“In the annexed figure PQRS denotes a plate of brass, accurately divided in the limb DQ, into ½ degrees, ½ minutes, and 1/12 minutes, by a diagonal scale; and the ½ degrees, and ½ minutes, and 1/12 minutes, counted for degrees, minutes, and 1/6 minutes. AB is a telescope three or four feet long, fixed on the edge of that brass plate. G is a speculum fixed on the brass plate perpendicularly as near as may be to the object-glass of the telescope, so as to be inclined forty-five degrees to the axis of the telescope, and intercept half the light which would otherwise come through the telescope to the eye. CD is a moveable index turning about the centre C, and, with its fiducial edge, showing the degrees, minutes, and 1/6 minutes on the limb of the brass plate PQ; the centre C must be over against the middle of the speculum G. H is another speculum, parallel to the former, when the fiducial edge of index falls on 0° 0′ 0″; so that the same star may then appear through the telescope in one and the same place, both by the direct rays and by the reflexed ones; but if the index be turned, the star shall appear in two places, whose distance is showed on the brass limb by the index.
Fig. 12.
“By this instrument the distance of the moon from any fixed star is thus observed: view the star through the perspicil by the direct light, and the moon by the reflexed (or on the contrary); and turn the index till the star touch the limb of the moon, and the index shall show on the brass limb of the instrument the distance of the star from the moon’s limb; and though the instrument shake by the motion of the ship at sea, yet the moon and star will move together as if they did really touch one another in the heavens; so that an observation may be made as exactly at sea as at land.
“And by the same instrument may be observed exactly the altitudes of the moon and stars, by bringing them to the horizon; and thereby the latitude and times of observation may be determined more exactly than by the ways now in use.
“In the time of the observation, if the instrument move angularly about the axis of the telescope, the star will move in a tangent of the moon’s limb, or of the horizon; but the observation may notwithstanding be made exactly, by noting when the line described by the star is a tangent to the moon’s limb, or to the horizon.
“To make the instrument useful, the telescope ought to take in a large angle; and to make the observation true, let the star touch the moon’s limb, not on the outside, but on the inside.”
This ingenious contrivance is obviously the very same invention as that which Mr. Hadley produced in 1731, and which, under the name of Hadley’s Quadrant, has been of so great service in navigation. The merit of its first invention must therefore be transferred to Sir Isaac Newton.
In the year 1672, Sir Isaac communicated to Mr. Oldenburg his design for a microscope, which he considered to be as capable of improvement as the telescope, and perhaps more so, because it requires only one speculum. This microscope is shown in the annexed diagram, where AB is the object-metal, CD the eye-glass, F their common focus, and O the other focus of the metal in which the object is placed. This ingenious idea has been greatly improved in modern times by Professor Amici, who makes AB a portion of an ellipsoid, whose foci are O and F, and who places a small plain speculum between O and AB, in order to reflect the object, which is placed on one side AP, for the purpose of being illuminated.
