Page:The American Cyclopædia (1879) Volume XV.djvu/252

 240 SPECTRUM lines, and also the invisible lines subsequently discovered beyond the violet and red ends of the spectrum, have been called "the Fraun- hofer lines." Subsequent observers modified at a distant slit with a telescope before which we have placed a grating, we see a white central image of the slit, just as if the gra- ting were not in front of the telescope; but we observe be- sides this central white line a series of spectra to its right and to its left. These spec- tra hav.e their violet ends placed toward the cen- tral image of the slit, and they are named in the or- der of their re- moval from the slit; as spectrum of the first or- der on the right or left, spectra of the second, third, fourth, &c., or- der on the right or left. These tpectra are often of great purity, so that hundreds FIG. 8.-KirchhoflT8 Spectroscope, by Bteinheil. of Fraunhofer lines can be seen with remarkable clearness. If the telescope, T E, be mounted on a divided circle, D, and the grating, G, placed in front of a collima- ting telescope, C, furnished with the slit at S, as in fig. 4, we can measure in the different Fraunhofer's instrument by substituting for the distant slit a collimating telescope (A, fig. 3); this consists of an achromatic lens with the slit at its principal focus. Kirchhoff with the spectroscope shown in fig. 3 has made an exquisite map of the spectrum, containing more than 3,000 lines. The relative positions of the lines in the spectra obtained from prisms formed of different refracting mate- rials, or even from the same material at dif- ferent temperatures, differ so much that maps made by different observ- ers are not comparable; hence recourse has been had to spectra formed by transmitting light through gratings. From measure- ments on these spectra, known as diffraction or in- terference spectra, can bo deduced the lengths of the waves of light correspond- ing to any tint in the spec- trum. The wave lengths are really given corre- sponding to the fixed lines in the spectrum; and as lines hold fixed po- sitions in reference to the colors in which they ex- ist, we have unchangeable wave lengths to which to refer any color that may be used in such practical purposes as the determination of indices of refraction or in observations in spectrum analysis. If we look spectra the angular distances of these lines from the centre of the image of the slit, and the angular distances from the centre of the image of the slit to the same line in spectra of different orders will be nearly as the number of the orders. Thus, if we call a this angle in FIG. 4. Spectrometer. the spectrum of the first order, it will be 2a, 3<z, 4a, &c., in the spectra of the succeeding orders. It necessarily follows from this fact that the length of the spectrum in any order