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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.
That size is specified as a visual angle, which is the angle, at the eye, under which the optotype appears. For 6/6 = 1.0 acuity, the size of a letter on the Snellen chart or Landolt C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree), which is a 43 point font at 20 feet. [10]
To help compare different orders of magnitude, this section lists lengths between 10 −2 m and 10 −1 m (1 cm and 1 dm). 1 cm – 10 millimeters; 1 cm – 0.39 inches; 1 cm – edge of a square of area 1 cm 2; 1 cm – edge of a cube of volume 1 mL; 1 cm – length of a coffee bean; 1 cm – approximate width of average fingernail
A common adjustment value in firearm sights is 1 cm at 100 meters which equals 10 mm / 100 m = 1 / 10 mrad. The true definition of a milliradian is based on a unit circle with a radius of one and an arc divided into 1,000 mrad per radian, hence 2,000 π or approximately 6,283.185 milliradians in one turn , and rifle scope ...
Under Marcus Vipsanius Agrippa, it was standardized as the distance of two steps (gradūs) or five Roman feet (pedes), about 1.48 meters or 4 feet 10 inches. One thousand paces were described simply as mille passus or passuum , now known as a Roman mile ; this is the origin of the English term "mile".
The 4 ft 9 in (1,448 mm) railways were intended to take 4 ft 8 + 1 ⁄ 2 in (1,435 mm) gauge vehicles and allow a (second) running tolerance. The Chester and Birkenhead Railway, authorised on 12 July 1837, used 4 ft 9 in (1,448 mm). [31] The London and Brighton Railway, authorised on 15 July 1837, used 4 ft 9 in (1,448 mm). [32]
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90-foot (27.43 m) radii on the elevated 4 ft 8 + 1 ⁄ 2 in (1,435 mm) standard gauge Chicago 'L'. There is no room for longer radii at this cross junction in the northwest corner of the Loop . The minimum railway curve radius is the shortest allowable design radius for the centerline of railway tracks under a particular set of conditions.