MOUNT WILSON SOLAR OBSERVATORY. 265 



The focal length of the 100-inch rairror is approximately 507.5 

 inches (12.88 m.), the clear aperture being very nearly 101 inches 

 (2.57 m.). The depth of the curve is about 1.25 inches (31.8 mm.) 

 at the center; the thickness of the finished glass is 12.75 inches (323.8 

 mm.) at the edge. At the center, where the difference is greatest, the 

 depth of the finished paraboloid differs from that of the nearest spheri- 

 cal surface (to which the glass was brought in preparation for para- 

 bolizing) by abiiost exactly 0.001 inch (0.025 mm.). The weight of 

 the finished glass is very nearly 9,000 pounds (4,082 kg.). 



The work of parabolizing was done almost entirely with the large 

 polishing machine and by the use of two pohshing or figuring tools of 

 90° sector form, one of 850, the other of 415 square inches area (the 

 area of the 101-inch glass surface is about 8,012 square inches) ; at least 

 95 per cent of the work was done with the larger tool. 



A system of progressive positions or settings of the tool, and of pro- 

 gressive strokes also, was used, similar to that devised while figuring 

 the 60-inch mirror. The system was greatly elaborated in this case, 

 however, and was used from the beginning of the parabolizing, so that 

 a great amount of experience was acquired before the work reached the 

 final and critical stage. This method made it possible to keep the 

 curve smooth at all stages of the work; in other words, it enabled the 

 opticians, from the beginning of parabolizing, and in each successive 

 day's figuring, to keep the percentage of the change of radius of curva- 

 ture, as compared with the total theoretical change, very nearly correct 

 for each zone. As a result it was possible to bring the parabolizing 

 almost to completion by the use of the machine and the large figuring- 

 tools alone. Less than 20 hours' work with small figuring-tools used by 

 hand was required at the very last stage to soften down several slight 

 protuberant zones which were too narrow for treatment with the 

 machine. 



Silvering the large mirror was accomplished without difficulty. 

 The large polishing machine was utihzed, as it permits rocking the 

 mirror for the various operations of silvering, and also tipping the mirror 

 for pouring off the solutions, both operations being accomplished by 

 the motor-driven mechanism. The work required 32 ounces of silver 

 nitrate; about 150 gallons of distilled water were used in the entire 

 process; 35 gallons of distilled water were required to fill the concavity, 

 and to this 9 gallons of dilute silver solution and 9 gallons of dilute 

 reducing solution were added. The time required for the deposition 

 of the silver was 15 minutes. Burnishing of the silver film was done 

 with the large pohshing machine and a cushioned burnishing tool 34 

 inches in diameter, covered with six selected chamois-skins. 



In addition to the work on the large mirror, much has been done on 

 the two convex mirrors for the same telescope, and on the apparatus, 

 both mechanical and optical, for testing these mirrors daily, in com- 



