1825.] Astronomical Society. 309 



Erection of Frauenhofer's large Refracting Telescope at the 

 Observatory of the Imperial University at Dorpat :" communi- 

 cated in a letter from Prof. Struve to Francis Baily, Esq. Presi- 

 dent. Prof. Struve received this telescope in November last, 

 and was happy to find that although it had travelled more than 

 300 German miles, its several parts had been so carefully packed 

 that none of them had sustained the slightest injury. When in 

 a perpendicular position, the height of the object glass is 16 feet 

 4 in. (Paris measure) from the floor, 13 feet 7 in. of which belong 

 to the telescope itself; so that the eye-glass stands 2 feet 7 in. 

 from the floor. The diameter of the object-glass is 9 Paris 

 inches (about 9f inches English). The weight of the whole in- 

 strument is about 3000 Russian pounds. It is so constructed 

 that it may be used as an equatorial. The upper part of the 

 instrument consists of the tube, with its axis of motion, two gra- 

 duated circles, and a variety of levers and counterpoises, pro- 

 ducing the most perfect equihbrium in every direction, and 

 providing against all friction. The dechnation circle is divided 

 from 10' to 10', but by means of the Vernier may be read off to 

 5". The instrument may be turned in dechnation with the 

 finger, and round the polar axis with still less force. 



The most perfect motion round the polar axis is produced by 

 means of clock-work, which is the principal feature of this 

 instrument, and the greatest triumph for the artist, the mecha- 

 nism being as simple as it is ingenious. A weight, attached to 

 a projection connected with the endless screw, overcomes the 

 friction of the machine. The clock vibrating in a circle regu- 

 lates the motion, by moving an endless screw connected with a 

 second wheel in the above projection. The weight of the clock 

 as well as that of the friction apparatus may be wound up without the 

 motion being interrupted. When the telescope is thus kept in 

 motion, the star will remain quietly in the centre, even when 

 magnified 700 times. At the same time there is not the least 

 shake or wavering of the tube, and it seems as if we were ob- 

 serving an immoveable sky. 



But the artist has done still more ; he has introduced a hand 

 on a graduated dial of the clock, by which the motion of the 

 latter can be instantly altered ; so that a star may be brought to 

 any point of the field of vision to which it may suit the observer 

 to carry it, accordingly as it is required to make the course of the 

 instrument go faster or slower than the motion of the heavens ; 

 and if once phiced, it may be kept in that position by returning 

 tlie hand to its original i)Osition. Tlie same mechanism is also 

 used to make the motion of the instrument coincide with that of 

 the Sun and Moon. 



.-This instrument has four eye-glasses, the least of which 

 magnifies 175 times, and the largest 700 times. 



