Observations of 500 Nebulae. 



11 



THE INSTRUMENT. 



The observations have been made at the University-Observatory at 

 Upsala with the same 13-foot telescope by v. Steinheil , which was used for 

 the measurement of the stargroup 20 Vulpeeulse, and as the instrumental 

 arrangements were in both instances exactly the same, and I have given a 

 detailed account of them in my essay on that cluster (Kongl. Svenska Ve- 

 tenskaps-Akademiens Handlingar. Band 11, N:o 3), I do not consider it 

 here necessary to repeat all those details. I therefore confine myself to 

 remarking that the equatoreal has always been sufficiently accurately adjusted, 

 both as regards the relative position of the axes and the instrument's orien- 

 tation with respect to the locality, to insure the outstanding errors being in- 

 sensible in the differential observations. 



In order to give some idea of the magnitude of the outstanding 

 errors in the instrument, I here give a few approximative mediumnum- 

 bers for the whole period of the observations. With regard to these 

 numerical data it should however be remarked, that the numbers in the va- 

 rious sections are not comparable with each other, the telescope having du- 

 ring that period been several times dismounted, and the adjustments in the 

 intervals for various reasons altered. 



The signs employed in the table below have the following significations. 

 A<p = the angle between the instrument's polar axis and the axis of the earth. 

 Aa=the angle between the instrumental and celestial meridians. 



i =the error of the angle between the instrument's two axes of rotation. 



c = the error of collimation of the optical axis. 



Periods. 



ACp 



Act 



i 



c 



Autumn 1862— Summer 1863 



-0'3 



+ 0'8 



-04 



-24 



Summer 186:3 — Autumn 1864 



-0,4 



+ 0,2 



-0,2 



+ 0,2 



Spring 1865— Spring 1866 



-0,5 



+ 0,2 



+ 1 



-1 



1866 Aug.— 1867 July 



+ 0,3 



-2,4 



+ 0,1 



+ 0,3 



1867 July-1868 June 



+ 0,3 



+ 0,5 



+ 0,2 



+ 0,2 



1868 July— 1870 June 



-0,1 



+ 0,2 



-0,1 



-0.3 



1870 Aug.— 1874 May 



+ 0,1 



+ 0,3 



0,0 



+ 0,2 



I ought also to add that the relative flexure of the tube on an average 

 amounts to 0',2 or 0',3, and has never sensibly affected the centering of 

 the glasses in the tube , of which I have convinced myself by direct exami- 

 nation with the telescope placed horizontally in opposite positions. A slight 



