Harper's new monthly magazine, 41 / 1870
The greatest invention, or rather discovery and invention combined, of modern times, in respect to practical utility, is probably the electric telegraph. The one most important, in respect tot he development of scientific truth, is the spectroscope. The history of this instrument, now attracting so much attention is briefly this:
It was about fifty years ago that Frauenhofer, a distinguished German optician residing in Munich, by means of some very perfect optical apparatus, and some new and delicate observations which he made with it, discovered the existence of certain very mysterious lines, some dark and some bright - the latter of various colors - crossing the solar spectrum, among the colored bands which form it. As ordinaly observed, by means of a common prism, the different colors are blended together, passing into each other by insensible gradations. The reason of this is that in the ordinary spectrum the images produced by the different colored rays, or rather the images produced by rays of different refrangibility, on which the effect of different colors is produced in the human sensorium, are superposed upon each other, and their confines blended. The great point was so to perfect the apparatus as to separate these component portions of the spectrum as much as possibe by the use of the most perfect lenses and prisms, and by the nicety of the arrangements for making the observation.
This Frauenhofer succeeded in doing, and thus brought the lines above referred to fully into view. This was in 1815. He studied these lines very attentively, and made a very carefully prepared colored drawing, representing the spectrum, as he developed it, with the principal lines laid down upon it. He counted in the spectrum more than six hundred of these lines, and he laid down upon his map three hundred and dudty-four of them. The principal of them he designated by letters of the alphabet, and they continue to be known as Frauenhofer's lines, and to be distinguished severally by the letters which he gave them, to this day.
The discovery of these lines, and the exhibition of the map delineating them, copies of which were multiplied and disseminated throughout Europe, excited great attention; but the nature and the meaning of the lines remained for many years a profound and and inscrutable mystery.
It was at length discovered that while the position and character of the lines in the spectrum formed from the sun were always the same, yet they varied greatly in spectra of rays issuing from different artificial sources of light, though constant for each particular source. And it was finally ascertained that they depend on the chemical constitution and physical condition - as whether solid, liquid, or gaseous - of the ignited substance from which the light came; and also in some degree upon the character of the intervening media through which the light passed on its way from the source to the instrument. It would lead to too much detail to enter here into particulars in respect to these points, the object of this article being only to give a summary idea of the nature of the discovery, about which so much is said at the present day, and of the form and appearance of the instrument, sufficient for the purposes of the general reader.
The engraving represents one of the various forms of the instrument. it consists of a stand with three branches, each supporting a telescopic tube; all three of the tubes being directed toward a glass prism which occupies the centre of stand. This prism is covered, in the actual instrument, by a metallic box to protect it from injury - openinigs being left in the sides of the box opposite to the orifices of the three telescopic tubes.
The tube to the right is directed toward the source of light, whatever it may be, the spectrum of which is to be examined. It is represented in the engraving as receiving the light from a gas flame, the flexible pipe by which the gas is supplied being seen below upon the table. The head of the stand supporting the burner is provided with the means of intermingling various substances with the flame, in order to examine the spectra which they severally produce.
This telescope may also be turned toward any natural source of light - as the sun, for example, or a star, or the aurora borealis, or a portion of the Milky-way.
The rays emanating from the source of light, whatever it may be, pass into this right-hand tube through a very narrow vertical slit, not seen, of course, in the engraving. The ribbon-like beam thus formed, passing through the tube, strikes upon the prism, and the various rays separated by the prism, through their different refrangibility, enter the object-glass of the tube on the left, where an image of the spectrum is formed, and can be seen, magnified, by the observer, who places his eye at the left hand of this tube, where the eye-glass is placed. There is a screen placed here near the end of teh tube, to protect the eye from rays coming to it on the outside of the instrument.
There is a third tube, smaller than the others, seen in the centre, and directed toward th spectator. In the orifice of this tube in the engraving can be indistinctly seen a small micrometer scale, illuminated by a candle placed upon a stand. The light from this arrangement passes through the small tube, and the image of the micrometer scale is reflected from one of the surfaces of the prism, and enters the left-hand tube, and is seen by the observer superposed upon the spectrum, and in such a position as to enable him to measure the distances from one spectral line to another, and to determine with precision the place of any one under examination.
The object of this article is simply to give to the general reader an idea of the construction and form of the instrument and the mode of making observations with it, and not to enter at all into a discussion of the results which have been obtained. It may, however, not be out of place to state the three general principles which form the foundation of the science of spectral analysis - or perhaps, rather, the three great divisions within which all the phenomena are comprised. The distinction depends on the conditions under which the light to be examined emanates.
1. It may emanate from an incandescent liquid or solid body.
2. It may emanate from an incandescent gaseous body.
3. It may emanate from an incandescent liquid or solid, and pass through an incandescent gaseous medium on its way.
In the first case, that is, where the source of the light is an incandescent liquid r solid body, the spectrum is contunuous, showing no traversing lines of any kind.
In the second case, that is, where the source of light is an incandescent substance in a gaseous state, the spectrum produced, instead of being continuous, consists of a certain number of variously colored bands, which are always rigorously the same for the same substance, but vary infinitely in number, color, and position when different substances are compared.
In the third case, that is, where light comes from an incandescent solid or liquid - and so would, if not modified in its passage, give a continuous spectrum - and passes, in its course, through a flame, that is, through a gaseous substance in a state of incandescence, the bright lines which the substance of the incandescent gas would have produced if it had been solid are changed into dark ones. That is, the gaseous substance intercepts and absorbs the peculiar luminous vibrations which it would have emitted if it had been the source of light instead of being the medium through which a brighter light from another source beyond it was transmitted.
Thus, if the spectrum given by any light, when examined in the spectroscope, consists of a continous longutudinal band of prismatic colors, it is known that the source of the light must be an incandescent solid. If it consists of transverse bars or bands of color on a dark back-ground, the observer infers that the source of the light is an incandescent gas; and the number, positions, and colors of these bands show what the substance is, provided it is one the spectrum of which is known. If, in the third place, the spectrum consists of a longutudinal band of prismatic colors tracersed by dark lines or bars crossing it at various intervals, the indication is that the ight under examination, coming from some source beyon, traverses an incandescent gas or flame in its way; and the dark lines, in this case, are found to correspond for any susbtance, in breadth and position, with the bright colored lines seen when the substance itself is the source of light instead of the medium through which a brighter light from an incandescent solid or liquid beyond passes on its way.
These transverse bars, whether bright, as in the second case, or dark, as in the third, are exceeding delicate, though perfectly distinct, and are very sharply defined; so that by means of a micrometer, which forms a part of the instrument, the position of any one can be very precisely determined, and its identity ascertained. And by means of them those who have attained to the necessary knowledge and skill required to read and interpret these wonderful revelations can determine, with apparent certainty, a great many facts in relation to the elementary constitution and physuical condition of the substance which constitute the sources of the light, not only of those which are near, but also, with equal facility, those which are inconceivably remote. The test is applied with same success to the light emitted from artificial sources in the laboratory, and to that coming to us from the aurora borealis, from the disk, or from the marginal conusactions of the sun, and from the remotest nebula or star.
Every different substance, so fas as is known, has its spectrum marked by its own peculiar and charasteristic lines. Even those which have never been analyzed, and so are classed by the chemists as simples substances, have usually more than one, whoch seems to indicate that they are really compounds, and that their actual elements have not been ascertained. This being so, and as the photosphere of the sun contains a great number of elements, the number of these lines in the solar spectrum is very great. It requires, however, great delicacy and exactness in the prism, and in the arrangement of the apparatus for observing them, to bring them into view. Sir Isaac Newton, as early as 1675, succeeded in forming a spectrum which showed the prismatic colors; but his apparatus and his method were too imperfect to bring the lines into view. Dr. Wollasten, in the early part of this century, was the first to do this, and to him is accordingly ascribed the honor of first discovering the existence of the lines. Afterward, by means of great improvements made in the construction of optical instruments, the celebrated optician Frauenhofer, as we have already seen, brought a great many more into view, and made a map of the spectrum, in which he laid down the positions of about six hundred of them. This map excited great attention throughout Europe, as has already been stated, and the lines were long known as Frauenhofer's lines, though no one had the least idea of the cause or of the significance of them, excepting that surmises were made by different observers that there was some connection between the position and character of the lines and the chemical constitution of the substances from which the light emanated. The number of lines since brought to view in the solar spectrum is more than six thousand!
Nothing can be more wonderful than the inconceivable delicacy of the test which the spectrum, fully developed and microscopically observed, affords. It detecs the presence of quantities of an element so exceedingly minute as to be wholly inappreciable by any other means. One of the most curious illustrations of this fact is afforded by the discovery of two new metals in a certain German mineral water, by Professor Bunsen, in 1860 - metals which existed in the water in quantities so exceedingly minute that by no other possible means than spectral analysis could their presence have been detected. Bunsen was led to suspect the existence of some new element in the water by observing two bright lines in the spectrum produced by a flame in which the alkalies left by the evaporation of a portion of the water had been introduced - which lines he had never observed in any other spectrum. The ordinary chemical tests gave no indications of the presence of such elements. This Bunsen attributed to the minuteness of the quantities; and in order to increase the quantities so as to bring the substances within the reach of the usual methods of analysis, he went to work to evaporate no less than forty-five tons of the water, and from the residue thus obtained he succeeded in obtaining an appreciable quantity of the metals in question. The names given to them are caesium and rubidium.
Another very curious instance of the delicacy of this test is afforded in the case of sodium, the spectrum of which is very remarkable, and is characterized by one line in particular by which exceedingly minute quantities can be detected. Now sodium is one of the most universally diffused substances in nature. It is one of the constituent elements of common salt, and so exists in enormous quantities in the ocean. And as two-thirds of the surface of the earth is covered with water, and as the winds are continually carrying up a fine spray from the crests of the waves into the air, the water of this spray in evaporating leaves the air full of infinitesimally minute particles of salt, which pervade the atmosphere every where, and form, as it were, a portion of its very substance. And although the quantity of sodium thus present is too small to be detected in ordinary cases by any of the usual chemical tests, it reveals itself at once in the spectrum whenever the minutest quantity of dust which has subsided from the atmosphere is thrown into the flame.
It would require many months' study to understand fully the character and the working of the instrument, and the nature and extent of the revelations which it has made, and which it is still making, in respect to the chemical and physical constitution of the distant bodies which form the sources of natural light. The knowledge which it brings to us is different in its character, as well as more subtile and precise in its minuteness, in comparison with the knowledge which we obtain by the means. With the telescope the observer sees the form and the colors of the object which comes into the field of view, and so vastly magnified that detals are clearly distinguished that are wholly invisible to the naked eye. With the spectroscope he sees no form, and no color characteristic of the object which he is studying; but simply a beautiful iridescent band, across which are drawn at intervals certain delicate and well-defined lines, which by their position and character form for him a perfectly intelligible language, which expresses the result of almost accurate, and thoroughly trust-worthy analysis of the hidden constitution of the material of which the object is composed.
Nor is it wholly to the constitution and character of distant and inaccessible objects that the revelations of the spectroscope have been confined. Some very important truths have been brought to light by it in regard to the chemical constitution of substances in the laboratory which have long been subject to the closest analytical examination, and many facts have been elicited which had baffled all preceding ofes of investigation. In a word, the "spectral analysis" opens an entirely new avenue of expoloration for man into the realms of nature around him - an avenua ehich runs in an entirely different direction from all previously known, and extends to an infinitely greater distance than any of them.