490 M. A. J. Angstrom on a new Determination 



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 per- 

 pendicularly 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 coin- 

 cided 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 



esin© = mX, 



where e, or the distance between two scratches on the grating, 

 had, according to the above remark, the value 



6 = 0-000166954 of a Par. inch, 



X denotes the required wave-length, © the observed angle, and 

 m the order of the spectrum. 



As the values of X 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 measure- 

 ments was found to be inappreciable. The changes in the tem- 

 perature 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 like- 

 wise omitted this correction. 





