through the capillaries, and it can now be seen easily by any student who will examine the web of a frog's foot. The largest blood vessels seen are the small arteries bringing the blood from the heart, the finest are the capillaries through which the blood passes into the small veins which carry it back to the heart and thus complete the circuit.

Harvey's work was an excellent example of the application of the inductive method of study laid down by Francis Bacon as the essential principle of scientific progress; but Harvey did not begin to teach the circulation of the blood until 1619, and as Bacon died seven years later he may be forgiven the omission of any reference to it in his writings, though he must have known of it. Harvey's opinion of Bacon's doctrines is represented by his remark, "He writes philosophy like a Lord Chancellor."

John Hunter, who was born exactly a hundred years after the publication of Harvey's work on the circulation of the blood, paid no heed to Bacon's mechanical system of arriving at scientific truth. This great anatomist, biologist and surgeon, whose studies of the human frame raised surgery from the art of the barber-surgeon to the dignity of a science, and whose observations and experiments embraced every object which he could secure in the animal and vegetable kingdoms, did not trouble about Baconian principles in his inquiries and interpretations. He had a passion for knowledge, and possessed the creative and constructive attributes which enabled him to make discoveries and to coordinate them. Writing to Jenner about certain observations and experiments he desired him to make, he said, "Be as particular as you possibly can"; and that sentence epitomises his scientific method. He

was rationally sceptical and critical of so-called facts, and was always ready to put them and principles to the test of experiment.

The principles of our art are not less necessary to be understood than the principles of other sciences; unless, indeed, the surgeon should wish to resemble the Chinese philosopher whose knowledge consisted only in facts. In that case, the science must remain unimproved until new facts arise. In Europe, philosophers reason from principles and thus account for facts before they arise. John Hunter.

On one occasion there was a discussion at a meeting at which Hunter was present as to the structure of certain organs in the digestive system of birds. The meeting adjourned without settling the question, and on the next assembly quotations from the works of Aristotle, Hippocrates, Galen and other authorities were brought forward by the advocates of particular views in support of their opinions. Hunter had not gone to the early masters for confirmation, but had dissected the organs and exhibited them on a plate. All the books in the world and all the speculations of philosophers throughout the ages are but feather-weights in comparison with ocular evidence of this kind.

The desire to test and measure for himself instead of accepting the observations and systems of ancient astronomers as fixed standards of reference led to the foundation of modern astronomy of precision by Tycho Brahe (1546-1601). While a boy of thirteen at the University of Copenhagen this young Danish noble was impressed by the observation of a partial eclipse of the sun, which had occurred punctual to prediction. He then determined to train himself for an astronomer, and when a youth of seventeen we find him making measurements of a conjunction of the planets Jupiter and

Saturn on August 17, 1564, by means of an ordinary pair of compasses. For a few years after, his attention was given to other scientific subjects, but the appearance of a remarkable new star in the constellation of Cassiopeia in 1572 re-awakened his activity and fixed his career.

Thenceforth, he devoted his life to the accurate determination of the positions and motions of celestial bodies, and started a renaissance of astronomical measurement. Until he began observations at his observatory at Uraniborg on the island of Huen, the astronomy of the ancients had remained practically undisturbed. No advance had been made in the knowledge of the positions of the fixed stars or of the moon's motion-so important for navigation; and the positions of the planets could not be foretold with anything like reasonable accuracy.

No astronomer had yet made up his mind to take nothing for granted on the authority of the ancients; but to determine everything himself. Nobody had perceived that the answers to the many questions which were perplexing astronomers could only be given by the heavens, but that the answers would be forthcoming only if the heavens were properly interrogated by means of improved instruments capable of determining every astronomical quantity anew by systematic observations. The necessity of doing this was at an early age perceived by Tycho Brahe. Dr. J. L. E. Dreyer.

For twenty-five years Tycho Brahe patiently and diligently measured the positions of stars and other bodies upon the celestial sphere, using instruments of his own design and attaining an accuracy of observation little short of marvellous. Upon the basis of these observations Kepler constructed his three famous laws of planetary motion.

Johann Kepler was born at Weil, in the Duchy of Würtemberg, in 1571. Refused as a divine, he pursued

the study of mathematics and was appointed professor at Gratz in 1594, being then twenty-four years of age. But five years later he was, as a Protestant, expelled from Styria and went to join Tycho Brahe at Prague, where for a number of years he endeavoured to fit Tycho Brahe's wonderfully accurate observations into the theories then held as to motions of bodies in the solar system.

In the process of discovery of the three fundamental laws known by his name, Kepler was led to make many fantastic hypotheses. But all through he was guided by the principle that God who made the world had established fixed laws throughout his works, laws that are often so definite as to be capable of expression in exact numerical terms. In accordance with these views he sought for numerical relations in the disposition of the planets and their arrangement, in respect to their number, their times of revolution and their distances from one another. Each hypothesis he made, however fanciful, he tried by a rigorous test whenever possible, and as soon as he found that the facts were not in accordance therewith he abandoned it, and without hesitation proceeded to try others, which he submitted to the same severe ordeal, to share perhaps the same fate. He says, "After many failures, I was comforted by observing that the motions in every case seemed to be connected with the distances; and that when there was a great gap between the orbits there was the same between the motions." He was at length led to the discovery of his well-known "Harmonic" law (the squares of the periodic times of revolution of the planets are as the cubes of their mean distances from the Sun).

The misery in which Kepler lived forms a painful contrast with the services which he performed for Science. The pension

on which he subsisted was always in arrears; and though the three emperors, whose reigns he adorned, directed their ministers to be more punctual in its payment, the disobedience of their commands was a source of continual vexation to Kepler. When he retired to Silesia, to spend the remainder of his days, his pecuniary difficulties became still more harassing. Necessity at length compelled him to apply personally for the arrears which were due; and he accordingly set out in 1630, when nearly sixty years of age, for Ratisbon; but in consequence of the great fatigue which so long a journey on horseback produced, he was seized with a fever, which put an end to his life. Sir David Brewster.

The astronomical views of early Greek philosophers were summed up by Aristotle in the fourth century B.C. There were many speculations as to the form of the earth and the nature of celestial phenomena, but few have any scientific significance. "Some say," remarked Aristotle in one of his works, "that the earth rests on water. We have ascertained that the oldest statement of this character is the one accredited to Thales the Milesian [639-546 B.C.], to the effect that it rests on water, floating like a piece of wood or something of that sort." Aristotle himself stated plainly that the earth is a round globe, and Pythagoras (566-470 B.C.) is believed to have taught that the earth moves in the heavens, but Aristarchus of Samos, in the third century B.C., was the first philosopher to suggest that the sun is the centre of our system and that the earth revolves around it in a year. When, however, four hundred years later, Ptolemy of Alexandria built up his system of astronomy, he adopted the doctrine of an immovable earth in the centre of the universe, and this was accepted as true until the time of Copernicus.

The earliest serious criticism of the views of Aristotle and Ptolemy appear to have been made by Oresme, who became Bishop of Lisieux, Normandy, in 1377, and