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of Fraunhofer’s grating by
0·00061 of a Par. inch,
or by diminishing the number of lines by 5.
That the second decimal is wrong in the above breadth (= 9·0155 lines) of Nobert’s grating is not probable; far more so is the supposition of an error of about half this magnitude in the estimation of the breadth of Fraunhofer’s grating, especially since the microscope, forty years ago, had not reached its present high degree of perfection. Fraunhofer, moreover, was compelled to strengthen the extreme lines of his grating, in order to see them more distinctly when measuring, a circumstance which may possibly have affected the positions of these two lines.
Besides the fact that my measurements agree with the results which Fraunhofer obtained by means of the grating No. 4, there is another reason in favour of the assumption that the differences (α–γ) arise from an incorrect value of e in Fraunhofer’s glass grating. For the above-cited memoir of Fraunhofer’s contains measurements made with another glass grating for which e had the considerably greater value of
0·0005919 of a Paris inch.
Fraunhofer made no use of these measurements, probably because this grating proved to be far less perfect, the spectra on one side of the axis being twice as intense as those on the other. On calculating these measurements, however, we obtain the following values corresponding to the lines from D to G:—
These values, compared with the series (α), indicate a constant difference; here, however, the differences amount only to
1·25, 1·14, 1·13, 1·63,
or to about one-third of those last given.
Now, since this last grating was nearly five times as coarse as the former, and probably also broader, it must have been easier to determine accurately its corresponding e. This circumstance
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.