Ptolemy saw that the shape of the earth was globular, and he demonstrated this by the arguments which we employ at the present day. He also saw how this mighty globe was poised, in what he believed to be the centre of the universe. He admitted that the diurnal movement of the whole heavens could be accounted for by the revolution of the earth upon its axis, but he assigned reasons for the deliberate rejection of this view. The earth according to him was a fixed body; it possessed neither rotation nor translation, but remained constantly at rest at the centre of the universe. The sun and the moon he supposed to move in circular orbits around the earth in the centre. The movements of the planets were more complicated, as it was necessary to account for the occasional retrograde motions as well as for the direct motions. The ancient geometry refused to admit that any movement, except circular, could be perfect, and accordingly a contrivance was devised by which each planet revolved in a circle, while the centre of that circle described another circle around the earth. It must be admitted that this scheme, though so widely divergent from what is now known to be the truth, did really present a fairly accurate account of the movements of the planets. Such was the system of Astronomy which prevailed during the Middle Ages, and which was only finally overturned by the great work to which Copernicus devoted his lifetime. The discovery of the true system of the universe was nearly simultaneous with the discovery of the New World by Columbus. The first principles which were established by the labours of Copernicus, stated that the diurnal movement of the heavens was really due to the rotation of the earth on its axis. He showed the difference between real motions and apparent motions; he proved that all the appearances of the daily rising and setting of the sun and the stars could be just as well accounted for by the supposition that the earth rotated, as by the more cumbrous supposition of Ptolemy. He showed that the latter supposition would attribute an almost infinite velocity to the stars, and that the rotation of the entire universe around the earth was really a preposterous supposition. The second great point, which it is the immortal glory of Copernicus to have demonstrated, assigned to the earth its true position in the fabric of the universe. He transferred the centre, about which all the planets revolve, from the earth to the sun; and he established the somewhat humiliating truth, that our earth is after all merely one of the system of planets revolving around the sun, and pursuing a track between the paths of Venus and of Mars. Such was, in brief outline, the great revolution which swept from astronomy those distorted views of the earth's importance, arising from the fact that we are domiciled on that particular planet. The achievement of Copernicus was soon to be followed by the invention of the telescope, that wondrous instrument by which the modern science of astronomy has been created. To the consideration of this most important subject we may well devote the first chapter of our book. CHAPTER I. THE ASTRONOMICAL OBSERVATORY. Early Astronomical Observations-The Observatory of Tycho Brahe-The Pupil THE earliest traces of the Astronomical Observatory are as little known, as the earliest discoveries in astronomy itself. Probably the first application of instruments to the observations of the heavenly bodies, consisted in the extremely simple operation of measuring the length of the shadow cast by the sun at noonday. The variations in the length of this shadow from day to day, and its periodical maxima and minima, furnished valuable information in the early attempts to investigate the movements of the sun. But even in very early times there were astronomical instruments employed which possessed considerable complexity, and showed no small amount of astronomical knowledge. The first great advance in this subject was made by the celebrated Tycho Brahe, who was born in 1546, three years after the death of Copernicus. His attention seems first to have been directed to astronomy by the eclipse of the sun which occurred on the 21st August, 1560. It amazed his reflective spirit to find that so surprising a phenomenon admitted of actual prediction, and he determined to devote his life to the study of a science possessed of such wonderful precision. In the year 1576 the King of Denmark had established Tycho Brahe on the island of Huen, and had furnished him with the splendid observatory of Uraniberg. It was here that Tycho assiduously observed the places of the heavenly bodies for some twenty years, and accumulated the observations which were destined, in the hands of Kepler, to lead to the great discovery of the planetary movements. Compared with our modern astronomical equipment the great instruments of Tycho are but quaint and primitive apparatus. In his days the telescope had not yet been invented, and he could only determine the places of the heavenly bodies in a comparatively crude manner; but his skill and patience in a great degree compensated for the imperfection of his instruments, and with him it may be said that the epoch of accurate astronomical observation commences. The application of the telescope by Galileo gave a most wonderful impulse to the study of the heavenly bodies. This extraordinary man stands out prominently in the history of astronomy, not alone for his connection with this supreme invention, but for his achievements in the more abstract parts of astronomy. It was Galileo who first laid with any solidity the foundation of the science of Dynamics, of which astronomy is the most splendid illustration; and it was he who expounded and upheld the great doctrine of Copernicus, and thereby drew down upon himself the penalties of the Inquisition. The structure of the eye itself, and more particularly the exquisite adaptation of the pupil, presents us with an apt illustration of the principle of the telescope. To see an object, it is necessary that the light from that object should enter the eye. The portal through which the light enters the eye is the pupil. In daytime, when the light is abundant, the iris gradually decreases the size of the pupil, and as the portal is thus contracted, less light can enter. At night, on the other hand, when the light is scarce, the eye requires to grasp all it can. The pupil then expands, more and more light is admitted according as the pupil grows larger, until at length the pupil is dilated to its utmost extent. The admission of light is thus controlled in the most perfect manner. The stars send us their feeble rays of light, and those rays form an image on the retina; but, even with the most widely-opened pupil, it may happen that the image is still not bright enough to excite the sensation of vision. Here the telescope comes to our aid: it catches all the rays in a beam of dimensions far too large to enter the pupil, and concentrates those rays into a small beam which can enter the pupil. We thus have the image on the retina intensified in brilliancy; in fact, it is illuminated with nearly as much light as would be obtained through a pupil as large as the object-glass of the telescope. Rays of light from the Star Objective Focus Eye piece In our astronomical observatories we find two entirely different classes of telescopes. The more familiar forms are those known as refractors, in which the operation of condensing the rays of light is effected by refraction. The same object can, however, be attained in a wholly different manner by the aid of the laws of reflection, and accordingly many telescopes, including the most gigantic instruments yet erected, are known as reflectors. The character of the refractor is shown in Fig. 1. The rays from the star fall upon the object-glass which is at the end of the telescope, and after passing through it they are refracted into a converging beam, so that all intersect at the focus. Diverging from thence, they encounter the eye-piece, which has the effect of again reducing them to parallelism. The large cylindrical beam which poured down on the object-glass is thus concentrated into a small one, which can enter the pupil. The composite nature of light requires a more complex form of objectglass than the simple lens here shown. In modern telescopes we employ what is known as the achromatic object-glass, which consists of one lens of flint glass and one of crown glass, combined together. To the Fig. 1.-Principle of the Refracting Telescope. It will thus be apparent, that the larger the object-glass, the |