M. F. Zollner on the Spectroscopic Reversion-Telescope. 419 



them appear distinctly side by side like a vernier and scale. For 

 this purpose we have only to move the arms of the objective 

 more or less from one another, and not, as before, to shift them 

 micrometrically parallel to the plane of division after the manner 

 of the heliometer. 



The reflection-prism is placed in the receiver E, and is move- 

 able in front of the halves of the objective by means of the screw 

 F. The screw G permits the telescope to be moved round the 

 axis K, and thereby effects, as remarked above, the relative dis^ 

 placement of the two spectra. The magnitude of this angular 

 motion can be read off on a graduated arc H with the aid of the 

 lens L. 



At I the rays from the system of prisms enter the telescope. 

 By means of the screw-thread M the instrument can be com- 

 bined with any spectral apparatus ; and for the purpose of ad- 

 justing the refracting edges of the dispersing and reflecting 

 prism (E), it can be turned upon its longitudinal axis by means 

 of the annular appendage I. 



The Merz spectroscope contains two direct-vision prism-systems, 

 which can be used either singly or combined. As in the usual 

 arrangement of that kind of spectroscope the collimator is at all 

 times immoveably fixed to the prism-piece, the strong dispersion 

 of the prisms used limits the observation to a proportionally 

 small portion of the spectrum. I therefore proposed to M. Merz 

 to remedy this inconvenience by making the collimator-telescope 

 in like manner moveable and its position controllable by an index 

 and graduated arc. This alteration, which I had previously had 

 applied to the spectroscopes manufactured here, answered the 

 purpose completely. It is obvious that then the graduated arc 

 on the collimator served not to measure, but merely to mark the 

 best position of the collimator for a certain part of the spectrum. 



The magnitude of the dispersion and the clearness of the 

 images produced by the spectroscope made for me and recently 

 sent to me from Munich are so considerable that, between the 

 two sodium lines in the solar spectrum, besides the nickel line, 

 there is seen distinctly a finer and more refrangible line, and that, 

 too, when the sun is at his greatest altitude. 



This instrument is converted into a reversion-spectroscope by 

 merely substituting for its eyepiece-cover one containing a small 

 reflection- prism which covers exactly half of the aperture. The 

 reflecting face stands parallel to the optic axis of the instrument, 

 and thus, being at the same time parallel to the refracting edges 

 of the dispersion-prisms, effects a partial inversion of the spec- 

 trum. When the instrument with such a reversion-eyepiece is 

 directed to a candle-flame impregnated with sodium, two par- 

 tially overlapping spectra are seen, which move in opposite di- 



2E2 



