TRANSACTIONS OF THE SECTIONS. 



43 



whose periodic time is ■- ; where t is the periodic time of an undulation in the 



rC 



sether consisting of waves one niilliraetre long. If we use Foucault's determination 

 of the velocity of light, viz. 298,000,000 metres per second, the value of this 

 constant is 



T = 3'3557 twelfth-seconds, 



meaning by a twelfth-second a second of time divided by 10^^, which, in other 

 words, is the millionth part of the millionth of a second of time. 



Thus the proposed numbers give the same information as a list of direct wave- 

 lengths, and in a more commodious form for theoretical purposes; while at the 

 same time the map of the spectrum dravni to this scale is to be preferred for use 

 in the laboratory, because it represents the spectrum formed by a prism with com- 

 paratively little distortion. This will be apparent from the following Table of the 

 wave-numbers of the principal lines of the solar spectrum : — 



On tlie Wave-hngtha of the Spectra of the Hydrocarhotis. 

 By Professor William Swan, LL.D., F.E.S.E. 



The author stated that in 1856 he had communicated to the Royal Society of 

 Edinbm-gh a paper, piiblished in vol. xxi. of their Transactions, entitled " On the 

 Prismatic Spectra of the Flames of Compounds of Carbon and Hydrogen." In 

 his observations on these substances he made use of an arrangement (employed by 

 him still earlier in 1847) identical with that which, since the publication of Kirch- 

 hoff and Bunsen's researches in Spectrum-analysis, is familiarly known as a " Spec- 

 troscope," namely, an observing telescope, a prism, and a collimator, receiving the 

 light to be examined through a narrow slit at its principal focus. 



The observations published in 1856 consist of carefully observed minimimi devi- 

 ations of fom-teen dark lines of the sun spectrum, and of twelve bright lines of the 

 hydrocarbon spectra, which bright lines were found to be identical in fifteen dif- 

 ferent hydrocarbons examined. No absolute coincidences between the lines in the 

 solar and terrestrial spectra were observed, except that, long before discovered by 

 Fraunhofer, between the double sun-line D and the double yellow line of ordinary 

 flames, noiv, wherever it may be seen, referred to sodium. 



The yellow line was generally present in the hydrocarbon spectra ; but, from a 

 careful quantitative experiment, it was ascertained that the 2,500,000th part of a 

 troy grain of sodium rendered its presence in a flame sensible : and the conclusion 

 was then distinctly stated, it is believed for the first time, that whenever or where- 

 ever the double yellow line appears it is due to the presence of minute traces of 

 sodium. 



In this state the observations of 1856 had remained until lately, when the author 

 was requested by his friend Professor Piazzi Smyth to compute the wave-lengths 

 of some of the hydrocarbon lines. As no exact coincidence existed between these 

 and the lines of the solar spectrum, it was necessary to have recourse to some pro- 

 cess of interpolation ; and that which suggested itself to the author was founded 

 \ipon Lagrange's well-known Interpolation theorem. In order to verify as far as 

 possible the results, the computation of the wave-lengths of the hydrocarbon lines 

 was repeated by interpolating between diSerent groups of sun lijies ; and the dis- 

 crepancies between the numbers so obtained in no case extended beyond the place 

 of units in Angstrom's scale of wave-lengths, where unity expresses the ten mil- 

 lionth part of a millimetre. The subject was brought before the Association in order 



