M. F. Zollner on the Spectroscopic Reversion-Telescope, 421 



satisfactorily by VogeFs observations*. For at the rising of the 

 sun a point in the equator moves toward it with a velocity of 

 about jig of a geographical milef, and at sunset with about the 

 same velocity from it, so that in the course of twenty-four hours 

 the point undergoes, in consequence of the rotation, a variation 

 of about y'q of a mile in the velocity of its motion relative to the 

 sun. But this quantity would alter the position of the sodium 

 lines ^J()^ of their distance, and hence, with the aid of the rever- 

 sion-prism, amount to 200 ^^ *^^ ascertained distance. Now, 

 since the mean value derived above from ten observations shows 

 a probable error of only 3 J^g of the said distance, it is evident 

 from this how near we have already approached to the successful 

 solution of the problem. How great an importance these ob- 

 servations may acquire in regard to ascertaining both the velocity 

 of light and, through the connexion of this with the constant of 

 aberration, the parallax of the sun, cannot a priori be estimated, 

 depending entirely on the progressive improvement of spectro- 

 scopic instruments. 



Relative to the facility of the general application of the rever- 

 sion principle in spectrometric investigations, I take leave, in 

 conclusion, to state that I have succeeded, by a combination of 

 two reflection-prisms immediately behind the eyepiece-cover, in 

 constructing a reversion eyepiece which perfectly fulfils the con- 

 dition of exact superposition of the two spectra. The reflecting 

 hypotenuse surfaces of the prisms are simply placed perpendi- 

 cular to each other, one of them being parallel to the width, and 

 the other parallel to the length of the spectrum. One half of 

 the eyepiece-aperture is supplied with light from the first, the 

 other with light from the second prism ; so that the first prism 

 effects the reversion of the spectrum, while the other, with cor^ 

 responding inclination of the plane of reflection, accomplishes 

 the juxtaposition of the, as regards the order of the colours, un- 

 changed spectrum. The action of this eyepiece is surprising: 

 for example, when combined with a Erowning^s miniature spec- 

 troscope it is at once converted into a measuring-apparatus 

 which, by micrometric motion of the reversion-prism, permits 

 the determination of the positions of the lines by coincidence 

 with all the delicacy desirable in so compendious an instrument. 



As soon as a sufficient number of measurements have been 

 accomplished according to this method upon star- spectra, I will 

 take leave to communicate to the Royal Society the results of my 

 observations. 



* Conf. Ber. d. K. Sachs. Ges. July 1, 18/1 ; Phil. Mag. S. 4. vol. xliii. 

 p. 47 ; and ' Beobachtungen angestellt aiif der Sternwarte des Kammerhein 

 von Billow zu Bothkamp von Dr. H. C. Vogel, Astronom der Sternwavte.' 

 Leipzig [Engelmann), 1872. 



t 1 German = 4 English geographical miles. 



