Cotemporary with Tycho Brahé and Galileo, and to some extent the associate and successor of the former, was John Kepler, one of the most eminent astronomers who have appeared in any age, and to whom the science is indebted for much of its present perfection. He was born December 27, 1571, at Wiel in Wurtemberg, and was descended of a noble but reduced family. His father, originally an officer of distinction in the army of Wurtemberg, was, at the time of young Kepler’s birth, in the humble capacity of a small inn-keeper; and thus, as is too often the case with genius, our philosopher had to struggle into fame through poverty and the vicissitudes of his father’s fortune. Poor, unbefriended, of a weakly constitution, and one of the most diminutive of children, Kepler received the rudiments of knowledge at the Monastic school of Maulbrunn, where he gave early indications of talent, and of that irrepressible spirit which, amid the severest obstructions, was never diverted from the main object of its pursuit. After his father’s death, which took place in his eighteenth year, he left Maulbrunn, and succeeded in entering the college of Tubingen. Here he completed the course of study then prescribed—​first philosophy and mathematics, and then theology; taking the degree of Bachelor in the year 1588, and that of Master of Philosophy in the year 1591. Of apt inquiring powers as a divine, and of more than average eloquence as a preacher, Kepler could now have readily succeeded in the church; but mathematics and the exact sciences were his favorite themes; and it may be fairly questioned if ever he turned a single thought to the clerical profession, beyond what the curriculum of the university compelled. In 1593–4, his reputation as a geometrician had so increased, that he was invited to fill the mathematical chair in the university of Gratz, in Styria. Here he pursued his astronomical studies with the most commendable zeal, devoting himself especially to the investigation of the physical causes of the motion of the celestial bodies.

Shortly after his installment, he married a lady descended from a noble family, and was beginning to enjoy that domestic happiness and studious quiet so congenial to his wishes, when persecution on account of his religion compelled him to leave Gratz, to which, however, he was afterwards recalled by the states of Styria. Meanwhile Tycho Brahé, who had come to Germany, and was comfortably settled under the munificent patronage of Rodolphus, fixed upon Kepler as a suitable assistant, and soon induced him, by urgent letters and flattering promises, to accept of the situation. Compelled in a great measure by the unsettled state of affairs in Austria, Kepler speedily repaired to Prague, and applied himself, in conjunction with Tycho, to the completion of the Rodolphine Tables, which were first published at Ulm in 1626. At Tycho’s recommendation, he was established at that place; but as his office and science did not afford him a subsistence, he studied medicine, in order to gain a livelihood by its practice. The emperor had assigned him a salary, but in the period of trouble which preceded the Thirty Years’ War, it was not paid. Even when he was appointed imperial mathematician by Matthias, Rodolphus’ successor, his hopes of recovering his arrears were disappointed. Fresh controversies with the clergy, and the disturbed state of the country, made his situation very uncomfortable: he therefore left Lintz, repaired to Ratisbon, declined an invitation to England, was confirmed by the succeeding emperor, Ferdinand, in the office of imperial methematician, and afterwards went to Ulm to superintend the printing of the Rodolphine Tables. In 1627 he returned to Prague, and received from the emperor six thousand guilders. He finally became a professor at Rostock, on the recommendation of Albert, duke of Wallenstein, but did not receive the promised compensation. In 1630 he went, by permission of the emperor, to Ratisbon, to claim payment of the arrears of his pension; but he was there seized with a violent fever, supposed to have been brought upon him by too hard riding; and to this he fell a victim in the month of November, in the fifty-ninth year of his age. In 1808, a monument, consisting of a Doric temple enshrining his bust, was erected to his memory in Ratisbon by Charles Theodore Von Dalberg.

Kepler is represented by his biographers as a man of small stature, thin, of a weak constitution, and defective sight; but of somewhat gay and sportive manners. He was attached to his science with the most fervent enthusiasm; he sought after truth with eagerness, but forgot, in the search, the maxims of worldly prudence. To him were allotted but a scanty share of what are commonly esteemed the pleasures of life; but he endured all calamities with firmness, being consoled by the higher enjoyments which science never fails to impart to her true and cordial votaries. ‘As an astronomer,’ says Lalande, ‘he is as famous in astronomy for the sagacious application which he made of Tycho’s numerous observations (for he was not himself an observer), as the Danish philosopher for the collection of such vast materials.’ To him, says another authority, the world is indebted for the discovery of the true figure of the planetary orbits, and the proportions of the motions of the solar system. Like the disciples of Pythagoras and Plato, Kepler was seized with a peculiar passion for finding analogies and harmonies in nature; and though this led him to the adoption of strange and ridiculous conceits, we shall readily be disposed to overlook these, when we reflect they were the means of leading him to the most important discoveries. He was the first who discovered that astronomers had been mistaken in ascribing circular orbits and uniform motions to the planets, since each of them moves in an ellipse, having one of its foci in the sun; and after a variety of fruitless efforts, he, on the 15th of May 1618, made his splendid discovery, that the squares of the periodic times of the planets are always in the same portion as the cubes of their mean distances from the sun. The sagacity of this wonderful man, and his incessant application to the study of the planetary motions, pointed out to him some of the genuine principles from which these motions originate. He considered gravity as a power that is mutual between bodies; that the earth and moon tend toward each other, and would meet in a point so many times nearer to the earth than to the moon as the earth is greater than the moon, if their motions did not prevent it. His opinion of the tides was, that they arise from the gravitation of the waters towards the moon; but his notions of the laws of motion not being accurate, he could not turn his conceptions to the best advantage. The prediction he uttered at the end of his epitome of astronomy, has been long since verified by the discoveries of Sir Isaac Newton; namely, that the determination of the true laws of gravity was reserved for the succeeding age, when the Author of Nature would be pleased to reveal these mysteries.

NEWTON.

The year in which Galileo died, was that in which Isaac Newton was born. This eminent individual, who was destined to establish the truth of the discoveries of his illustrious predecessors, Copernicus and Galileo, was born on the 25th of December 1642, at Coltersworth, in Lincolnshire, where his father cultivated his own moderate paternal property. After receiving the rudiments of education, under the superintendence of his mother, he was sent, at the age of twelve, to the grammar school at Grantham, where the bias of his early genius was shown by a skill in mechanical contrivances, which excited no small admiration. Whilst other boys were at play, his leisure hours were employed in forming working models of mills and machinery; he constructed a water-clock from an old box, which had an index moved by a piece of wood sinking as the drops fell from the bottom, and a regular dial-plate to indicate the hours.

On his removal from school, it was intended that he should follow the profession of a farmer, but his utter unfitness for the laborious toils of such a life was soon manifested. He was frequently found reading under a tree when he should have been inspecting cattle, or superintending laborers; and when he was sent to dispose of farming produce at Grantham market he was occupied in solving mathematical problems in a garret or hay-loft, whilst the business was transacted by an old servant who had accompanied him to town. These strong indications of the bias of his disposition were not neglected by his anxious mother; she sent him again for a few months to school, and on the 5th of June 1660, he was admitted a student of Trinity College, Cambridge.

The combination of industry and talents, with an amiable disposition and unassuming manners, naturally attracted the notice of his tutors, and the friendship of his admiring companions; amongst these was Isaac Barrow, afterwards justly celebrated as a preacher and a mathematician. Saunderson’s Logic, Kepler’s Optics, and the Arithmetic of Infinites by Wallis, were the books first studied by Newton at Cambridge. He read the geometry of Descartes diligently, and looked into the subject of judicial astrology, which then engaged some attention. He read little of Euclid, and is said to have regretted, in a subsequent part of his life, that he had not studied the old mathematician more deeply.

The attention of Newton, while he was pursuing his studies at Cambridge, was attracted to a branch of natural philosophy hitherto little understood—​namely, light. It was the opinion of the celebrated philosopher Descartes that light is caused by a certain motion or undulation of a very thin elastic medium, which he supposed pervaded space. Newton overturned this theory. Taking a piece of glass with angular sides, called a prism, he caused the sun to shine upon it through a small hole in the shutter of a darkened apartment. By this experiment he found that the light, in passing through the glass, was so refracted or broken, as to exhibit on the wall an image of seven different tints or colors; and after varying his experiments in a most ingenious way, he established the very interesting facts, that light is composed of rays resoluble into particles, that every ray of white light consists of three primary and differently colored rays (red, yellow, and blue), each of which three is more or less refrangible than the other. This remarkable discovery laid the foundation of the science of optics.

In 1665, the students of the university of Cambridge were suddenly dispersed by the breaking out of a pestilential disorder in the place. Newton retired for safety to his paternal estate: and though he lost for a time the advantages of public libraries and literary conversation, he rendered the years of his retreat a memorable era in his own existence, and in the history of science, by another of his great discoveries—​that of the theory of gravitation, or the tendency of bodies towards the center of our globe. One day, while sitting in his garden, he happened to see an apple fall from a tree, and immediately began to consider the general laws which must regulate all falling bodies. Resuming the subject afterwards, he found that the same cause which made the apple fall to the ground, retained the moon and planets in their orbits, and regulated, with a simplicity and power truly wonderful, the motions of all the heavenly bodies. In this manner was discovered the principle of gravitation, by a knowledge of which the science of astronomy is rendered comparatively perfect.

On his return to Cambridge in 1667, he was elected Fellow of Trinity College; and two years afterwards, he was appointed professor of mathematics in the place of his friend Dr. Barrow, who resigned. His great discoveries in the science of optics formed for some time the principal subject of his lectures, and his new theory of light and colors was explained, with a clearness arising from perfect knowledge, to the satisfaction of a crowded and admiring audience. He was elected a Fellow of the Royal Society in 1671, and is reputed to have been compelled to apply for a dispensation from the usual payment of one shilling weekly, which is contributed by each member towards the expenses. He had at this period of life no income except what he derived from his college and professorship, the produce of his estate being absorbed in supporting his mother and her family. His personal wishes were so moderate, that he never could regret the want of money, except as much as it limited his purchases of books and scientific instruments, and restricted his power of relieving the distresses of others. About the year 1683, he composed his great work, The Principia, or Mathematical Principles of Natural Philosophy. In 1688, the memorable year of the Revolution, he was chosen to represent the university in parliament, and the honor thus conferred on him was repeated in 1701. His great merit at last attracted the notice of those who had it in their power to bestow substantial rewards, and he was appointed warden of the Mint, an office for which his patient and accurate investigations singularly fitted him, and which he held with general approbation till his death. Honors and emoluments at last flowed upon him. Leibnitz, having felt envious of the discoveries of Newton, tried to revenge himself by transmitting a problem, which he thought would show his superiority, by baffling the skill of the English mathematician. It was received by Newton in the evening, after his usual day’s labor at the Mint, and he solved it before he retired to rest. After this there was no further attempt made to traduce his fame. In 1705 he received the honor of knighthood from Queen Anne.