Here is a short passage from the book describing some of the conditions in the Caltech optical shop as people were polishing the 200-inch mirror:
The obsession with cleanliness in the optical shop was more than many men could stand. The floors were swept and washed daily. A worker rolled a magnet over the floor daily, sometime several time a day, to pick up even tiny specks of metal. If a speck was found it was put into an envelope, and the search began for the culprit machine. Was it a chip off a gear? Abrasion of some metal part that no one heard because of the noise of the grinding machines? A foreign speck off the shoes or uniform of a careless worker? Whatever the cause, it had t be found. A speck of metal under a polishing tool on the surface of a disk could make a scratch that might destroy months of work.
The image that came to my mind of a worker rolling a magnet over the floor to pick up stray bits of metal didn't equal the reality of the device that they actually used. I present to you the Electro-Magnetic Sweeper:
An amazing looking device! Here it is in action next to the 200-inch Pyrex disc:
Even more amazing to me is the fact that both of the people in the photo above the glass are smoking pipes! Both photos are from December 1938.