386 



NATURE 



[July 12, 19 17 



In his report for the year 1912 Prof. Hale had , 

 the pleasure of announcing a great additional gift j 



Fig. I. — Lowest section of loo-in. telescope tube, ready for transportation to Mt. Wilson. 



to the Carnegie Institution by Mr. Carnegie, 

 accompanied by the hope that adequate provision 

 would be made for the completion of the loo-in. tele- 

 scope. Plans for the mounting and for the observa- 

 tory were accordingly prepared in elaborate detail, 

 and work on the mirror was energetically con- 

 tinu(pd. In addition, a 6o-in. plane mirror, to be 

 used in testing the lOO-in. at its principal focus, 

 was put in hand. Pending the trials of the 

 mirror, however, actual construction work was 

 not commenced until 1913. The smaller parts of 

 the mounting, including the driving clock, were 

 undertaken in the workshops of the observatory, 

 and the larger by the Fore .River Shipyards at 

 Quincy, Mass. The dome, 100 ft. in diameter, 

 was undertaken by the Morava Construction 

 Works in Chicago. 



Not the least of the difficulties to be faced was 

 that of transporting materials and heavy instru- 

 mental parts to the summit of the mountain, which 

 is about 6000 ft. above sea-level. For about 

 half the distance of nine miles from Pasadena it 

 became necessary to widen the bed of the mountain 

 road from 3 ft. to 8 ft., and special motor 

 trucks, with excess water capacity to guard 

 against overheating the engines on the steep 

 mountain grades, had to be provided. As an 

 instance of the extensive demands on the trans- 

 portation service, more than 650 tons of steel for 

 the dome, some of the pieces being 24 ft. long, 

 with a maximum weight of ^\ tons, were conveyed 

 to the summit during the spring and summer of 

 191 5. At a later stage, parts of the telescope 

 weighing more than nine tons were safely trans- 

 ported. Fig. I shows a section of the telescope 

 tube in course of transit. 



All difficulties, however, appear to have been 

 NO. 2d8Q. VOL. Qq1 



successively overcome, and with the aid of Prof. 



Hale's annual reports a general description of the 

 instrument can now be given. 

 First, with regard to the great 

 mirror itself. The work of chang- 

 ing the spherical surface into a 

 paraboloid occupied about a year, 

 and for many months tests were 

 made daily, both at the centre of 

 curvature and at the principal 

 focus. In general, the tests at 

 the centre of curvature were 

 found to be most useful in deter- 

 mining the total amount of para- 

 bolisation, and under the best 

 conditions of air in the testing 

 room it was possible to determine 

 , the radius of curvature of a 

 zone within one-thousandth of 

 an inch. The tests made at the 

 focus, with the aid of the 60-in. 

 flat, were invaluable for detect- 

 ing and correcting slight zonal 

 errors of surface, and by a com- 

 bination of the two tests a higher 

 degree of accuracy of surface was 

 secured than would have been pos- 

 sible with either test alone. The 

 parabolising was almost entirely 



performed by mechanical means, the final figuring 



by hand tools occupying less than twenty hours. 



Fig. 2. — Partially erected mounting of loo-in. telescope, December, igifi 

 (The cutting of the teeth of the large worm gear lias since been completed.) 



After the final figuring, the Hartmann test wasi 

 applied photographically, in otder to check the 



