MOUNT WILSON SOLAR OBSERVATORY. 233 



THE OPTICAL SHOP. 



The completion of the two convex mirrors used in conjunction with 

 the 100-inch muTor in the Cassegrain and coude combinations occupied 

 the first part of the year in the optical shop, which has remained under 

 the charge of Mr. Ritchey. These miiTors, after being ground and 

 polished to a spherical form, were hyperbolized with the aid of the 

 large miri'or. The 60-inch plane mirror was utilized to reflect the 

 parallel beam, and, by a slight rotation of this mirror, the field at the 

 equivalent focus of the system could be examined at any desired dis- 

 tance from the axis. 



As soon as the work of figuring the convex mirrors had been finished, 

 there was no further necessity for retaining the 100-inch mirror in 

 Pasadena. Arrangements were accordingly made for its transporta- 

 tion to Mount Wilson, and this operation was carried out early in July. 

 The mirror was crated in a strong box lined with building paper and 

 supported on its edge by a hea\'y' frame-work bolted to the bed of the 

 motor truck. To reduce the amount of vibration, numerous springs 

 were inserted between the box and the frame-work. The top of the 

 mirror box when placed on the truck was about 14 feet from the ground 

 and its weight, including the support, was 7.5 tons. The trip up the 

 mountain was made without incident, the specially geared truck per- 

 forming its work without difficulty. 



The two large oval plane mirrors to be used in the 100-inch telescope 

 have been shaped and fine-ground. One of these, the Newtonian flat, 

 is exceptionally thick and its edge has been beveled in order to reduce 

 the amount of intercepted light. Tw^o mkrors 9 inches in diameter 

 have been completed for use in the coude form of the telescope when 

 only the center of the field is employed. For spectroscopic work this 

 will obviate the necessity of resilvering the large oval plane at frequent 

 intervals. 



Additional work in the optical shop has included two 12-inch plane 

 mirrors for use in the Snow telescope spectrograph, several speculum 

 metal planes for diffraction gratings, and six small mirrors for special 

 experiments. 



INSTRUMENT SHOP. 



The work of the instrument shop, as in previous years, has been 

 under the direction of Mr. Ayers, foreman, with Mr. Jacomini as chief 

 instnmient maker. Much difficulty has been encountered in securing 

 a suitable end- thrust bearing for the ruling-machine, the artificial 

 rubies used for this purpose frequently breaking dow^n, owing to minute 

 flaws and bubbles near the surface. Recent experiments, however, give 

 promise of a solution of this problem. Mr. Jacomini has also given 

 much time to questions connected with the erection of the 100-inch 

 telescope; the cutting of the large driving worm-wheel has been carried 

 out under his supervision. 



