then laid upon the table; when the glass was laid above them and rubbed, the pieces of paper leaped from the one part of the glass to the other, and twirled about in the air. Notwithstanding this explicit account of the experiment, it entirely failed at the Royal Society, and the secretary was desired to request the loan of Sir Isaac's apparatus, and to inquire whether or not he had secured the papers from being moved by the air, which might have somewhere stole in. In a letter, dated 21st December, Sir Isaac recommended to the society to rub the glass "with stuff whose threads may rake its surface, and, if that will not do, to rub it with the fingers' ends to and fro, and knock them as often upon the glass." These directions enabled the society to succeed with the experiment on the 13th January, 1676, when they used a scrubbing brush of short hog's bristles, and the heft of a knife made with whalebone.

Among the minor inventions of Sir Isaac Newton, we must enumerate his reflecting instrument for observing the moon's distance from the fixed stars at sea. The description of this instrument was communicated to Dr. Halley in the year 1700; but, either from having mislaid the manuscript, or from attaching no value to the invention, he never communicated it to the Royal Society, and it remained among his papers till after his death in 1742, when it was published in the Philosophical Transactions. The following is Sir Isaac's own description of it as communicated to Dr. Halley.

12

"In the annexed figure PQRS denotes a plate of brass, accurately divided in the limb DQ, into degrees, minutes, and minutes, by a diagonal scale; and the degrees, and minutes, and minutes, counted for degrees, minutes, and minutes. AB is a telescope three or four feet long, fixed on the edge of that brass plate. G is a speculum fixed

near as

on the brass plate perpendicularly 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 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.

"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 Аа

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.

In another letter to Mr. Oldenburg, dated July 11th in the same year, he suggests another improvement in microscopes, which is to "illuminate the object in a darkened room with the light of any convenient colour not too much compounded: for by that means the microscope will, with distinctness, bear a deeper charge and larger aperture, especially if its construction be such as I may hereafter describe."* This happy idea I have some years ago succeeded in realizing, by illuminating microscopic objects with the light of a monochromatic lamp, which discharges a copious flame of pure yellow light of definite refrangibility.†

In order to remedy the evils arising from the weak reflecting power of speculum metal, and from its tarnishing by exposure to the air, Sir Isaac proposed to substitute for the small oval speculum a triangular prism of glass or crystal ABC. Its side

* Sir Isaac does not seem to have afterward described this construction.

† See Edinburgh Transactions, vol. ix. p. 433, and the Edinburgh Journal of Science, July, 1829, No. I. New Series, p. 108.

Fig. 14.

a

AB ba he supposes to perform the office of that metal, by reflecting towards the eye-glass the light which comes from the concave speculum DF, fig. 13, whose light he supposes to enter into this prism at its side CB bc, and lest any colours should be produced by the refraction of these planes, it is requisite that the angles of the prism at Aa and Bb be precisely equal. This may be done most conveniently, by making them half right angles, and consequently the third angle A at Cc a right one. plane AB ba will reflect all the light incident upon it; but in order to exclude unnecessary light, it will be proper to cover

The

B

b

it all over with some black substance excepting two circular spaces of the planes Ac and Bc, through which the useful light may pass. The length of the

prism should be such that its sides Ac and Bc may be square, and so much of the angles B and b as are