254 SPECTRUM ANALYSIS 7 Argus, p Lyra, &c., show the lines of hydro- gen bright instead of dark, as though surround- ed by hydrogen glowing with a heat more in- tense than that of the central orb itself around which the hydrogen exists. Secchi's observa- tions were comparatively rough, and the infer- ence that particular lines, as those of hydrogen for example, are really present depended in his case simply on the general correspondence of a set of lines with the set belonging to the element. But Huggins and Miller, in England, showed, by the direct comparison of stellar with terrestrial spectra, that certain elements exist in particular stars. Thus they found in the spectrum of Aldebaran lines correspond- ing with those of hydrogen, sodium, magne- sium, iron, tellurium, calcium, bismuth, anti- mony, and mercury. In the spectrum of Be- telgeuse they recognized the lines of sodium, magnesium, iron, bismuth, and calcium, but found those of hydrogen wanting. They dis- covered that (at least in the instances exam- ined by them) the colors of the double stars are due to the existence of stellar atmospheres exercising an elective absorption. For exam- ple, the spectrum of the orange component of the well known double star Albirco shows dark bands in the blue and violet ; while the spec- trum of the blue component shows many strong lines in the orange and red. The nebulae show FIG. 5. Spectrum of Nebula (H 4,874). SPECULUM sunlight. Tempers comet (1866) was the first analyzed with the spectroscope, by Huggins in England. Winnecke's (1868) was the first to which careful analysis was applied, with the curious result of observing that the bands agree in position with those obtained as the spectrum of carbon, by passing the electric spark through olefiant gas. Huggins obtained the same result (which was confirmed by Pro- fessors Harkness and Young in America) from the study of Encke's comet in 1872. The first large and long-tailed comet studied with the spectroscope was Coggia's comet of 1874. Huggins gives the following account of the spectroscopic analysis of this comet : " When the slit of the spectroscope was placed across the nucleus and coma, there was seen in the instrument a broad spectrum, consisting of the same three bright bands exhibited by comet II., 1868, crossed by a linear continuous spec- trum from the light of the nucleus. On the continuous spectrum of the nucleus I was not able to distinguish with certainty any dark lines of absorption, or any bright lines, other than the three bright bands. Besides these spectra, there was also present a faint broad continuous spectrum between and beyond the bright bands. When the slit was moved on to different parts of the coma, the bright bands and the faint continuous spectrum were found to vary in relative in- tensity. When the slit was brought past the nucleus on to the com- mencement of the tail, the gaseous spectrum became rapidly fainter, two orders of spectra. One class, including the clusters, resolvable and suspected resolva- ble nebula, besides other nebulaa which proba- bly are resolvable into stars though no signs of the fact can be detected with the telescope, show a spectrum resembling the stellar spec- trum in general characteristics, though usually too faint to be assigned to any given order of stellar spectra. The other class, which in- cludes all the irregular and planetary nebulas, besides most of the elliptic irresolvable nebula the ring nebula in Lyra, the dumb-bell nebu- la, and others, presents the remarkable phe- nomenon of a spectrum of three bright lines (in a few cases four lines are seen). Fig. 5 is the spectrum of the nebula known as H 4,374. This is a small but bright object, and it is the nebula to which Huggins in 1864 first direct- ed his telespectroscope. One line agrees in position with a hydrogen line, another with a nitrogen line, but the third line has not yet been shown to coincide with a line of any known element. Comets show a mixed spec- trum, the nucleus, coma, and tail each giving a combination (though in varying proportions) c a discontinuous or band spectrum, and a continuous spectrum due probably to reflected the continuous until, at a short dis- tance from the nucleus, spectrum predominated so strongly that the middle band only, which is the brightest, could be detected on it." The planets, shining by reflected light, can only re- veal under the spectroscope the possible pres- ence of absorptive vapors in their atmosphere. (See MAES.) SPECULUM (Lat., mirror), a term commonly applied to concave metallic reflectors, such as are used in reflecting telescopes for concen- trating the rays of light from distant lumi- nous bodies, and presenting the image of these in their focus. Their perfection consists in large surface, whereby they collect the great- est quantity of light; in the highest possi- ble polish,_ whereby it is reflected with least loss; and in the most exact parabolic curva- ture, rendering the image distinct and precise. In a speculum of 6 ft. diameter, a variation even at its edge from the true parabolic curva- ture, so minute as to escape detection with any except the most refined means of measurement, may render the whole useless. The metallic alloy best adapted for the requirements of specula was first employed for this purpose by Sir Isaac Newton, and is similar to that used by the ancient Egyptians for mirrors. It con-
