July 12, 1917] 



NATURE 



387 



visual observations, and to provide a permanent 

 record of the surface. The larg^est deviation of 

 the observed from the theoretical focal length for 

 any of the zones, except a small useless portion 

 at the centre, was 014 mm., or about one part 

 in 92,000. 



The clear aperture of the mirror is nearly 

 loi in., and its focal leng^th 42 ft. 3^ in. ; the thick- 

 ness at the edge is i2| in. The depth of the cur\e 

 is about i^ in., and to some it may come as a sur- 

 prise to learn that at the centre, where the dif- 

 ference is greatest, the depth of the finished para- 

 boloid differs from that of the nearest spherical sur- 

 face, which was that first given to the mirror, by 

 only one-thousandth of an inch. The weight of 

 the finished g^lass is a little more than four tons. 



Although the area of the . . 



surface to be silvered was 

 8012 sq. in., this operation was 

 accomplished without difficulty. 

 About 150 gal. of distilled 

 water and 32 oz. of silver 

 nitrate were used in the entire 

 process ; 35 gal. of distilled 

 water were required to fill the 

 concavity, and to this 9 gal. of 

 dilute silver solution and 9 gal. 

 of dilute reducing solution were 

 added. Deposition was com- 

 plete in fifteen minutes.^ 



Two convex mirrors have 

 been prepared for use with the 

 large mirror. One is 28J in. 

 in diameter and more than 

 6^ in. thick ; it has a radius of 

 curvature of 28 ft. io| in. , and 

 in Cassegrain combination will 

 give an equivalent focal length 

 of about 150 ft. The second 

 convex mirror, to be used 

 alternatively, is 25 in. in dia- 

 meter, with a radius of curva- 

 ture of about 22 ft. II in., giv- 

 ing an equivalent focal length 

 of 251 ft. when in combination 

 with the large mirror. We 

 believe that arrangements have 

 also been made for observa- 

 tions at the principal focus. 



Few details of the mounting- have been given 

 in Prof. Hale's reports, but from the photograph 

 reproduced in Fig. 2 it would seem that the " Eng- 

 lish equatorial " construction has been adopted. 

 In this arrangement, the polar axis takes the form 

 of a long rectangle, turning- on an axis parallel to 

 the longer sides, and the telescope tube is pivoted 

 so as to turn on an axis parallel to the shorter 

 sides. These two movements correspond respec- 

 tively with motions of the telescope in Right Ascen- 

 sion and declination. The illustration shows the 

 polar axis, with one of the sections of the tube in 

 position. The tube is in four such sections, and 

 has a diameter of 1 1 ft. The greater part of the pres- 

 sure on the bearings, due to the immense weight 

 of the moving parts, will be relieved by the flota- 

 XO. 2489, VOL. 99] 



tion method first introduced by Dr. Common, and 

 afterwards adopted for the 60-in. reflector at 

 Mt. Wilson. Large circular floats, concentric 

 with the polar axis, are provided for this purpose, 

 one at the top and another at the bottom of the 

 polar axis. As will be seen in Fig. 2, in the case 

 of the lower float, each of these revolves in a 

 nearly semicircular trough, and the intervening 

 space will be filled with mercury. The driving 

 clock is described as a highly perfected piece of 

 mechanism. It required more than half a ton of 

 bronze castings, and nearly i^ tons of iron cast- 

 ings. It is provided with a driving weight of two 

 tons. 



The pier which supports the telescope measures 

 20 ft. by 45 ft. at the ground level, and is 



Fic. 3. — Completed dome for the loo-in. telescope, from the summit of the i5o-f4. tower telescope. 



33 it. in height. The top consists of a circular 

 concrete floor 1 1 in. thick and 54 ft. in 

 diameter, being supported on the east and west 

 sides by massive reinforced-concrete brackets ex- 

 tending outwards from the pier. A metal wall 

 about 8 ft. high reaches from the edge of this 

 floor to the level of the main steel floor of the 

 building, and the joint between the two may be 

 made air-tight by means of a water seal if found 

 desirable. The pier itself is a remarkable struc- 

 ture. It Is hollow in construction, with three heavily 

 reinforced floors at different elevations. The first, 

 at a distance of 16 ft. from the ground, is in- 

 tended for a large water tank, to form a reservoir 

 for a water circulation system enveloping the 

 loo-in. mirror for the maintenance of constant 



