Its scanning window is crown glass
prism and there is special anti-scratching coating on the scanning surface.
The history of those practices, and the abidingly strange rigid liquidity of glass itself, get a full hearing; among the book's consistent pleasures is its mastery of craft-language (like the euphonious "roundels of crown glass
"  observed in a Durer engraving).
* Crown Glass
: One of two main types of optical glass.
Moreover, their index of refraction--1.53--is closer to that of crown glass
than other thermoplastics; thus, their use in place of glass requires minimal design changes.
Extreme made its mark by producing some of the most respected target scopes in the business, employing German-engineered crown glass
with world-famous Zeiss multilayer, non-reflective coatings for scratch resistance, crystal-clear target image and superior low-light performance.
Pilkington plc originated in 1826 as the St Helens Crown Glass
Company founded with the technical knowledge of John William Bell and finance from three of the most influential local families - the Bromilows, the Greenalls and the Pilkingtons.
For most designs, prisms made of barium light crown glass
(Bak4) are preferred over the industry-standard borosilicate crown glass
Archaeological evidence which would support an occupation in the early 19th century includes coins and medals dating to the War of 1812 or earlier, creamware or "Old Blue" transferware ceramics, fragments of crown glass
windows, blown and hand-moulded glass bottles, metal buttons stamped or cast from a mould, and gun parts related to this period.
Flint glass (containing lead) produced a rather wider spectrum than crown glass
(ordinary window glass) did.