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iron white-hot, is passed through a prism, the spectrum is continuous, and consists of a series of distinct colors, varying from red on one side to violet on the other.

2d. If the light of a burning gas containing any volatilized substance be passed through a prism, the spectrum is not continuous, but is ornamented by bright-colored lines—sodium giving two yellow lines, strontia a red one, silver two beautiful green ones. Each element produces a definite series which can be readily recognized as its test.

3d. If a light of the first kind be passed through one of the second, the spectrum will be found to be crossed by dark lines. Thus, if the white light of a burning match be passed through a flame containing sodium, instead of the vivid yellow lines so characteristic of that metal, two black lines will exactly occupy their place. A gaseous flame absorbs the rays of the same color that it emits.

The Spectroscope.—This instrument consists of two small telescopes, with a prism mounted between their object-glasses, in the manner shown in the cut. The rays of light enter through a narrow slit at A and are rendered parallel by the object-glass. They then pass through the prisms at C, are separated into the different colors, and entering the second telescope at D, fall upon the eye at B. A third telescope is sometimes attached, which contains a minutely accurate scale for measuring the distances of the lines. In addition, a mirror may throw in a ray of sunlight or starlight at one side of the slit, and so we can compare the spectrum of the sunbeam with that of any flame we desire.

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Revelations of the spectroscope concerning the sun.— The spectrum of the sunbeam is not continuous, but is crossed by a large number of dark lines, called, from their discoverer, Fraunhofer's lines. It is therefore concluded that the sun's light is of the third class just named, and that it is produced by the vivid light of a highly heated body shining through a flame full of volatilized substances. But not only does spectrum analysis thus shed light on the physical constitution of the sun, but these lines are so distinctive, so marked and varied, that the very elements of which the sun is composed may be discovered. Thus, for example, iron gives a spectrum of some 70 lines, differing in intensity and relative length. These are bright when iron vapor is burning, and dark when white light is passed through such burning vapor. In the solar spectrum we have the perfect coincidence of 70 dark lines, line for line and strength for strength. The conclusion is irresistible that iron is contained in the sun's atmosphere. The following include all the elements that are now known to exist in it:

Sodium,

Calcium,
Barium,
Magnesium,

Stars Are Suns.—The same method of analysis has been applied to the stars. Their spectra also are marked by dark lines. Their constitution is therefore like our sun; they contain also the same familiar elements. Aldebaran seems the most like our earth. It has at least nine elements known to chemists:

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Betelgeuse contains many elements known to us, but no hydrogen.—What a world that must be without water! Arcturus, Butherford says, closely resembles our sun.

We thus trace in the faintest star that trembles in the measureless depths of space the same elements that compose the food we eat and the water we drink. We know that we are akin to nature everywhere—that we are a part of a system vast as the universe.

Spectra Of Nebula.—Instead of being marked with dark lines, as are the spectra of the stars, many of these exhibit bright lines. Their spectra are of the 2d kind. This proves the nebula to consist, not, like the stars, of an intensely heated solid body shining through aluminous atmosphere, but of a glowing mass of gas. Out of 60 nebulae examined by Mr. Huggins, 20 exhibited the bright liues belonging to the gases, and all contained nitrogen.

It is possible in this manner even to decide the relative brightness of the different nebulae. The dumb-bell nebula was found to emit a light only about one twenty-thousandth part that of a common wax-candle. If this matter be a "sun-germ," how immensely must it become condensed before its rushlight glimmering can rival the dazzling brilliancy of even our own sun 1

The Solar Flames, which before were seen only at an eclipse, can now be examined at any time. The sun is a sea of fire. Flames travel over its surface faster than the earth in its orbit: one shot out 80,000 miles and disappeared in ten minutes. Such tremendous convulsions surpass all terrestrial phenomena.

TIME.

Sidereal Time.—A sidereal day is the exact interval of time in which the earth revolves on its axis. It is found by marking two successive passages of a star across the meridian of any place. This is so

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absolutely uniform, that the length of the sidereal day has not varied -^ of a second in 2,000 years. The sidereal day is divided into twenty-four equal portions, which are called sidereal hours, and each of these into sixty portions, termed sidereal minutes, etc.

Astronomical clocks are regulated to keep sidereal time. The day commences when the vernal equinox is on the meridian. Therefore, the time by the sidereal clock does not in any way point out the hour of the ordinary day. It only indicates how long it is since the vernal equinox crossed the meridian, and thus always shows the right ascension of any star which may happen to be on the meridian at that moment. The hours of the clock are easily reduced to degrees (see p. 38). The astronomer always reckons the hours of the day consecutively up to twenty-four.

Solas Time.—A solar day is the interval between two successive passages of the sun across the meridian of any place. If the earth were stationary in its orbit, the solar day would be of the same length as the Bidereal; but while the earth is turning around on its axis, it is going forward at the rate o. 360° in a year, or about 1° per day. When the earth has made a complete revolution, it must therefore perform a part of another revolution through this additional degree, in order to bring the same meridian vertically under the sun. One degree of diurnal revolution is about equal to four minutes of time.

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