SPECTRUM (in optics) is the name applied to the coloured image of the sun, or a narrow luminous object, when the light proceeding from it is transmitted through a prism, and either allowed to fall on a screen at some distance, or received directly into the eye. [Duna Hoar.] A similar coloured image, produced by diffraction, may be obtained without a prism [Eurrneartex or Liana], and to it the term spectres; equally applies.
The fixed lines of the solar spectrum have been already mentioned in the former of the articles just quoted. Much light has recently been thrown on their origin by the labours of Professor Kirchoff. Fraunhofer long ago observed that the spectrum of a candle, or of a spirit-lamp, exhibits a double bright line, which exactly coincides in position with the double dark line D of the solar spectrum ; and Sir David Brewster noticed a similar correspondence in the case of defla grating nitre. (` Report of the British Association,' for 1842, p. 15.) An imitation of the fixed lines of the solar spectrum, as to their general character, is obtained by subjecting light to the absorbing action of certain gases, as was first discovered by Sir David Brewster, in the case of nitrous acid gas. If the light of a lamp, which, if unmodified, would give a continuous spectrum, be transmitted through this gas, and then analysed, the spectrum is seen to be traversed by numerous dark lines or narrow bands, which by their sharpness of definition, narrowness, and apparently capricious arrangement, remind one of the lines of the solar spectrum. That some of the fixed lines of the spec trum seen when the sun is near the horizon are to be attributed to absorption by the earth's atmosphere, is evident from the fact that they are not seen when the sun is high ; but that all have not this origin seems to follow from the fact, observed by Fraunhofer, that different fixed stars have lines of their own, whereas were all the lines due to absorption in the earth's atmosphere, the same system ought to be seen whether the source of light were the sun or a fixed star.
During the researches in which he was engaged, in common with Professor Bunsen, on the application of the prismatic analysis of flames to qualitative chemical analysis, Professor Kirchoff was led to discover that flames which exhibit in their spectra bright lines of certain definite refrangibilities at the same time absorb energetically light of those precise degrees of refrangibility. (Poggendorffs Annalen,'
vol. cx., p.161; or Phil. Mag.' for August, 1860.) He further connected this phenomenon with Prevost's theory of exchanges. He found that if behind a flame giving bright lines in its spectrum were placed a brilliantly luminous body of higher temperature, giving itself alone a continuous spectrum, the compound light, consisting partly of light emitted by the flame, partly of light emitted by the luminous body behind it and transmitted through the flame, exhibited on analysis dark lines in the places of the bright lines giver' by the flame alone ; the regions of those lines, as compared with the neighbouring regions, suffering more in luminosity by absorption of the light from the luminous body than they gained by the light emitted by the flame. A particular instance of this simultaneous emission and absorption of rays of definite refrangibility had been observed in the case of the voltaic arc many years before by Foucault (` L'Institut,' for Feb. 7, 1849), who, however, had not further followed the observation, nor connected it with the theory of exchanges. Now the outer portions of the atmosphere of the sun or a star, and the inner portions, or else the solid or liquid body itself, may be conceived to be in the same relative condition as the flame in the above experiment and the luminous body behind it; and thus we are led to connect the dark lines seen in the spectra of light coming directly from the sun or a fixed star, with the presence in their atmospheres, in a state of incandescence, of those elements, which, when present in flames, cause them to give out bright lines of the mine refrangibility. It is found that the intro duction of small quantities of various metallic salts into a flame causes Its spectrum to exhibit bright lines, depending upon, and characteristic of, the metal introduced ; and by comparing these lines with the dark lines of the solar spectrum we may infer the presence or absence of such metals therein. Thus Professors Bunsen and Kirchoff have con cluded that the solar atmosphere contains potassium and sodium, that lithium is absent, or present in comparatively small quantity, and sc forth.