A SILVERED GLASS TELE SCO PI! 



45 



ever, to It-t it rc^t on the edi^e I, I," . when not in use. At c there is an iron catch 

 on the arch, by which the- lever c. that raises the dome, is held down. The fulcrum 



Fig. 41. 



The Dome Arch. 



is at d. The lever is hinged near c, so that when by being depressed it should have 

 come in the way of the telescope below, the lower half g can be pushed up, the 

 l>;irt. from c toward <1 still holding the dome supported. 



The arch can be set across the observatory in any direction, north and south, east 

 and west, or at any intermediate position, because the abutments where the ends 

 rest, are formed by a ring / /' /", fastened round the circular aperture, through the 

 stationary part of the roof. 



AVhrn the telescope is not in use, and the dome is let down, so that there is no 

 longer an interval of a quarter of an inch between it and the rest of the roof, it 

 U routined inside by four clamps and wedges. Otherwise, owing to its lightness, it 

 woidd be liable to be blown away. These clamps a, 

 1 i^. II. are three sides of a square, made of iron one 

 inch square. They catch above by a point in the 

 wooden basis-circle of the dome b, and below are 

 tightened by the wedge c. 



When the dome is raised it is prevented from moving 

 laterally and sliding off by three rollers, one of which 

 is seen at /, Fig. 40. These catch against its inner edge, 

 and only allow slight play. At first it was thought ne- 

 cessary to have a subsidiary half arch at right angles to 

 the other to hold it up, but that is now removed. 



All the parts work very satisfactorily, and owing to 

 the care taken to get the roof-circle and basis-circle flat and level, no leakage takes 

 place at the joint, and even snow driven by high winds is unable to enter. 



c. The Observer's Chair. 



This is not a chair in the common acceptation of the word, but is rather a movable 

 platform three feet square, capable of carrying two or more persons round the 

 observatory, and maintaining them in an invariable position with regard to the tele- 

 scope eyepiece. 



A Dome Clamp. 



