478 THE POPULAR SCIENCE MONTHLY. 



permit the construction of the apparatus, at once extensive and deli- 

 cate, which it employs. 



Astronomy as a description of motions, on the contrary, only re- 

 quired 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 dis- 

 credit. It should be said that the beauty and importance of the dis- 

 coveries 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 pho- 

 tography. 



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 tele- 

 scope would never have permitted us otherwise than as a matter of 

 probability to consider the planets as like the earth in form, constitu- 

 tion, 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 defini- 

 tively 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-de- 

 termined phase in the history of human ideas respecting the universe, 

 and it is characterized by the great name of Galileo. 



