248 SCIENCE PROGRESS 



amount of parabolisation. This test consists in measuring the 

 curvature of successive zones of the surface from the edge to 

 the centre by a specially delicate optical method known as the 

 " knife-edge " test and comparing the values obtained with 

 the computed values. (2) The mirror was tested at its focus 

 with the aid of the 60-inch plane mirror, made specially for the 

 purpose. With a perfect paraboloid a source of light placed in 

 its focus will emit light which the mirror will reflect as a parallel 

 beam of light. This will be reflected back along its path by a 

 perfectly plane mirror, at right angles to the axis, and brought 

 to a focus again. The deviation from a true focus gives an 

 indication of the defects in figure of the mirror. This method 

 of testing is invaluable for detecting and correcting slight zonal 

 errors of surface. A combination of the two tests enables a 

 degree of certainty to be obtained which cannot be obtained by 

 either alone. Special care was necessary throughout the testing 

 to avoid irregular changes of figure due to temperature 

 variation. 



When the parabolising had been completed, a photographic 

 test was made by the well-known Hartmann method, as a check 

 on the visual methods, and to provide a permanent record of 

 the surface. Outside the central 8-inch zone (which is covered 

 by the projected area of the Newtonian and convex mirrors) 

 the largest deviation of the observed from the theoretical 

 focal lengh for any zone was 0*14 mm., or only one part 

 in 92,000. 



As an instance of the immense amount of work involved in 

 completing this mirror, it may be mentioned that for testing 

 the 60-inch flat, which it was necessary to construct for testing 

 the 100-inch mirror, three spherical mirrors of 14 inches aperture 

 and 16 feet radius, of 36 inches aperture and 50 feet radius, and 

 of 27 inches aperture and 135 inches radius respectively, had 

 to be constructed, besides numerous grinding and polishing 

 tools. 



The mirror as completed has an aperture of nearly 101 

 inches, a focal length of slightly over 42 feet, a thickness of 1 275 

 inches at the edge and 1 1*50 inches at the centre, and a weight 

 of over 4 tons. Plate I. shows the mirror on the polishing 

 machine. 



The silvering was done without difficulty, the mirror being 

 used to form its own bath. Thirty-two ounces of silver nitrate 



