In 1917, a crew drives an early Mack truck up the Mount Wilson Toll Road, with the precious 100-inch mirror crated and standing vertically in the back. Photo: Carnegie/ Huntington Library

One Hundred Years Ago, July 1st, 1917, the 100-Inch Mirror Arrived atop Mount Wilson!

The order for the 4.5 ton disk of glass was sent on September 14th, 1906, to the Saint Gobain Glass company in France, and almost eleven years later the finished 100-inch mirror arrived on the mountain. The mirror was the heart of the telescope and without it all the other work would have been wasted. In the fall of 1907, two attempts to cast the glass disk failed. On December 31. 1907, the disk that would become the telescope’s mirror was poured and the annealing process began, in which the glass was cooled slowly to prevent cracking. On February 19th, 1908, the glass was removed from the mold and examined. It was not pretty. It had three obvious layers, like a cake, and was filled with bubbles and swirls from the three ladles of glass that were poured. (Scroll to the bottom of the page to see photos.)

After months of negotiations between Mount Wilson and Saint Gobain, the disk was shipped on October 22, 1908, with the provision that the payment of $11,000 would only be made if it was usable. The disk traveled across the Atlantic aboard the steamer St. Andrew. From Hoboken, New Jersey, it was shipped to New Orleans, and then made its way by rail to Pasadena. It arrived on December 7, 1908, the same day the 60-inch mirror (also from Saint Gobain) was installed in its telescope on the mountain. The following day the disk was unpacked. When the glass was unveiled, the director of the Observatory, George Ellery Hale and George Willis Ritchey, the optician who was to grind the mirror, declared it useless because of all the defects. They wanted a perfect, homogeneous, stable piece of glass in which to grind the optical surface. John Hooker, who was paying for the mirror, was equally disappointed. He sent Ritchey to France to discuss arrangements for another casting.

More attempts were made to make a perfect disk, but they all failed. During this period, Hale had a major nervous breakdown, and his wife wrote to Walter Adams, the number two man at the Observatory, “I wish that the glass was in the bottom of the ocean.” Finally, in desperation, the flawed disk was reexamined and tests indicated that it might make a stable mirror after all. On  March 4th, 1913, Hale wrote Saint Gobain with the news that the disk was, after all the doubts, a good one. A number of original letters documenting these events can be found below, many from 1909 when alternative ways of casting were desperately being sought. All are in English, accept the first one from Ritchey confirming the order. We will add other documents in French as they are translated. Photos below: Huntington Library/Carnegie

Original Communications from the Observatory to Saint Gobain in France, as PDF files.

Special thanks to Amandine Metraux, the Archivist at Saint Gobain in France and Norm Vargas with CHARA on Mount Wilson for obtaining these documents.

The March 4th , 1913 letter from Hale expressing his final acceptance of the mirror disk from Saint Gobain and his gratitude for all their efforts to make a better one. His graceful wording of the letter is typical, yet still remarkable given that his worries over the 4.5-ton piece of glass caused his severe mental breakdown in 1910.

This edge-on photo was taken during the recent re-aluminizing of the 100-inch mirror. The three layers are clearly visible, as are the multitude of bubbles and swirls. Photo: Nik Arkimovich


Seen up close, a section of the disk resembles the stars of a distant galaxy. Photo: Nik Arkimovich