306 Description of Fraunhofer's Telescopes. 



sanction of some of the most eminent practical astronomers in 

 Europe, and are now considered as constituting an instrument 

 of incalculable value for general astronomical observations. 



The splendid telescope, which we have already mentioned 

 (see p. 174) as made for the observatory of Dorpat, is shown 

 in Plate VII., where the instrument is repi'esented as mount- 

 ed parallactically upon a stand, the telescope being balanced 

 in every position. The hour circle is divided by two verniers 

 into Jour seconds of time, and the declination circle into ten 

 seconds. The equatorial axis is put in motion by a clock having 

 two sets of wheel- work, so that the telescope follows by itself the 

 diurnal motion of the stars. But it may also be turned freely 

 by the hand in every direction, or by means of an endless 

 screw. The friction of the equatorial axis is diminished by 

 friction rollers, so that the telescope, though its weight was 

 about thirty- six quintals of Bavaria, could be moved by the 

 pressure of a single finger. 



The figure in Plate VII. represents the telescope as seen 

 from the side on which the clock is placed. 



The object-glass is thirteen and one third feet (Pied de Ro 

 de Paris) in focal length, and its aperture is nine inches. 



It has eight astronomical eye-pieces, beside the following 

 micrometers. 



1. A repeating line micrometer, with a circle of position, 

 whose two verniers give a single minute. This micrometer is 

 furnished with a mechanism for illuminating the lines, the field 

 remaining obscure, so that these lines appear to be luminous 

 stripes on a dark ground. These lines are, as we are inform- 

 ed, cut upon glass with a diamond point. As these lines ap- 

 pear like so many silver threads suspended in the heavens, the 

 transits of the smallest stars across them may be observed. 



2. Two micrometers, each of which consists of two free 

 rings. 



3. Two micrometers with one free ring. In all these mi- 

 crometers, the rings, which are accurately turned out of brass, 

 are fixed upon plates of glass, so that they seem to be suspend- 

 ed in the field of the telescope. By observing the immersions 

 and emersions of the stars at the inner and outer circumfer- 

 ences of the rings, the differences of right ascension and de- 

 clination of two stars are determined. 



