TO KNOW THE STARRY HEAVENS 



339 



inches in diameter and when examined 

 was found to have been cast in three 

 layers. This was clue to the fact that 

 there were no crucibles large enough 

 to hold such a mass of molten glass and 

 the pourings into the mould had been 

 made as closely together as possible, 

 but the expert optician could detect 

 flaws that made it seem unwise to at- 

 tempt the grinding of the disc. Two 



rust be deposited and cause scratches. 

 The brushes revolved very slowly and 

 with an even motion, working from the 

 center to the circumference of the disc. 

 After the first cutting had been clone 

 and the depression was about an inch 

 and a quarter deep in the center, the 

 grinding was clone with even greater 

 care the emery solution was allowed 

 to stand for a time and the liquid 



THE MACHINERY USED TX THE ROUGH GRINDING OF THE 100-INCH DISC. 

 Note the relative sizes of the child and the disc. 



more attempts were made at St. Go- 

 bian, but word was sent to Pasadena 

 that neither of the later attempts was 

 as successful as the earlier one. Pro- 

 fessor Ritchey then made a trip to 

 France, but found that the mechanical 

 obstacles at the factory made it impos- 

 sible to pour so large a piece of glass 

 from one crucible. Finally it was de- 

 cided that work should proceed on the 

 disc at Pasadena. It was placed upon 

 the bed-plate of the grinding machine 

 and Mr. W. L. Kinny and Mr. Dalton, 

 to whose skill its final perfection is 

 due, began the preliminary grinding. 

 The surfaces were smoothed until it 

 measured ioo inches in diameter and 13 

 inches in thickness. 



This smoothing process was followed 

 by the. actual grinding of the concave 

 surface. This was done by applying 

 flour emery in water with wooden 

 brushes covered with cheesecloth. No 

 metal was allowed to touch the surface 

 of the glass, lest minute particles of 



siphoned off so that only the very fine 

 particles were allowed to touch the 

 glass. 



After the grinding was finished there 

 remained the gigantic task of polish- 

 ing the surface and this included a ser- 

 ies of the most delicate and exhaustive 

 tests. One of the earlier of the tests 

 was to determine the effect of changes 

 of temperature upon the glass. The 

 temperature of the shop was lowered 

 to 30 and then in a few days raised to 

 105 . In spite of the many defects in 

 the structure of the glass, it was found 

 that it was not harmed by this range 

 when the change was gradual. Rapid 

 changes, however, did effect the figure 

 and as the polishing advanced it was 

 found that the heat from the friction 

 made it necessary to discontinue the 

 work for the day after a couple of 

 hours. 



While the glass was being polished 

 an elaborate series of experimental 

 tests were made each day to determine 



