POTASSir.M, IiriJIDlUM, CAESIUM, AND LITHIUM 555 



up into thinner lines, is seen instead of the system of sodium lines, owing 

 to the small dispersive power of the prism and the considerable aper- 

 ture of the slit of the object tube. 



This conformity of the bright lines formed by sodium with the 

 dark lines of the solar spectrum cannot be accidental. This conclusion 

 is further confirmed by the fact that the bright lines of other metals 

 correspond with dark lines of the solar spectrum. Thus, for example, 

 a series of sparks passing between the iron electrodes of a Ruhmkorff 

 coil gives 450 very distinct lines characterising this metal. All these 

 450 bright lines, or the whole spectrum corresponding with iron, is re- 

 peated, as Kirchhoff showed, in the solar spectrum as dark Frauenhof er 

 lines which occur in exactly the same situations as the bright lines 

 occur in the iron spectrum, just as the sodium lines correspond with 

 the band D in the solar spectrum. Many observers have in this 

 manner simultaneously studied the solar spectrum and the spectra of 

 different metals, and discovered in the former lines which correspond 

 not only with sodium and iron, but also with many other metals. 27 The 

 spectra of such elements as hydrogen, oxygen, nitrogen, and other 

 gases may be observed in the so-called Geissler's tubes that is, in 

 glass tubes filled with rarefied gases, through which the discharge of a 

 Ruhmkorff J s coil is passed. Thus hydrogen gives a spectrum composed 



37 The most accurate investigations made in this respect are carried on with spectra 

 obtained by diffraction, because in this case the position of the dark and bright lines does 

 not depend on the index of refraction of the material of the prism, nor on the dispersive 

 power of the apparatus. The best that is, the most general and accurate method of 

 expressing the results of such determinations consists in determining the lengths of the 

 waves corresponding to the rays of a definite index of refraction. We will express this 

 wave length in millionth parts of a millimetre (the ten millionth parts are already 

 doubtful, and fall within the limits of error). In order to illustrate the relation between 

 the wave lengths and the positions of the spectrum, we will cite the wave lengths cor- 

 responding with the chief Frauenhofer lines and colours of the spectrum. 



Frauenhofer line ABC D Eb F G H 



Wave length . 761'0 687'5 656'6 589'5-588'9 527'8 518'7 486'5 481'0 897'2 



Colour . . red orange yellow green blue violet 



In the following table are given the wave lengths of the light rays (the longest and 

 most distinct, see later) for certain elements, those in black type being the most clearly 

 defined and distinct lines, which are easily obtained either in the flame of a Bunsen's 

 burner, or in Geissler's tubes, or in general, by an electric discharge. These lines refer 

 to the elements (the lines of compounds are different, as will be afterwards explained, 

 but many compounds are decomposed by the flame or by an electric discharge), and 

 furthermore to the elements in an incandescent and rarefied gaseous state, for 

 the spectra sometimes vary considerably with a variation of temperature and 

 pressure. 



It may be said that the red colour corresponds with lines having a wave length of 

 fromJBO (with a greater wave length the lines are hardly visible, and are ultra red) to 650, 

 the orange from 650 to 590, the yellow from 590 to 520, the green from 520 to 490, the 



