0?i the Const rmtion of a large Refracting Telescope. 41 



culminations, in the observations for 1822, any that had been 

 fit for it. The volume for 1821 I have not yet seen; and to 

 employ earlier ones I consider unadvisable, since it is pro- 

 bable that the instrument now gives quite different polar di' 

 stances from what it used to do. 



However, I am not of opinion that an imperfection in the 

 meteorological instruments is absolutely necessary for the ex- 

 jilanation of the difference between the two series of observa- 

 tions : on the contrary, I believe that this difference may be 

 also explained from the supposition already made, — that the 

 bend of the instruments is such as maj- be removed by the 

 apphcation of counterpoises invariable in all situations relative 

 to the horizon. From this supposition follows the formula 

 a sin z -Y h cos z. However, I have reason to think that this 

 gi'ound may be essentially erroneous, which I intend to ex- 

 plain in another place. 



For the present, the question respecting the difference be- 

 tween the Greenwich and Kiinigsberg observations seems to 

 me to stand thus : 



1°. That there can be no doubt but that the mural circle 

 at Greenwich (probably by the strengthening of the telescope 

 undertaken in 1821) has now given larger polar distances 

 than before ; as is proved by the catalogues of 1 8 1 3 and 1 822. 



2°. That the difference has been so far reduced from the 

 perceptible magnitude, which it had according to the standard 

 catalogue (partly by Mr. Pond's own subsequent catalogue, 

 partly by M. Olufsen's calculation of the observations of 1 822, 

 made after my refractions), that the remaining part may be 

 readily explained from very probable causes. 



Bessel. 



V. On the Construction of the large Refracting Telescope just 

 completed. By M. Fraunhofer. Read at a Meeting of 

 the Royal Bavarian Academy of Sciences, the lOth of Jidy 

 1824 *. 

 T^HE instrument, of which I have the honour of speaking, 

 ■*■ is destined for the Im)icrial Observatoi'y at Dorpat. It is 

 the largest of its kind, and new in various parts of its con- 

 struction. 



The largest telescopes hitherto used were those constructed 

 with metal mirrors. But since even the most perfect of these 

 mirrors reflect but a small portion of the light it receives (the 

 larger portion of it being absorbed), the mirror telescopes must 



* From Scluimadier's Attron. Nnvhrirhlcn, Nos. 74 and T!^. 

 Vol. 66. No. ?,'21. July 182.5. F be 



