the spectroscope man is thus able to analyze samples of matter on the earth and determine their composition. Joseph Fraunhofer (1787-1826) observed a number of distinct dark lines of varying width vertically across the continuous spectrum which He carefully noted the position of about he obtained with sunlight. 600 of these lines, which have been named Fraunhofer lines in his honor; but he failed to explain their significance. Kirchhoff's discovery of bright-line spectra in 1859, coupled with the later momentous discovery that light rays from a luminous gas, when passed through a cool, nonluminous layer of the same gas, would produce dark bands at the exact position of the bright lines produced by the gas under ordinary conditions, provided the key to a vast storehouse of knowledge concerning the universe. At last the Fraunhofer lines were explained. They must have been produced by the passing of the sun's rays through the envelope of cooler gas surrounding the sun, which itself is thought to be a huge mass of luminous gas. The bright-line spectra of known elements on the earth were found to coincide with the position of many of the Fraunhofer lines, so that it was soon learned that such elements as sodium, carbon, iron, copper, ' An element is one of the 92 different kinds of matter that cannot be decomposed into simpler kinds of matter by ordinary chemical means.
MANY INSTRUMENTS SUPPLEMENT TELESCOPE 71 calcium, nickel, zinc, magnesium, oxygen, hydrogen, and many others were common to both the earth and the sun. The spectrometer also enables the astronomer to measure star distances. The same Fraunhofer lines from different stars were observed to vary in width. By checking these star distances by the method of parallax,^ it was found that the width of the Fraunhofer lines revealed the intrinsic or actual brightness of a star. Knowing the apparent brightness of a star as measured by means of a telescope and the eye or a photoelectric cell, and knowing its actual brightness as shown by the Fraunhofer lines, it is easy to estimate star distances. One could compute distances on the earth in the same way by the use of lights of equal brightness, such as airplane beacons, and then measuring the apparent brightness of all of the lights in sight. The knowledge of the actual and apparent brightness of the stars enables one to estimate their distance from the earth, for it is known that the intensity of light varies inversely as the square of the distance from the observer. The velocity of a source of light can also be observed by means of the spectrometer. Here again the Fraunhofer lines or emission lines come into use; they are observed to shift their position slightly. In the study of sound it is found that the pitch of a musical note will be raised or lowered according to whether the source of the sound is approaching or receding from us. This effect, called the Doppler effect, is found in light also. It was noticed that the position of spectral lines produced by light from a moving object is shifted from the position they take when the light-source is standing still. If the source of the light is approaching the observer, the lines are shifted toward the violet end of the spectrum; and if it is receding, the lines are shifted toward the red end. The amount of the shift depends upon the speed at which the light-source is moving. By comparison of a spectrophotograph of a particular star with that of a standard spectrophotograph, not only the relative speed but the direction of the motion of the star can be determined. By way of summary, let us note that the spectrometer, in conjunction with the telescope and a camera, enables the astronomer to determine the composition of a star, its temperature, speed, and direction of motion relative to the earth. ' The method of parallax is used to determine star distances, using the principle employed by surveyors in determining the distances across lakes, etc. If a given star is observed at two intervals, six months apart, so that the observations are made on opposite sides of the earth's orbit, and the angle of the telescope relative to the plane of the earth's orbit is measured, then by trigonometry the distance from the earth to the star can be determined. This method is accurate only for the nearer stars. The angles are so small for more distant stars that results are unreliable for stars more than 300 light-years away.