244 SCIENCE PROGRESS 



It may in fact be stated that the amount of clear sky is 

 more than can fully be utilised. 



The construction of the ioo-inch telescope was made possible 

 by the generous offer of an American millionaire, Mr. John D. 

 Hooker of Los Angeles, who presented to the Carnegie Institute 

 of Washington in the year 1906 the sum of 45,000 dollars for 

 the construction of the largest mirror that could be obtained 

 for mounting in a reflecting telescope for the Solar Observatory, 

 with the hope that the diameter of the mirror would reach 

 100 inches. The 5-foot telescope for the observatory was then 

 nearing completion and the experience gained in the construction 

 of this instrument gave promise that the practical difficulties 

 might be successfully overcome. There were carefully con- 

 sidered by Prof. Hale, Director of the observatory. They were 

 as follows : 



(1) The manufacture of a suitable glass disc. For large 

 mirrors, experience indicates that the thickness should be 

 somewhere about one-seventh or one-eighth of the diameter 

 in order that sufficient rigidity may be obtained and that it 

 may be possible so to mount the mirror that the figure will not 

 be distorted. Thinner discs are also more liable to suffer 

 temporary disturbance of figure through change of temperature. 

 A glass disc 100 inches in diameter and 13 inches thick weighs 

 over 4^ tons, and the practical difficulties of casting so large 

 a disc in one piece are enormous. The glass must be sufficiently 

 homogeneous to be tolerably free from internal strains, which 

 might cause it to fracture either during working or subsequently, 

 particularly if sudden changes of temperature occurred. It 

 must also be free from bubbles which, if present, might be 

 exposed by the grinding. 



There are very few glass works which would think of attempt- 

 ing the task. Some of the French makers seemed to have a 

 special aptitude for the purpose and the St. Gobain Company, 

 who had long experience and full understanding of the require- 

 ments, expressed their willingness to undertake it. 



(2) The production of a perfect paraboloidal figure. These 

 large mirrors are not, as might be expected, spherical in figure. 

 The surface which possesses the property that a beam of parallel 

 light falling upon it is reflected so that every ray passes accur- 

 ately through a point is a paraboloid. A sphere does not 

 possess this property, the defect being known as spherical 



