The windows are made of optical glass similar to that used in the largest telescopes.
In 1914 the imports of optical glass were $641,000 in value.
In April, 1917, the most serious problem in the situation was the manufacture of optical glass.
Nevertheless, disks of optical glass, both crown and flint, have been produced up to 39 in.
optical glass when it comes into the workshop has the appearance of small blocks of rough ice of a greyish hue.
These were cast by Mantois, of Paris, whose superiority to the American manufacturers of optical glass is recognized.