NEWTON 441 in the Philosophical Transactions on various parts of the science of optics, and, although some of his views have been found to be erroneous, and are now almost universally rejected, his investigations led to discoveries which are of permanent value. He succeeded in explaining the colour of thin and of thick plates, and the inflexion of light, and he wrote on double refraction, polarization, and binocular He also invented a reflecting sextant for observ- vision. ing the distance between the moon and the fixed stars, the same in every essential as the instrument which is still in everyday use at sea under the name of Hadley s quadrant. This discovery was communicated by him to Dr Halley in 1700, but was not published, or communi cated to the Eoyal Society, till after Newton s death, when a description of it was found among his papers. In March 1673 we find Newton taking a somewhat pro minent part in a dispute in the university. The public oratorship fell vacant, and a contest arose between the heads of the colleges and the members of the senate of the university as to the mode of electing to the office. The heads claimed the right of nominating two persons, one of whom was to be elected by the senate. The senate on the other hand insisted that the proper mode was by an open election. The duke of Buckingham, who was the chancellor of the university, endeavoured to effect a com promise between the contending parties. He suggested an expedient which, he says, "I hope may for the present satisfy both sides. I propose that the heads may for this time nominate and the body comply, yet interposing (if they think fit) a protestation concerning their plea that this election may not hereafter pass for a decisive pre cedent in prejudice of their claim," and, " whereas I under stand that the whole university has chiefly consideration for Dr Paman of St John s and Mr Craven of Trinity College, I do recommend them both to be nominated. For it is very reasonable that in this nomination, before the difference be determined between you, the heads should have regard to the inclination of the body, especially seeing you all agree in two men that are very worthy, and very fit for the place." The heads, notwithstanding this reasonable and conciliatory suggestion of the chancellor, nominated Dr Paman and Mr Ralph Sanderson of St John s, and the next day one hundred and twenty-one .members of the senate recorded their votes for Craven and ninety-eight for Paman. On the morning of the election a protest in which Newton s name appeared was read, and entered in the Regent House. But the vice-chancellor admitted Paman the same morning, and so ended the first contest of a non-scientific character in which we find Newton taking part. On March 8, 1673, Newton wrote to Oldenburg, the secretary of the Royal Society : Sir, I desire that you will procure that I may be put out from being any longer Fellow of the Royal Society: for though I honour that body, yet since I see I shall neither profit them, nor (by reason of this distance) can partake of the advantage of their assemblies, I desire to withdraw." Oldenburg must have replied to this by an offer to apply to the Society to excuse Newton the weekly payments, as in a letter of Newton s to Oldenburg, dated June 23, 1673, he says, " For your proffer about my quarterly payments, I thank you, but I would not have you trouble yourself to get them excused, if you have not done it already." Nothing further seehis to have been done in the matter until January 28, 1675, when Oldenburg informed "the Society that Mr Newton is now in such circumstances that he desires to be excused from the weekly payments." Upon this " it was agreed to by the council that he be dispensed with, as several others are." Very soon after this that is, on February 18, 1675 Newton was formally admitted into the Society. The most probable explanation of the cause why Newton wished to be excused from these payments is to be found in the fact that, as he was not in holy orders, his fellowship at Trinity College would lapse in the autumn of 1675. It is true that the loss to his income which this would have caused was obviated by a patent from the crown in April 1675, allowing him as Lucasian professor to retain his fellowship without the obligation of taking holy orders. This must have relieved Newton s mind from a great deal of anxiety about pecuniary matters, as we find him so soon after this event as November 1676 subscribing =40 towards the building of the new library of Trinity College. It is supposed that it was at Woolsthorpe in the summer of 1666 that Newton s thoughts were directed to the subject of gravity. Voltaire is the authority for the well-known anecdote about the apple. He had his infor mation from Newton s favourite niece Catharine Barton, who married Conduitt, a fellow of the Royal Society, and one of Newton s intimate friends. How much truth there is in what is a plausible and a favourite story can never be known, but it is certain that tradition marked a tree as that from which the apple fell, till 1820, when, owing to decay, the tree was cut down and its wood carefully preserved. Kepler had proved by an elaborate series of measure ments that each planet revolves in an elliptical orbit round the sun, whose centre occupies one of the foci of the orbit, that the radius vector of each planet drawn from the sun describes equal areas in equal times, and that the squares of the periodic times of the planets are in the same pro portion as the cubes of their mean distances from the sun. The fact that heavy bodies have always a tendency to fall to the earth, no matter at what height they are placed above the earth s surface, seems to have led Newton to conjecture that it was possible that the same tendency to fall to the earth was the cause by which the moon was retained in its orbit round the earth. Newton, by calcu lating from Kepler s laws, and supposing the orbits of the planets to be circles round the sun in the centre, had already proved that the force of the sun acting upon the different planets must vary as the inverse square of the distances of the planets from the sun. He therefore was led to inquire whether, if the earth s attraction extended to the moon, the force at that distance would be of the exact magnitude necessary to retain the moon in its orbit. He found that the moon by her motion in her orbit was deflected from the tangent in every minute of time through a space of thirteen feet. But by observing the distance through which a body would fall in one second of time at the earth s surface, and by calculating from that on the supposition of the force diminishing in the ratio of the inverse square of the distance, he found that the earth s attraction at the distance of the moon would draw a body through 15 feet in one minute. A less careful calculator might have been satisfied with the close approximation of these two results ; but Newton, on the contrary, regarded the discrepancy between the results as a proof of the inaccuracy of his conjecture, and " laid aside at that time any further thoughts of this matter." The idea thus laid aside was not finally condemned. In 1679 a controversy between Hooke and Newton, about the form of the path of a body falling from a height, taking the motion of the earth round its axis into consideration, led Newton again to revert to his former conjectures on the moon. The measure of the earth, which had hitherto been accepted by geographers and navigators, was based on the very rough estimate that the length of a degree of latitude of the earth s surface measured along a meridian was 60 miles. More accurate estimates had been made by Norwood and Snell, and more recently by Picard. At a meeting of XVII. 56
