Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

colours will vary.

      Fig. 149.—The Solar Spectrum.

      The Solar Spectrum is the name given to the coloured band formed by the decomposition of a beam of light into its elementary colours, of which there are seven. This is an easy experiment. A ray of light can be admitted into a darkened room through a hole in the shutter, and thus admitted will produce a white spot on the screen opposite, as at g in the diagram (fig. 149). If we interpose a prism—a triangular piece of glass—the “drop” of a chandelier will do—we cause it to diverge from its direct line, and it will produce a longer streak of light lower down. This streak will exhibit the prismatic colours, or the “colours of the rainbow”; viz., red (at the top), orange, yellow, green, blue, indigo (blue), and violet last. These are the colours of the Solar Spectrum. The white light is thus decomposed, and it is called mixed light, because of the seven rays of which it is composed. These rays can be again collected and returned to the white light by means of a convex lens.

      “White light,” said Sir Isaac Newton, “is composed of rays differently refrangible,” and as we can obtain the colours of the rainbow from white light, we can, by painting them on a circular plate and turning it rapidly round, make the plate appear white. Thus we can prove that the seven colours make “white” when intermingled. But Newton (1675) did not arrive at the great importance of his experiment. He made a round hole in the shutter, and found that the various colours overlapped each other. But, in 1802, Dr. Wollaston improved on this experiment, and by admitting the light through a tiny slit in the wood, procured an almost perfect spectrum of “simple” colours, each one perfectly distinct and divided by black lines.

      But twelve years later, Professor Fraunhofer made a chart of these lines, which are still known by his name. Only, instead of the 576 he discovered, there are now thousands known to us! To Fraunhofer’s telescope Mr. Simms added a collimating lens, and so the Spectroscope was begun; and now we use a number of prisms and almost perfect instruments, dispersing the light through each. We have here an illustration of a simple Spectroscope, which is much used for chemical analysis (fig. 150).

      In the spectrum we have long and short waves of light, as we have long and short (high and low) waves in music, called notes. The long or low notes are as the red rays, the high notes as the blue waves of light. (Here we have another instance of the similarity between light and sound.) But suppose we shut out the daylight and substitute an artificial light. If we use a lamp burning alcohol with salt (chloride of sodium), the spectrum will only consist of two yellow bands, all the other colours being absent. With lithium we obtain only two, one orange and one red. From this we deduce the fact that different substances when burning produce different spectra; and although a solid may (and platinum will) give all seven colours in its spectrum, others, as we have seen, will only give us a few, the portion of the spectrum between the colours being black. Others are continuous, and transversed by “lines” or narrow spaces devoid of light; such is the spectrum of the sun, and by careful and attentive calculation and observation we can get an approximate idea of the matter surrounding the heavenly bodies.

      Fig. 150.—The Spectroscope.

      We have said there are lines crossing the spectrum transversely; these are called Fraunhofer’s lines, after the philosopher who studied them; they were, however, discovered by Wollaston. These lines are caused by light from the lower portion of the sun passing through the metallic vapours surrounding the orb in a state of incandescence, such as sodium, iron, etc. One of Fraunhofer’s lines, a black double line known as D in the yellow portion of the spectrum, was known to occupy the same place as a certain luminous line produced by sodium compounds in the flame of a spirit lamp. This gave rise to much consideration, and at length Kirchkoff proved that the sodium vapour which gives out yellow light can also absorb that light; and this fact, viz., that every substance, which at a certain temperature emits light of a certain refrangibility, possesses at that temperature the power to absorb that same light. So the black lines are now considered the reversal of luminous lines due to the incandescent vapours by which the sun is surrounded. Thus the presence of an element can be found from black or luminous lines, so the existence of terrestrial elements in celestial bodies has been discovered by means of preparing charts of the lines of the terrestrial elements, and comparing them with the lines of stella spectra.

      We have supposed the beam of light to enter through a slit in the shutter, and fall upon a screen or sheet. The solar spectrum shown by the passage of the beam through a prism is roughly as below—

      Fig. 151.—Example of the Spectrum.

      Fraunhofer substituted a telescope for the lens and the screen, and called his instrument a Spectroscope. He then observed the lines, which are always in the same position in the solar spectrum. The principal of them he designated as A, B, C, D, E, F, G, H. The first three are in the red part of the spectrum; one in the yellow, then one in the green; F comes between green and blue, G in the indigo-blue, and H in the violet. But these by no means exhaust the lines now visible. Year by year the study of Spectrum Analysis has been perfected more and more, and now we are aware of more than three thousand “lines” existing in the solar spectrum. The spectra of the moon and planets contain similar dark lines as are seen in the solar spectrum, but the fixed stars show different lines. By spectrum analysis we know the various constituents of the sun’s atmosphere, and we can fix the result of our observations made by means of the Spectroscope in the photographic camera. By the more recent discoveries great studies have been made in “solar chemistry.”

      What can we do with the Spectroscope, or rather, What can we not do? By Spectroscopy we can find out, and have already far advanced upon our path of discovery, “the measure of the sun’s rotation, the speed and direction of the fierce tornados which sweep over its surface, and give rise to the ‘maelstroms’ we term ‘sunspots,’ and the mighty alps of glowing gas that shoot far beyond the visible orb, ever changing their form and size; even the temperature and pressure of the several layers and their fluctuations are in process of being defined and determined.” This is what science is doing for us, and when we have actually succeeded in ascertaining the weather at various depths in the atmosphere of the sun, we shall be able to predict our own, which depends so much upon the sun. Last year (1880) Professor Adams, in his address to the British Association, showed that magnetic disturbances, identical in kind, took place at places widely apart simultaneously. He argues that the cause of these identical disturbances must be far removed from the earth.

      “If,” he says, “we imagine the masses of iron, nickel, and magnesium in the sun to retain even in a slight degree their magnetic power in a gaseous state, we have a sufficient cause for all our magnetic changes. We know that masses of metal are ever boiling up from the lower and hotter levels of the sun’s atmosphere to the cooler upper regions, where they must again form clouds to throw out their light and heat, and to absorb the light and heat coming from the hotter lower regions; then they become condensed, and are drawn back again towards the body of the sun, so forming those remarkable dark spaces or sunspots by their down rush to their former levels. In these vast changes we have abundant cause for those magnetic changes which we observe at the same instant at distant points on the surface of the earth.” So we are indebted to the Spectroscope for many wonderful results—the constitution of the stars, whether they are solid or gaseous, and many other wonders.

      The manner in which we have arrived at these startling conclusions is not difficult to be understood, but some little explanation will be necessary.

      The existence of dark lines in the solar spectrum proves that certain rays of solar light are absent, or that there is less light. When we look through the prism we perceive the spaces or lines, and we can produce these ourselves by interposing some substance between the slit in the shutter before mentioned and the prism. The vapour of sodium will answer our purpose, and we shall find a dark line in the spectrum, the bright lines being absorbed by the vapour. We can subject a substance to any temperature we please, and