SPECTRUM 



617 



tines, and named the more important of them by the 

 early letters of the alphabet. These are shown in 



VIOLET 



INDIGO 



BLUE 



GREEN 



YELLOW 



ORANGE 



RED 



Fig. 1. 



the first spectrum in the table of spectra. They 

 are the standard lines with which it is usual to 

 compare the line characteristics of other spectra. 



For the careful observation of these lines the 

 spectroscope or spectrometer has been constructed. 

 It consists essentially of a prism or train of prisms, 

 P; a collimator, C, at the focus of whose lens, L, is 

 placed a narrow slit, S, parallel to the edge of the 

 prism ; and a telescope, T, for producing a magni- 

 fied image of the spectrum of the illuminated slit. 

 Nearly all transparent refractory substances give 

 similar spectra, although the dark lines may be 

 somewhat differently spaced in the different cases. 

 This arises from the fact that substances vary in 



Fig. 2. 



their dispersive as well as in their refractive powers 

 (see DISPERSION). The optical value of the spec- 

 troscope is that it gives us the means of accurately 

 determining the refractive indices of different sub- 

 stances for rays of definite wave-lengths. Now, 

 although refrangibility depends on wave-length, 

 being in general greater for the shorter wave, it does 

 not depend upon it according to any simple or com- 

 mon law. Hence in prismatic spectra the character- 

 istic lines are not spaced in accordance with any 

 simple relation to the wave-lengtlis of the corre- 

 sponding rays. If, however, we substitute for the 

 prismatic part of the sj>ectroscope a diffraction- 

 grating, we obtain a spectrum in which the rays are 

 spaced according to a law of extreme simplicity. 



A diffraction-grating is formed by ruling a series 

 of fine lines on a glass or metal surface. For the 

 production of a good spectrum it is necessary that 

 the line* should be equidistant and so close that 

 several thousands go to an inch. If the image of 

 an illuminated slit be viewed by a telescope through 

 or after reflection from such a grating, a remark- 

 able appearance is presented. A central luminous 

 line is seen, just as if no grating existed, and for 

 tome distance on either side the field is dark. Bat 

 soon on totli sides spectra appear, with their blue 

 ends nearest the central line. Still farther to left 

 and right secondary spectra appear, their blue ends 

 overlapping the red ends of the primary spectra. 

 These are followed by a third but fainter set, and 

 o on. These successive spectra are due to the 

 Interference (q.v.) of the rays emanating from the 



discontinuous wave-front which has been made 

 so discontinuous at the grating. The absolute 

 position and breadth of the spectra depend on 

 the closeness of the lines of the grating ; bub 

 the relative positions of the coloured rays in 

 any spectrum depend only on the wave-lengths. 

 Thus in the solar spectrum produced by a dif- 

 fraction-grating Fraunhofer's lines are so dis- 

 tributed that their distances from the central 

 luminous line above mentioned are propor- 

 tional to the wave-lengths of the correspond- 

 ing rays of light. This spectrum is accord- 

 ingly called the Normal Spectrum. Compared 

 with it, the ordinary prismatic spectrum is 

 much crushed towards the red end and ex- 

 tended towards the violet end. A rough com- 

 parison is shown in fig. 3, the principal Fraun- 

 nofer lines lieing given in the two spectra, 

 which are of the same total length. 



Professor Rowland, by means of his concave 

 gratings, or gratings marked on a concave cylin- 

 drical surface of speculum metal, has produced 

 remarkably fine spectra. Because of the slight 

 concavity the grating focusses the spectrum clearly 

 at a particular distance, so that the object-glass of 

 the telescope may be dispensed with. 



We have now to consider the significance of the 

 dark lines in the solar spectrum. These gaps may 

 be imagined as originating in two ways. They 

 may l>e absent in the sunlight from the very 

 beginning, or they may be absorbed by some sub- 

 stance through which the ray passes from the sun 

 to the earth. As Brewster snowed long ago, many 

 of the lines are really due to absorption by the 

 earth's atmosphere, and are more marked when 

 the sun is low than when the sun is high. These 

 lines which are certainly due to absorption by the 

 earth's atmosphere are called telluric. Near the 

 Fraunhofer D lines there exists a very remarkable 

 group of lines known as the Rain-band. It is due 

 to water-vapour in the air, and gets very dark as 

 the humidity approaches saturation. The prin- 

 cipal lines in the solar spectrum are, however, not 

 telluric. Nor can they be explained as due to the 

 absorptive action of the ether, inasmuch as the 

 various spectra of stars, though broadly similar to 

 that of the sun, differ from it and from one 

 another greatly in detail. (Compare, for example, 

 the spectra of Sirius and of the sun in the table.) 

 In short, solar and stellar spectra are very charac- 

 teristic in the number and distribution of the 



REFRACTION 

 SPECTRUM. 



DIFFRACTION 

 SPECTRUM. 



Fig. 3. 



lines which cross what is otherwise a continuous 

 spectrum (see STARS). If then these lines are due 

 to absorption, it must be absorption in the atmo- 

 sphere enveloping each star or sun. That this is 

 the true explanation of the dark lines has been for 

 long regarded as established beyond a doubt. 



Previous, however, to the discovery of the prin- 

 ciple which lies at the basis of stellar and solar 

 spectroscopy, the great variety of spectra given by 

 different substances had been recognised. Some of 

 these are shown in the accompanying table of 

 spectra ; and, as suggested by Talbot and Herschel 

 in 1825, an obvious application of the prism is to 

 the qualitative determination of small quantities 

 of substances in minerals. In the accompanying 

 coloured plate, the characteristic spectra of' the 

 vapours of ten of the metals taken by themselves 

 may be compared with the very different spectra of 



