A brief account of some of the historical instruments exhibited at the Optical Convention, 1926 / by R.S. Clay.

  • Clay, Reginald Stanley, 1868-1954.
Date:
[1926]
Catalogue details

Licence: In copyright

Credit: A brief account of some of the historical instruments exhibited at the Optical Convention, 1926 / by R.S. Clay. Source: Wellcome Collection.

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    A BRIEF ACCOUNT OF SOME OF THE HIS¬ TORICAL INSTRUMENTS EXHIBITED AT THE OPTICAL CONVENTION, 1926 By R. S. CLAY, B.A., D.Sc. It is obviously impossible within the limits of a short paper to describe all the interesting instruments exhibited ; I, therefore, propose only to touch on a few of those of outstanding importance ; as far as possible, I will take them chronologically. The first to which I will call attention are the early navigating instruments. Of these we have an early astrolabe supposed to have belonged to the Spanish Armada. It was found in 1845 under a rock on the north side of Combe Hill in the island of Valencia, Co. Kerry, Ireland, within view of the place where three vessels of the Armada were wrecked. The instrument was made heavy at the bottom so that it would remain steady in the wind ; it was held up by the ring and the swinging arm was rotated until the sun’s image, formed through the pinhole in one sight, fell on the other sight. Thus the sun’s meridian altitude on that day could be determined, and from that, knowing its declination, the latitude was determined. Another instrument, also used to determine the sun’s meridian altitude, was the cross¬ staff, of which an original example is exhibited (the crosses are restored). The end of the staff was held against the bone at the side of the orbit of the eye and a cross was moved along the staff until its upper end was in line with the sun at the same time that its lower end was in line with the horizon. The staff is graduated to give the angle subtended. This staff is inscribed “ Hamon a St. Malo, 1687 There are several quadrants which were also used to obtain either the altitude of the sun or that of one of the brighter fixed stars. The quadrant was held up in a vertical plane and the sun or star sighted through the two pin¬ holes in the sight vanes. While so held, the thumb was placed on the string of the plumb-bob suspended from the top corner of the quadrant, and thus the slope of the edge of the quadrant was found, i.e. the altitude of the star. There is one by Senior, dated 1600, which is interesting as it was made and signed by the designer ; one by Sutton, dated 1660 ; one by T. Wright on stand—a rather unusual feature ; another, a small astronomical quadrant, by J. Sisson on stand ; a 12-inch one by Bird on a pillar stand, said to have been used by Captain Cook. Many quadrants used to have Gunter’s, Sutton’s, or other projection on them, and could be used to determine the position of the sun on the ecliptic, to find its time of rising and setting, to find the time of