420 M. F. Zollner on the Spectr^oscopic Reversion- Telescope. 



rections when the observing-telescope is moved by the iiiicroir.e- 

 tric screw, and afford an extraordinarily accurate observation of 

 the coincidence of homologous lines. In order to effect this 

 partial overlapping of the two spectra in observations of the dark 

 lines of the solar spectrum, it is necessary to cause divergent 

 rays likewise to fall upon the slit. Hence, if the spectroscope is 

 not combined with a telescope (in which case coincidence of the 

 plane of the slit with the optical image of the sun satisfies the 

 above-mentioned condition), a small lens of short focal distance 

 fixed in front of the slit iuiparts to the rays the requisite property. 



Fig. 2 represents the impression made by the two sodium 

 lines when the most powerful eyepiece in the described spectro- 

 scope is employed; n is the nickel line, and w the above-men- 

 tioned fainter line. 



In order to give an idea of the great accuracy attainable in the 

 determination of the positions of lines by the employment of the 

 reversion-prism, I take the liberty to add a number of measure- 

 ments of the distance of the sodium lines and of the others found 

 between them. The numbers given are parts of the circumfe- 

 rence of the screw ; on which it is to be remarked that for these 

 measurements the thread was not sufficiently fine, as the tenth 

 part of the values given had to be estimated. The letters placed 

 together at the head of each column denote the lines which were 

 brought to coincidence in the measurements. 







Series 



I. 



Series 2. 



aa^. 



hby 



bb^-aa,. 



l{bb^-aa^). 



aa^. 



a«i. 



a.,. 



ab^. 



257 



'6\Q 



5-9 



2-95 



! 26-0 



27-4 



28-0 



29-0 



257 



317 



6-0 



3-00 



260 



27-5 



280 



291 



25-8 



317 



5-9 



2-95 



1 260 



27-6 



281 



29-0 



25-9 



31-8 



5-9 



2-95 



i 26- 1 



27-6 



281 



291 



25-9 



31-8 



5-9 



2-95 



1 261 



27-6 



28-1 



29-1 



26-0 



31-8 



5-8 



2-90 



\ 26 2 



27-5 



281 



29-1 



25-9 



31-8 



5-9 



295 



26 2 



27-6 



28-2 



290 



25-8 



31-9 



61 



3-05 



1 26-2 



277 



281 



29-1 



26-0 



31-9 



5-9 



2-95 



26-1 



275 



28-2 



291 



25-9 



319 



60 



300 



26-2 



27-5 



281 



290 









Mean 2-965 +0009 



2611 



27-55 



2810 



2906 



















It will be seen that the delicacy of the measurements is extra- 

 ordinarily great, and well justifies the hope that, with the speedy 

 improvement of spectroscopic instruments, my endeavours to 

 demonstrate the rotation of the earth from the displacement of 

 the lines in the solar spectrum will before very long be successful, 

 in like manner as the rotation of the sun has been already shown 



