2$e> SCIENCE PROGRESS 



work, 32 feet by 16 feet, which constitutes the polar axis. The 

 heavy lower end of the tube is very short, so that it can swing 

 through this frame for motion in declination. This design 

 gives a good support to the telescope tube and avoids the use 

 of heavy counterpoises, as in the German type of mounting. 

 The weight of the polar axis is taken by mercury flotation, the 

 two mercury tanks being seen in Plate II. at the ends of 

 the polar axis. This gives a very efficient anti-friction 

 apparatus. 



The foundations for the dome and telescope pier were com- 

 menced in 191 3. The pier proper measures 20 feet by 45 feet 

 at its base and is 33 feet high. The top is extended on all 

 sides with a circular floor 52 feet in diameter, which is supported 

 by heavy reinforced concrete brackets. From this floor a 

 circular wall rises to the level of the main floor of the steel 

 building. A special extension pier was added with a top 

 sloping at an angle equal to the latitude of the observatory, 

 to carry the spectrograph, etc., when the telescope is used in 

 the coude* form. The dome is a handsome structure with a 

 diameter of about 100 feet. The height of its top is 105 feet. 

 To prevent as much as possible variations of temperature inside, 

 the dome has a double roof of sheet iron with an intervening 

 air space of about 2 feet. Louvres are arranged at the top of 

 the dome to allow the escape of heated air. The shutter is 20 

 feet wide and very massive. It is divided vertically and con- 

 sists of two portions moving on horizontal tracks at the top 

 and bottom. Along the faces of the two sections are cushions 

 to render them air tight. The dome is shown in Plate III., with 

 the shutter open. This plate gives an excellent idea of the 

 beautiful situation of the observatory. This is not without 

 its disadvantages, and when the heavy parts of the mounting 

 had been constructed it was necessary first to widen the 

 mountain road up from Pasadena before they could be 

 brought to the observatory. Particularly delicate was the 

 operation of bringing the finished mirror from the Pasadena 

 workshop to the mountain-top. It was packed in a special 

 manner, and supported by springs to prevent any possibility 

 of its fracturing through jolting. 



The mounting of the mirror is an important matter, as with 

 an improper mounting distortion of figure would result. The 

 mirror is supported at its back surface by a number of weighted 



