This point of rotation allowed the angle between the index arm and the radial arm to be changed. At the point of rotation of the radial arm, a single mirror was mounted. A telescopic sight was mounted on the index arm. The three were arranged in a triangle as in the image on the right. The device consisted of three primary components, an index arm, a radial arm and a graduated chord. This instrument was first described in 1666 and a working model was presented by Hooke at a meeting of the Royal Society some time later. It used a single mirror to reflect the image of an astronomical object to the observer's eye. Hooke's instrument was a single-reflecting instrument. The lines of sight are represented by the red dashed line. The index with telescope mounted is shown in black, the radius arm with the mirror (grey) attached in blue and the chord in green on white. It does not accurately depict the fine details of the instrument but rather the basic functionality. Robert Hooke's single-reflecting instrument Representative drawing of Robert Hooke's reflecting instrument. This instrument appears to have been used for approximately 100 years, mainly in the Zeeland Chamber of the VOC (The Dutch East India Company). Invented in 1660 by the Dutch Joost van Breen, the spiegelboog (mirror-bow) was a reflecting cross staff. ![]() Joost van Breen's reflecting cross-staff However, it had little influence outside of the Netherlands. The van Breen instrument was the exception, in that it was used by the Dutch. These were little used or may not have been built or tested extensively. Some of the early reflecting instruments were proposed by scientists such as Robert Hooke and Isaac Newton. Any further improvements required a completely new technology. What followed is a series of inventions and improvements that refined the instrument to the point that its accuracy exceeded that which was required for determining longitude. ![]() Those that considered the problem realized that the use of specula (mirrors in modern parlance) could permit two objects to be observed in a single view. Requiring the observer to observe two objects with two divergent lines of sight increased the likelihood of an error. The deficiency of prior instruments was well known. The solution to this problem was seen to require an accurate means of measuring angles and the accuracy was seen to rely on the observer's ability to measure this angle by simultaneously observing two objects. The driving force behind the developments discussed here was the solution to the problem of finding one's longitude at sea. The purpose of reflecting instruments is to allow an observer to measure the altitude of a celestial object or measure the angular distance between two objects. While reflecting instruments are used in many professions, they are primarily associated with celestial navigation as the need to solve navigation problems, in particular the problem of the longitude, was the primary motivation in their development. In particular, the use of mirrors permits one to observe two objects simultaneously while measuring the angular distance between the objects. Reflecting instruments are those that use mirrors to enhance their ability to make measurements. For reflecting instruments in astronomy, see Reflecting telescope.
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