Chemical and Physical Papers. 65 



the line D to somewhat beyond the line F, and his pupil and as- 

 sistant, Hofmann, with the same apparatus and by the same meth- 

 ods, observed and mapped the rest of the spectrum from a little 

 below A to a little beyond Gr. From the result of their labors we 

 have what are commonly referred to as Kirchhoff numbers. Thalen 

 with different instruments subsequently extended this scale to H. 

 The Kirchhoff scale numbers are given in the right-hand margin 

 of the diagram, and with these as ordinates, and the same abscissas 

 before used, the Kirchhoff curve is drawn; and by comparing its 

 ordinates with those having the same abscissa on the Duboscq 

 curve, the identity of any metallic line seen by the Duboscq instru- 

 ment may be established. The most exact designation of spectrum 

 lines is doubtless by wave-lengths. The Swedish physicist. Ang- 

 strom, reduced the Kirchhoff numbers to wave-lengths, as well as 

 greatly extended his observations, and the numbers taken from his 

 charts are the abscissas in the wave-length curve here shown. 

 The unit is one divided by 10 seventh power, expressed in milli- 

 meters, decimally 0.00000001 mm. It will be seen that the curve 

 is tolerably regular and convex from the axis of abscissas. The 

 wave-lengths are given in the right-hand margin, from 400 to 800 

 in units, as above indicated. We may thus find in any observation 

 in this single-prism spectroscope the corresponding Bunsen chart 

 numbers, the Kirchhoff numbers, or the wave-lengths. 



Wollaston, in 1802, appears to have been the first to observe the 

 dark lines in the solar spectrum which are always produced if the 

 light falling on the refracting prism issues from a narrow slit par- 

 allel to the axis of the prism itistead of a circular aperture, as in 

 Newton's experiments. These lines were not carefully studied till 

 1814, when Fraunhofer, a celebrated Munich optician, took up the 

 subject and carefully studied these lines. He designated seven 

 diffnrent lines or groups of lines by the first seven letters of the 

 alphabet, which continue to be the common names of these lines. 

 He counted in the spectrum, obtained by his instruments, 574 lines. 

 He devised a grating, made by winding fine platinum wire on the 

 threads of two parallel screws, soldering the wire to them, and then 

 cutting the wire away on one side. Letting his narrow beam of solar 

 light fall on this grating, and making use of the principle of inter- 

 ference developed by Young, Arago, and Fresnel, he was able to 

 measure the wave-lengths of many of these dark lines. It was not, 

 however, till 1860 that Kirchhoff, by a splendid induction, ex- 

 plained the cause of these dark lines and demonstrated that the 

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