Spectral line

“ The dark lines take their appropriate place, and are recognized on the ruled scale. Turn it on one edge, that is approaching us one and a quarter miles a second by the revolution of the sun on its axis, the spectral lines move toward the violet end; turn the spectroscope toward the other edge of the sun, it is receding from us one and a quarter miles a second by reason of the axial revolution, and the spectral lines move toward the red end. Turn it near the spots, and it reveals the mighty up-rush in one place and the down-rush in another of one hundred miles a second. ”

“ With improvements in the methods of experiment and research the number of lines has increased to hundreds and even thousands. The light from a glowing [Pg 50] solid or liquid element forms, on the other hand, a continuous spectrum, i.e. a spectrum which has no dark lines. An illustration of the solar spectrum with the strongest Fraunhofer lines is given at the end of the book.
In contrast to the solar spectrum with dark lines on a bright background are the so-called line spectra, which consist of bright lines on a dark background. ”

“ It is no doubt important to recognize that the two types of spectra seem to represent two extreme cases of one formula, the significant difference being that in the line spectrum the distance between lines diminishes as we recede from the head, while in the case of the band it increases, at any rate to begin with. But, on the other hand, no one pretends to have found the rigorous expression for the law, and the appropriate approximation may take quite different forms when constants which are large in one case are small in the other. ”

“ In this sense space is not Euclidean, but "curved." It follows from the second of the relations above that the interval between two beats of the unit clock ( = 1) corresponds to the "time" in the unit used in our system of co-ordinates. The rate of a clock is accordingly slower the greater is the mass of the ponderable matter in its neighbourhood. We therefore conclude that spectral lines which are produced on the sun's surface will be displaced towards the red, compared to the corresponding lines produced on the earth, by about 2 • 10-6 of their wave-lengths. ”

“ The reversing layer, comprising the layers above the photosphere, where the general opacity has greatly decreased and selective opacity begins to be appreciable, is responsible for the lines in the spectrum, which form the major part of the material of stellar spectroscopy. When the energy flowing out through the reversing layer in any specified wave-length is less than the energy in the neighboring continuous background, an absorption line is produced in the spectrum. ”

“ This structure is most easily seen in the part of the spectrum between E and F; above F the lines are crowded so closely that it is difficult to resolve them, and below E they appear to grow wider, more diffuse, and fainter. It seems to indicate that the principal absorption which darkens the center of a sun-spot is not such as would be caused by minute solid or liquid particles—by smoke or cloud—which would give a continuous spectrum; but it is a true gaseous absorption, producing a veritable dark-line spectrum, in which the lines are countless and contiguous. ”

“ In comparing, with the same spectroscope, the spectrum of the electric arc and the solar spectrum, we observe that the former spectrum displays a more considerable number of bright rays than the solar spectrum of dark rays. Since it is nearly certain that the spectral bands belong to the spectrum of carbon—for they are observed when the electric arc shines in a vacuum, that is, when carbon alone is in ignition—it follows that the spectrum of this element contains more rays than the entire solar spectrum. ”

“ 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. ”

“ The possibility of arranging the majority of stellar spectra in homogeneous classes that constitute a continuous series, is an indication that the composition of the stars is remarkably uniform—at least in regard to the portion that can be examined spectroscopically. The fact that so many stars have identical spectra is in itself a fact suggesting uniformity of composition; and the success of the theory of thermal ionization in predicting the spectral changes that occur from class to class is a further indication in the same direction. ”

“ 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. ”

“ On comparing the spectra of sunlight and of the light proceeding245 from the incandescent vapour of iron, it became apparent that at least sixty bright lines in the spectrum of iron coincided with dark lines in the sun’s spectrum. Such coincidences could never be observed with certainty, because, even if the lines only closely approached, the instrumental imperfections of the spectroscope would make them apparently coincident, and if one line came within half a millimetre of another, on the map of the spectra, they could not be pronounced distinct. ”

“ This photograph was made about ten seconds after the end of totality. The thin line of the photosphere, which had then emerged from behind the moon, was drawn out by the prism into the bright band, which constitutes the larger portion of the picture. This is the ordinary solar spectrum. It will be noted that while in spectra as usually seen the lines are straight, since a straight slit is used, here the lines are arcs, since nature furnishes a crescent of light. An examination of these dark arcs shows that in nearly all cases they become bright lines at the upper edge of the spectrum. ”

“ The bright envelope nearly all disappears, while the inner dark cone is considerably developed, reaches a very high temperature, and exhibits a spectrum identical with that of the outer cone of the original flame. The brightness of the spectral bands is augmented by the rising of the temperature, and two new luminous bands, a red and a violet one, become visible in the spectroscope. At this moment we recognize that these bands are composed of a series of rays or bright lines, separated from one another by dark spaces. ”

“ The colour and the position in the spectrum both depend on the interval between one wave and the next, so that if a body giving out light of a particular wave-length, e.g. the blue light corresponding to the F line of hydrogen, is approaching the observer rapidly, the line in the spectrum appears slightly on one side of its usual position, being displaced towards the violet end of the spectrum; whereas if the body is receding the line is, in the same way, displaced in the opposite direction. ”

“ If two stars are revolving round one another, their motions towards and away from the earth are changing regularly and are different; hence, if the light from both stars is received in the spectroscope, two spectra are formed—one for each star—the lines of which shift regularly relatively to one another. If a particular line, say the F line, common to the spectra of both stars, is observed when both stars are moving towards (or away from) the earth at the same rate—which happens twice in each revolution—only one line is seen ”

“ With regard to the well-known statement that solids and liquids give continuous spectra, while gases give line-spectra, it must be remarked that metallic vapors show in nearly all cases a continuous spectrum before they condense. Oxygen gives a continuous spectrum at the lowest temperature at which it is luminous. Examining liquids and solids by the method of absorption, we find that many of them show discontinuous spectra, presenting fairly narrow bands. It is not denied that the nearness of molecules does not affect the spectrum. ”

“ Radial-velocity spectrograms are secured with instruments of much lower power. Close solar and laboratory lines, of different intensities, clearly separated on Rowland's plates, are blended on stellar plates. For this and other reasons, the effective wave-lengths on the two classes of plates are different. The difficulty of assigning correct wave-lengths in the case of plates taken with a single-prism spectrograph is even greater: whole groups of separate lines are blended into one apparent line, and lines actually single are very few indeed. ”

“ Let us use the term spectral index (s) to define a number expressing the spectral character of a star. Then we may conveniently define this conception in the following way. Let A0 correspond to the spectral index s = 0.0, F0 to s = +1.0, G0 to s = +2.0, K0 to s = +3.0 M0 to s = +4.0 and B0 to s = -1.0. Further, let A1, A2, A3, &c., have [Pg 18] the spectral indices +0.1, +0.2, +0.3, &c., and in like manner with the other intermediate sub-classes. Then it is evident that to all spectral classes between B0 and M there corresponds a certain spectral index s. ”

“ A discontinuous spectrum, or bright-line spectrum as it is familiarly called, indicates that the molecules of the source of light are not crowded together, and therefore the light must come from an incandescent gas. A continuous spectrum shows only that the molecules are crowded together, or are so numerous that the body to which they belong is not transparent and gives no further information. The body may be solid, liquid, or gaseous, but in the latter case the gas must be under considerable pressure or of great extent. ”

“ These variations in spectra depend upon the fact that any substance that is heated sufficiently to emit light whether in the heavens or on the earth, will give a spectrum. If it be a solid or liquid body, we shall have what is called a continuous spectrum—that is, a colored band bright from end to end, with no sign of any dark or bright lines about it. ”

“ Bolometric methods may be used with facility and advantage in the investigation of the distribution of intensities in continuous or semi-continuous spectra but difficulties are met with in the case of line spectra. Good results in this respect have been obtained by B. W. Snow [7] and by E. P. Lewis, [8] lines as far as 11,500 having been measured by the latter. More recently F. Paschen [9] has further extended the method and added a number of infra-red lines to the spectra of helium, argon, oxygen and other elements. ”

“ The spectroscopic analysis, when applied to the same problem, walks upon slippery ground and must take heed lest it also fall. The alleged displacement is a nice quantity, and instrumental sources of error have been pointed out which may explain away the whole of it. I lay no stress upon the large difference between Vogel and Huggins in the quantity of motion which spectrum analysis ascribes to Sirius, inasmuch as the direction of the motion is the same. We do not yet know all the elements which the earth contains. The spectroscope has already added four to the number. ”

“ The whole spectrum of the moving object, in fact, is very slightly shoved hither or thither, according as it is travelling towards or from the eye; but, for convenience of measurement, one line is usually picked out from the rest, and attention concentrated upon it. The application of this method to the stars, however, is encompassed with difficulties. It needs a powerfully dispersive spectroscope to show line-displacements of the minute order in question; and powerful dispersion involves a strictly proportionate enfeeblement of light. ”

“ Glow throughout quiescent, and no stratification in the tube. A compound-prism spectroscope, taking in the whole of the spectrum, showed in the capillary stream, from yellow to red, a fairly bright wedge, having a dark band in the centre, and six bright columns, with dark lines at intervals, shading off on either side. On the more refrangible side of the yellow, the spectrum was composed of a set of bright bands and lines in the green, blue, and purple, one line only (in the green) standing out very bright. In the yellow and red no bright line stood out alone. ”

“ If the slit in a spectroscope is lighted with a yellow light from such a flame, and if a grating normal to the direction of the rays is substituted for the prism, then in the telescope there is seen a yellow image of the slit, and on each side of it one, two, three or more yellow images. If sunlight is used the central image is white, since all the colours are here assembled. The other images become spectra because the different colours are unequally refracted. ”

“ Analyzed by the aid of the spectroscope, the luminous cone gives a brilliant and continuous spectrum—that is, one having the appearance of a ribbon exhibiting all the colors of the rainbow, while the exterior, faintly luminous envelope gives a discontinuous spectrum formed of three bright bands—one yellow, one green, and one blue.
As only solid incandescent bodies are capable of giving a continuous spectrum, we conclude that carbon in the solid state is incandescent in the luminous envelope of the flame. ”