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2. Cheval mirror - Wikipedia

   en.wikipedia.org/wiki/Cheval_mirror

   The size of individual mirrors was still small: the 17 seemingly large window-like panes in the Hall of Mirrors are in fact stitched from 357 small pieces of mirror glass. [12] In 1687, Bernard Perrot [ fr ] developed a process of glass casting that enabled first truly large glass mirrors, impossible to make using the traditional glass blowing ...

3. File:Comparison optical telescope primary mirrors.svg

   en.wikipedia.org/wiki/File:Comparison_optical...

   Dotted lines show sizes of round mirrors that would have had equivalent light-gathering ability. The telescopes shown on this comparison chart are listed below, ordered in each sub-section by (effective) mirror/lens area, low to high, and then by actual/planned first light date, old to new. The "present-day" status is given as of the beginning ...

4. Snellen chart - Wikipedia

   en.wikipedia.org/wiki/Snellen_chart

   Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters. In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres.

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6. Hale Telescope - Wikipedia

   en.wikipedia.org/wiki/Hale_Telescope

   The Hale's 200 in (510 cm) mirror was near the technological limit of a primary mirror made of a single rigid piece of glass. [ 22 ] [ 23 ] Using a monolithic mirror much larger than the 5-meter Hale or 6-meter BTA-6 is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it.

7. Angular diameter - Wikipedia

   en.wikipedia.org/wiki/Angular_diameter

   Angular sizes measured in degrees are useful for larger patches of sky. (For example, the three stars of the Belt cover about 4.5° of angular size.) However, much finer units are needed to measure the angular sizes of galaxies, nebulae, or other objects of the night sky. Degrees, therefore, are subdivided as follows: 360 degrees (°) in a full ...