While glass does exhibit creep under the right conditions, apparent sagging in old windows may instead be a consequence of obsolete manufacturing processes, such as that used to create crown glass, which resulted in inconsistent thickness.
42.
He was also instrumental in producing a flint and crown glass disc, measuring 29-inches ( 74 cm ) in diameter, which ironically, was purchased by the French government to be used in a telescope.
43.
The company charter permitted it to manufacture " crown glass of all kinds in the towns of Boston and Cambridge . " At that time, about 40 glass factories existed in the United States, though most had few employees.
44.
Values of " V " range from below 25 for very dense flint glasses, around 34 for polycarbonate plastics, up to 65 for common crown glasses, and 75 to 85 for some fluorite and phosphate crown glasses.
45.
Values of " V " range from below 25 for very dense flint glasses, around 34 for polycarbonate plastics, up to 65 for common crown glasses, and 75 to 85 for some fluorite and phosphate crown glasses.
46.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain, hence the name " bullseye . " Optical distortions produced by the bullseye could be reduced by grinding the glass.
47.
A Tessar comprises four elements in three groups, one positive crown glass element at the front, one negative flint glass element at the center and a negative plano-concave flint glass element cemented with a positive convex crown glass element at the rear.
48.
A Tessar comprises four elements in three groups, one positive crown glass element at the front, one negative flint glass element at the center and a negative plano-concave flint glass element cemented with a positive convex crown glass element at the rear.
49.
Glass has the ability to crown glass ), or increased ( to as much as 1.8 ) with high-density materials such as ( classically ) lead oxide ( see flint glass and lead glass ), or in modern uses, less toxic oxides of zirconium, titanium, or barium.
50.
For the construction of an achromatic collective lens ( f positive ) it follows, by means of equation ( 4 ), that a collective lens I . of crown glass and a dispersive lens II . of flint glass must be chosen; the latter, although the weaker, corrects the other chromatically by its greater dispersive power.
