Spectroscope

“ The white light of the sky, when passed through a spectroscope, is drawn out into a long rainbow band, and thereby enormously reduced in intensity. The light of the prominences, on the contrary, is concentrated in the radiations characteristic of hydrogen and helium gas, and the great dispersing power of the spectroscope merely separates more and more widely the colored images which correspond to these radiations, without greatly reducing their intensity. ”

“ So valuable a means of research has this process of analysis proved to be, that the discoveries made by the spectroscope appear marvellous. The spectroscope was first made known as a refined instrument for the analysis of light by two Germans, a physicist and a chemist, Kirschoff and Bunsen. In 1860, the latter succeeded in detecting and separating two new alkaline bodies from all other bodies from the waters obtained from the Durkeim springs, less than 0·0002 part of a milligramme of which can be detected by spectrum analysis. ”

“ Ordinary incandescent vapours from highly heated substances give discontinuous spectra, i.e. spectra in which the rays of coloured light are quite limited, and they appear in the spectroscope only as lines of the breadth of the slit. These are called line-spectra, and every chemical element possesses in the incandescent gaseous state its own characteristic lines of certain colour and certain refrangibility, by means of which that element can be recognised. ”

“ Conversely the phenomena of the spectroscope all depend on the fact that the vibrations of atoms and molecules can propagate waves through the ether, as well as absorb ether-waves into their own motions, and thus give spectra distinguished by bright or dark lines peculiar to each substance, by which it can be identified. Whatever ether may be, this much is certain about it: it pervades all space. ”

“ But then it may be asked, How comes it that the prism and the spectroscope are so useful to astronomers? In answer we may say, that if we knew no more about the black lines in the spectra of the sun and stars than we knew forty years ago, the spectroscope ought still to be an astronomical instrument, because it is our duty to observe every fact in nature, even if we cannot explain it. ”

“ Let’s see now, how a real spectroscope is made, for it isn’t always convenient to have a pitch dark room, nor is it a very exact way to hold the prism to your eye when examining the light of burning sodium, or hydrogen gas, or other substances. ”

“ We saw in a previous chapter how the spectroscope splits up light-waves into their colours. But nature is constantly splitting the light into its different-lengthed waves, its colours. The rainbow, where dense moisture in the air acts as a spectroscope, is the most familiar example. A piece of mother-of-pearl, or even a film of oil on the street or on water, has the same effect, owing to the fine inequalities in its surface. ”

“ If upon the spectrum were printed the word BLOOD in the largest and blackest of capitals, it could not be more legible to an ordinary reader than are the two blood-bands to the eye of a spectroscopist. There is nothing like them in nature, and whether it be by association of ideas, or by absolute fact, these two bars have a strangely menacing look about them. Not only that, but if the blood should be that of a person suffocated with carbonic acid gas, the Spectroscope will say so.
Some years ago a man owed his life to the Spectroscope. ”

“ Every chemical element known, then, has a distinctive spectrum of its own when it is raised to incandescence, and this distinctive spectrum is as reliable a means of identification for the element as a human face is for its owner. Whether it is a substance glowing in the laboratory or in a remote star makes no difference to the spectroscope; if the light of any substance reaches it, that substance will be recognised and identified by the characteristic set of waves. ”

“ And soon the microscope unfolds the beautiful forms of the Polycystina shells, the minute fibril of the muscle, and the components of the blood of life. The telescope brings near the world of stars, and resolves the bright mist into clusters of distinct orbs. The balance weighs quantities of matter too small for the touch to appreciate. And lastly, the spectroscope takes the picture, so to speak, of chemical phenomena too distant to be realised by these means ”

“ The history of the particular spectroscope he employed is in one respect specially interesting to myself, as the extension of spectroscopic power was of my own devising before I had ever used or even seen a powerful spectroscope. The reader is aware that spectroscopes derive their light-sifting power from the prisms forming them. The number of prisms was gradually increased, from Newton’s single prism to Fraunhofer’s pair, and to Kirchhoff’s battery of four, till six were used, which bent the light round as far as it would go. ”

“ What I have ventured to call modern spectroscopy is the union of the old science with the modern ideas of the dynamical theory of gases, and includes the application of the spectroscope to the experimental investigation of molecular phenomena, which without it might for ever remain matters of speculation or of calculation.
A body, then, is made up of a number of atoms. These are hardly ever, perhaps never, found in isolation. Two or more of them are bound together, and do not part company as long as the physical state of the body remains the same. ”

“ Fundamentally, spectroscopic analysis depends upon the principle of refraction, of which we have spoken in connection with the atmosphere. Although the most powerful 147spectroscopes are now made on a different plan, the working of the instrument can best be comprehended by considering it in the form in which it was first invented, and in which it is still most often used. In its simplest form the spectroscope consists of a three-angle prism of glass, through which a ray of light is sent from the sun, star, or other luminous object to be examined. ”

“ Such an instrument is called an analyzing spectroscope while one without the lens is called an integrating spectroscope, since it furnishes to each point of the slit a sample of the radiant energy coming from every part of the source of light, and thus produces only an average spectrum of that source without distinction of its parts. When a spectroscope is attached to a telescope, as is often [Pg 141] done (see Fig. 49) , the eyepiece is removed to make way for it, and the telescope objective takes the part of the analyzing lens. ”

“ By means of the spectroscope the wave-length of different kinds of light may be compared to within one ten-thousandth part. In this way it has been ascertained not only that molecules taken from every specimen of hydrogen in our laboratories have the same set of periods of vibration, but that light having the same set of periods of vibration is emitted from the sun and from the fixed stars. We are thus assured that molecules of the same nature as those of our hydrogen exist in those distant regions, or at least did exist when the light by which we see them was emitted.... ”

“ At the eye end of the telescope is attached the spectroscope and the micrometer, and the whole set of instruments so adjusted that just the edge of the sun is seen, making a half spectrum. The other half of the spectroscope projects above the solar limb, and is dark, so if an explosion throws up liquid jets, or flames of hydrogen, the astronomer at once sees them and with the micrometer measures their height before they have time to fall. ”

“ We may hope for abundant results from such a research.
The spectroscope is a joint gift from both chemistry and physics: the mysterious dark lines of Wollaston or Fraunhofer's spectrum, which must be so familiar to many here, have been shown to be variously illuminated by incandescent chemical substances; and by the great genius of Kirchhoff and Bunsen they have been made to bear witness to the presence of these substances, when inconceivably small in quantity. ”

“ Neither must it be supposed that the services of the spectroscope are confined to metals, for nearly all coloured matter can also be subjected to its scrutiny. Even the most minute substances, when examined by the microscope in conjunction with the prism, shew a particular spectrum by which{344} they can always be identified. Nor does the form of the substance present any difficulty in its examination, for a solution will shew the necessary absorption bands. Blood, for instance, can be discovered when in a most diluted form. ”

“ The spectroscope has convinced the chemist that a chemical analysis can be effected otherwise than by mixing substances in a testing-tube, and that he can react upon the gas of a distant comet as well as upon the hydrogen in the water which flows from a stop-cock in his laboratory. Modern analysis has shown that the limits of mathematical insight, which the synthetical geometrician supposed to be absolute, may be transcended indefinitely ”

“ As to nitrogen, its presence in such bodies is doubtful, so far as the spectroscope is able to inform us; but, as it exists in such quantities in the earth's atmosphere, the belief is strong, especially among those who accept the nebular hypothesis, that the failure to discover it there is due to our imperfect methods, or to our ignorance of the manner in which the phenomena of the spectroscope are to be interpreted. ”

“ The spectroscope, however simple or complex it may be, is an instrument which allows us to observe the image of the slit through which the light enters it, in the most perfect manner. If the light contains rays of every wave-length, then the images formed by each will be so close together that the spectrum will be continuous, that is, without break. If the light contains only certain wave-lengths, then we shall get certain, and not all, of the possible images of the slit, and the spectrum will be discontinuous. ”

“ Special precautions often are necessary to accomplish this purpose, for too high a temperature would invariably break up the compound molecule into its more elementary constituents. For some bodies I may employ the low temperature of an ordinary Bunsen burner. With others, a weak electric spark taken from their liquid solutions will supply a sufficient quantity of luminous undecomposed matter to allow the light to be analyzed by a spectroscope of good power. The spectrum of a compound body is never a line-spectrum. It is either a spectrum of bands or a continuous spectrum. ”

“ Any ordinary pocket spectroscope, with or without scale or a comparison prism over part of the slit, can in fact be fitted to an [Pg 185] adapter and used with the star focussed on the slit and a cylindrical lens, if necessary, as an eye-cap.
Such slit spectroscopes readily give the characteristics of stellar spectra and those of the brighter nebulæ or of comets. They enable one to identify the more typical lines and compare them with terrestrial sources, and save for solar work are about all the amateur observer finds use for. ”

“ Contortion of Lines on the Sun's Limb.—When the spectroscope is directed to the centre of the sun's disk, the distortion of the lines indicates only vertical motion in the sun's atmosphere; but, when the spectroscope is directed to the limb of the sun, displacements of the lines indicate horizontal motions in the sun's atmosphere. When a powerful spectroscope is directed to the margin of the sun's disk, so that the slit of the collimator tube shall be perpendicular to the sun's limb, one or more of the dark lines on the disk are seen to be prolonged by a bright line, as shown in Fig. ”

“ I should like to explain to you how I believe the spectroscope may contribute its share to the settlement of that question. In his first great paper on the molecular theory of gases, the late Professor Clerk Maxwell assumed that two molecules may actually come into contact, that they may strike each other, as two billiard-balls do, and then separate, according to the laws of elastic bodies. This theory is difficult of application when a molecule contains more than one atom, and, especially as it did not in the case of conduction of heat give results ratified by the experimental test. ”

“ When an astronomer wishes to examine the spectrum of any special part of the sun, he takes off the eye-piece of his telescope and screws the spectroscope upon the draw-tube. The spectroscope is [Pg 133] made exactly like the large one for ordinary work. The tube d (Fig. 49) carries the slit at the end nearest the telescope, and this slit must be so placed as to stand precisely at the principal focus of the lens where the sun's image is formed (see i, i, p. 44) . ”

“ It is thus manifest that the resources of the older astronomy were quite incapable of meeting the demands of astronomers when it became necessary to learn the movements of the stars towards us or from us, as well as the movements perpendicular to the line of vision, which had always been the subject of much investigation. It is just here that the spectroscope comes in to fill the vacant place in the equipment of the astronomer. ”

“ It was not until the invention of the spectroscope that the theory was universally abandoned. Untenable though it was, it was the first serious attempt to co-ordinate the isolated facts ascertained concerning phenomena of the solar disc. ”

“ Microscopes, opera glasses, and other magnifying instruments depend upon the same principles as the telescope.
One of the most astounding of man's tools is the spectroscope, an instrument used for analyzing light. Through a knowledge of chemistry scientists can establish scientific relations between different substances and the light which they emit. ”