2IO REPORTS ON INVESTIGATIONS AND PROJECTS. 



the figure of the horizontal mirror has been made truly spherical, it may 

 perhaps become necessary to discard the disk. As the French Plate Glass 

 Company, after repeated trials, has failed to cast a 100-inch disk of the de- 

 sired thickness (13 inches), a new furnace of different design may be re- 

 quired to give a successful result. An alternative plan, which also appears 

 promising, is to try an excellent 100-inch disk, about 7 inches (18 cm.) thick, 

 which is now at St. Gobain. This showed considerable flexure in a series of 

 accurate tests, very kindly made for us by the Count de la Baume Pluvinel, 

 but a suitable support system would probably eliminate difficulty from this 

 source. 



Although we feel confident that a thoroughly satisfactory 100-inch mirror 

 will ultimately be available, it has not seemed wise to begin the construction 

 of the mounting or dome while the present uncertainty exists. After an in- 

 spection of the works of the Fore River Shipbuilding Company and a full 

 discussion with their engineers of the design, a contract has been made with 

 this firm to build the mounting on a percentage basis. The drawings have 

 received much study, and are so far advanced that work can be undertaken 

 as soon as the question of the mirror is settled.* 



WORK OF THE INSTRUMENT SHOP. 



The most important new work undertaken during the year is a large ruling 

 machine for diffraction gratings. As already mentioned, it is essential to the 

 complete success of the 100-inch reflector that its accessory apparatus should 

 be no less perfect than the telescope itself. For this purpose, and also for 

 use with the spectrographs of the two tower telescopes, the Snow telescope, 

 the 60-inch reflector, and the physical laboratory, gratings of large size, and 

 sometimes of special design, are required. Although we are indebted to 

 Michelson for some excellent gratings of large size, and to Anderson for 

 smaller ones of great perfection, we are as much in need of a ruling machine 

 as we were of grinding and polishing machines for specula before our optical 

 shop was equipped with them. As soon as the ruling machine becomes avail- 

 able, many special experiments now out of reach will be rendered possible. 



Fortunately, it was not necessary to intrust this very difficult undertaking 

 to untried hands. Through the cordial cooperation of President Remsen and 

 Professor Ames, of Johns Hopkins University, Dr. J. A. Anderson has been 

 granted a year's leave of absence for work in Pasadena. The detailed draw- 

 ings of the machine have been made by Mr. Pease in accordance with Dr. 

 Anderson's plans and after a careful study of the Rowland engine, which 

 had been completely rebuilt under his direction. Dr. Anderson arrived in 

 Pasadena September 10, and the work will be pushed forward as rapidly as 

 its rigorous requirements permit. 



* Since the above was written, the cause of the change of figure observed when the 

 mirror is rotated has been discovered and eliminated. There is now every reason to 

 believe that the present mirror will prove suitable for use in the telescope. 



