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



which underlies this instrument, viz. the principle of the rever- 

 sion of spectra^ to general use in the determination of the posi- 

 tions of the hues in spectral analysis, I have striven to remove 

 the circumstances which permit to the reversion-spectroscope, 

 constructed three years since*, only a limited applicability. 

 The principal of these circumstances were the following : — first, 

 that it was absolutely necessary to employ prisms with direct 

 vision ; and secondly, that the relative displacement of the two 

 spectra could only be effected by means of the displacement of 

 the halves of the object-glass of the telescope. In both these 

 ways was the applicability of the principle confined to a certain 

 very limited portion of the spectrum. The generally immove- 

 able attachment of the illuminating object (the slit) to the ob- 

 serving-telescope, however, has enabled us to effect the reversal 

 of either the entire or divided spectrum simply by means of a 

 reversing-prism with total reflection. By this the relative dis- 

 placement of the two spectra is at the same time connected with 

 the variations of the angle of the plane of reflection to the optic 

 axis of the collimator j so that the mobility of the two halves of the 

 objective parallel to the plane of division is no longer necessary. 

 The alterations of angle of the face of the total-reflection prism 

 can be effected in two ways — either by micrometric motion of 

 the prism alone, or by moving the observing-telescope in combi- 

 nation with the prism. It would in general be advantageous to 

 combine the two motions, — the first, micrometric^ for differential 

 determinations, the second for determinations of position of all 

 the lines of the spectrum with the aid of an index and a graduated 

 arc. 



The reflecting prism can have two different positions in the 

 observing-telescope, viz. either with the object-glass or the eye- 

 piece. 



The advantages, corresponding to the particular purpose, ob- 

 tained by the use of the one or the other construction I have 

 discussed /. c. 



You will permit me now to lay before you a perfectly finished 

 specimen of a spectroscopic telescope with a reversion-objective, 

 and at the same time some measurements which have been ob- 

 tained with a reversion-eyepiece in another telescope, executed 

 by M. Merz from data supplied by me. 



The telescope with reversion-objective is represented in fig. 1 

 (Plate IV.), one third of the natural size. 



The telescope A contains at B the objective-halves moveable 

 perpendicularly to the plane of division. By this motion we can 

 either cause one of the spectra to overlap the other, or make 



* Berichte der K. Sachs. Ges. der Wiss. Feb. 6, 1869. Phil. Mag. 

 November 1869, p. 360. 



