238 CARNEGIE INSTITUTION OF WASHINGTON. 



appears that the difficulty of securing suitable edge-supports for such 

 mirrors has been exaggerated; a very simple device will serve per- 

 fectly if the pressure is applied in the proper plane. 



As a further test of the suitability of the glass for use in the tele- 

 scope, its figure was examined after the temperature of the room had 

 been reduced to 45° F., and maintained at this point for several days. 

 As no change in figure was observed, the temperature was raised to 

 95° F. and the tests repeated. These also showed the same figure as 

 before. It thus appears to be safe to use the disk under the actual 

 conditions of observation. 



As a check on the visual knife-edge tests, they were repeated photo- 

 graphically and the figure was further controlled by means of the 

 Hartmann test. The measures and reductions of the photographs 

 were made by Mr. Adams, Miss Lasby, and Miss Burwell. 



It is appropriate to express here our hearty acknowledgments of 

 the unflagging interest and persistence of the French Plate Glass 

 Company, which pursued costly experiments through several years 

 in the hope of producing a flawless disk. We are also indebted to the 

 Count de la Baume Pluvinel for his kindness in testing at St. Gobain 

 a 100-inch disk of less thickness than the one selected. 



Since the acceptance of the disk the work of figuring has gone on 

 steadily, and has now advanced to a point where parabolizing may 

 soon be undertaken. The figuring of the 60-inch plane mirror, for 

 use in testing the 100-inch, has also been carried on simultaneously 

 and it will be ready for use as soon as it is needed. 



In the work of designing the mounting much practical advantage 

 has been derived from the suggestions of Professor Peter Schwamb, 

 formerly of the Massachusetts Institute of Technology, who has been 

 engaged as our representative to superintend the work of construc- 

 tion at the Fore River Shipyards in Quincy, Massachusetts. Several 

 valuable suggestions offered by Professor Schwamb have been incor- 

 porated in the design, and the work of construction is now under way. 



The design for the steel building and dome for the 100-inch tele- 

 scope was completed about June 1 by Messrs. D. H. Burnham 

 & Co. of Chicago, and bids were secured from several contractors. 

 Unfortunately, however, the figures were considerably in excess of 

 the architects' estimates, on account of an increase in the market 

 price of steel. After waiting several months, a new set of bids is 

 being obtained, and the indications are that a very substantial 

 reduction in cost will thus be effected. As our building program 

 did not include the erection of the dome this year, there will be no 

 delay on account of this experience. It is expected that the steel 

 will be shipped to Pasadena early in 1914, and the work of hauhng it 

 to Mount Wilson can be undertaken in the spring in ample time for 

 the opening of our consti*uction season. 



