A macrometer is an instrument for measuring the size and distance of distant objects.
Distant in this sense means a length that can not be readily measured by a
calibrated
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a Standard (metrology), calibration standard of known accuracy. Such a standard could be another measurem ...
length. The optical version of this instrument used two
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s on a common
sextant
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of cel ...
. By aligning the object on the mirrors using a precise
vernier, the position of the mirrors could be used to compute the range to the object. The distance and the angular size of the object would then yield the actual size.
See also
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Rangefinder
A rangefinder (also rangefinding telemeter, depending on the context) is a device used to measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, such as photography an ...
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Theodolite
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...
References
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Surveying instruments
Surveying
Measuring instruments
Length, distance, or range measuring devices