THE

LONDON, EDINBURGH and DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

SUPPLEMENT to VOL. XXIX. FOURTH SERIES.

LXVII. On a new Determination of the Lengths of Waves of Light, and on a Method of determining, by Optics, the Translatory Motion of the Solar System. By Å. J. Angström^*, [With a Plate.]

I.

IN the Note on Fraunhofer’s lines which I had the honour of communicating to the Royal Academy in October 1861, I spoke of my intention of revising the lengths of luminous waves, as determined by Fraunhofer†, and of extending these determinations to all the remarkable lines of the spectrum, in order with their help to obtain the wave-lengths for the metal-spectra.

The weather last summer was, on the whole, scarcely favourable to such experiments on the solar spectrum, nor are these experiments by any means complete. Nevertheless, since my measurements of the principal lines of Fraunhofer are sufficiently numerous and self-accordant to secure my results from any essential change, I have deemed it of some interest to examine whether, and to what extent, these new determinations agree with those obtained by Fraunhofer himself—the more so because no new measurements on the wave-lengths of light have, to my knowledge, been made since Fraunhofer closed his wonderful investigations.

I employed in my experiments an optical theodolite constructed by Pistor and Martins in Berlin, and a glass grating made by the optician Nobert in Barth. The theodolite was provided with two telescopes, the second of which served as a sight-indicator (Sehzeichen). In reading off, two microscopes were used, and one division of the micrometer corresponded to an angle of 2″·1.

The eyepiece is also provided with a micrometer arrangement: the screw-head is divided into 100 parts; and when the telescope

* From Poggendorff's Annalen, vol. cxxiii. p. 489; to which journal the paper was communicated by the Author after its publication in the Oefverigt af K. Vet. Akad. Förh. 1863, No. 2.

† Poggendorff’s Annalen, vol. cxvii. p. 290.

Phil. Mag. S. 4. No. 199. Suppl. Vol. 29.     

is adjusted on an infinitely distant object, every scale-division corresponds to l″·308.

The glass grating prepared by Nobert is particularly well constructed. In a space 9·0155 Par. lines broad, there are 4501 lines drawn by a diamond. Errors of division, as tested by Nobert with a microscope which magnified 800 times, lie below 0·00002 of a Par. line.

The breadth, as given by Nobert, was obtained by comparison with a standard prepared by the mechanician Baumann of Berlin, and which was a copy of the one made by the same artist for Bessel.

As a proof of the excellence of this glass grating, I may state that Fraunhofer’s lines can be seen therewith in the third and fourth spectrum, and that in distinctness and richness of detail these lines far exceed those which are obtained by the refraction of light through a flint-glass prism.

During the observations the grating was always placed perpendicularly to the incident rays. This was accomplished, first, by always giving to the unscratched side of the grating a position such that the image of the heliostat-aperture reflected by it coincided with the aperture itself; secondly, by adjusting on the heliostat-aperture the moveable telescope used in the observations; and thirdly, by fixing the axis of the second telescope so as to coincide with the prolongation of the optic axis of the first.

The scratched side of the glass grating was always turned from the incident light and towards the moveable telescope, being placed in the middle over the rotation-axis of the instrument.

The observations were calculated according to the known formula

e sin Θ = mλ,

where e, or the distance between two scratches on the grating, had, according to the above remark, the value

e=0·000166954 of a Par. inch,

λ denotes the required wave-length, Θ the observed angle, and m the order of the spectrum.

As the values of λ thus obtained have reference to air, they must be dependent upon its temperature and barometric pressure;

I have consequently always noted these two elements, although under ordinary circumstances their influence on the measurements was found to be inappreciable. The changes in the temperature of the grating itself exercise a somewhat more important action; nevertheless since, at the time the observations were made (September and commencement of October), the temperature of the room only oscillated between 13° and 18° C., I have likewise omitted this correction.