permit the construction of the apparatus, at once extensive and delicate, which it employs.
Astronomy as a description of motions, on the contrary, only required the eyes and very simple instruments. Therefore the first astronomers began with that branch. At a later period, the science, ceasing to be purely descriptive, became geometric, and at last took a sublime flight; and by the application of the higher calculus we had the celestial mechanics.
During this long period, the physical branch of the science, to speak correctly, did not exist. Reduced to hypotheses that could not be verified, the theories of celestial physics had even fallen into discredit. It should be said that the beauty and importance of the discoveries with which the geometricians endowed the elder sister of our branch contributed no little to this result. Three great discoveries have, however, completely changed the situation, by giving to the physical branch arms which permit it at last to enter gloriously into the competition. I refer to the telescope, spectrum analysis, and photography.
The foundations of physical astronomy were laid in the invention of the telescope. Every one has heard of the emotion which filled Europe at the announcement of the discovery of an instrument which had the power of making distant objects appear as if they were near. It was at that time that Galileo, having only learned that such an instrument existed, discovered its arrangement, constructed one, turned it toward the sky, and, with this aid, fertilized by his genius, made a series of magisterial discoveries. These discoveries belong pre-eminently to physical astronomy, and form its first courses. If we except the sun and moon, which have a very sensible diameter, and admit of some observations without the aid of the telescope, all the stars appear to the eye only as brilliant points, and admit of no studies except of their motions. Therefore, an astronomy without the telescope would never have permitted us otherwise than as a matter of probability to consider the planets as like the earth in form, constitution, and office. But when it was seen that these brilliant and almost blazing points were resolved under the telescope into well-defined disks, showing indications of continents, clouds, and atmospheres; when satellites were perceived around those globes playing the same part to them as the moon plays to the earth then probabilities gave place to a clear certainty. Telescopes, then, are the instruments by means of which the constitution of the solar system has been definitively unveiled, and the earth has been assigned its part and its rank in the system of the planets. The discovery of the spots on the sun and of its rotation completed the conception of the solar system and prepared for the theory of its formation. Here is marked a well-determined phase in the history of human ideas respecting the universe, and it is characterized by the great name of Galileo.