108 Prof. Struve on the large Refracting Telescope 



But, it is equally well constructed for the more delicate mo- 

 tions. The declination circle is stopped by a spring, and se- 

 cured, by means of a micrometer-screw, against a strong iron 

 arm fixed to the frame. 



This screw is moved by a long Hooke's joint, which the 

 observer holds in his hand, while his eye is applied to the 

 glass. In this manner the telescope is pointed in declina- 

 tion with as much certainty as in the best meridian instru- 

 ments. 



For the purpose of producing the finer motions round the 

 polar axis, an endless screw is adapted to move in the hour 

 circle. A spring presses on this screw uniformly, and a lever 

 is employed to raise it out of the circle ; another Hooke's 

 joint is placed on the head of this screw, by which the mo- 

 tions are produced. 



But the most perfect motion round the polar axis is pro- 

 duced by means of clock-work, which is the principal feature 

 of this instrument, and the greatest triumph for the artist ; 

 the mechanism 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 circular arc, regulates 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, 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 seven hundred 

 times. At the same time, there is not the least shake or 

 wavering of the tube, and it seems as if we were observing 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 into the middle of the field, or to any other point of 

 it to which it may suit the observer to cany it, by rendering 

 the motion of the instrument, for a time, faster or slower than 

 that of the heavens, as the case may require ; and when once 

 placed, it may be kept in that position by returning the hand 



