362 M. P, Zollner on a New Spectroscope, together with 



The most important of these difficulties which have heretofore 

 hindered a definite solution of the problem in question, I think 

 I have overcome by a new construction of the spectroscope, the 

 first specimen of which I have the honour of laying before the 

 Society. 



The arrangement is essentially as follows : — The line of light 

 produced by a slit or by a cylinder lens is in the focus of a lens 

 which, as in all spectroscopes, first renders parallel the rays to 

 be dispersed. The rays then pass through two Amicus direct- 

 vision prisms, which I obtained of superior excellence from the 

 optical workshop of M. Merz in Munich. 



They are so fastened together that each of them transmits one- 

 half of the rays emerging from the object-glass of the collimator, 

 but so that the refracting edges are on opposite sides, and thus 

 the total mass of rays is decomposed into two spectra of opposite 

 directions. The object-glass of the observing-telescope, which 

 again unites the rays to an image, is cut at right angles to the 

 horizontal refracting edges of the prisms, as in the heliometer ; 

 and each of the two halves may be moved micrometrically, both 

 parallel to the line of section and also at right angles thereto. 

 Thus not only can the lines of one spectrum be successively made 

 to coincide with those of the other, but both spectra, instead of 

 being superposed, may be placed closed beside each other (so 

 that one is displaced in reference to the other like a nonius), or 

 they may be partially superposed. By this construction, not 

 only is the delicacy of the double image as a means for deter- 

 mining any change in position of the spectrum-lines utilized, but 

 any such alteration is also doubled, inasmuch as its influence in 

 the two spectra is exerted in opposite directions. 



The principle of the reversion of the spectra, fundamental to 

 the instrument described (for which I therefore propose the 

 name " Reversion- Spectroscope"), may be applied even without 

 using Aimer's systems of prisms. It is only necessary to reverse, 

 by reflection from a mirror or from a prism, one part of the pen- 

 cil of rays emerging from an ordinary prism, and then to observe 

 the whole pencil as above by means of a telescope provided with 

 a cut object-glass. This principle also dispenses with the simul- 

 taneous introduction of artificial sources of light for investigating 

 small alterations of refrangibility, and enables those changes to 

 be seen and measured by the alterations in position of perfectly 

 homogeneous objects. 



The series of measurements which were made with the 

 dark lines D of the solar spectrum, as well as with the bright 

 sodium-lines of the flame of a taper impregnated with salt, 

 and which I here adduce as a criterion of the capability of the 

 instrument, justify the hope that by means of this spectroscope 



