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tends to increase the probability of the existence of an error in the value of e corresponding to the finer grating.
The values of the wave-lengths obtained by means of Nobert’s grating, therefore, appear to me to merit a greater confidence than that which Fraunhofer’s can justly claim.
As already stated at the commencement of this paper, I have not limited my measurements to the principal lines of Fraunhofer. I have measured, with the circle, the angle Θ for all the stronger lines at a distance from each other of from 10′ to 20′, and determined with the eyepiece–micrometer the positions of the remaining intermediate lines. The measurements, moreover, were repeated in the second, third, and fourth spectra, in order to verify their exactitude.
The following Table contains some of these results, those wavelengths alone being given which correspond to the strongest and most prominent lines of the solar spectrum. Most of these lines belong to iron or to lime, and have consequently a double interest, since they present themselves also in the gas–spectra of these substances. In order to give the reader a visible image of the position and breadth of these lines in the solar spectrum, I have added a figure (Plate III. fig. 1), which correctly shows their respective positions as presented by a prism of sulphide of carbon having an angle of 60°. An arc of 2′ corresponds in the figure to a length of one millimetre.
Table II.—Wave-lengths, in hundred millionths (=1/108) of a Paris inch.