METHODS IN MODERN PHYSICAL ASTRONOMY. 479 



Was it possible at once to go beyond this ? Was it possible to 

 question the stars in their turn, and inquire if, like the sun, they had 

 a sensible disk, spots, a rotation, and planets revolving around them ; 

 was it possible, in short, to extend to the stellar universe the notions 

 we had already acquired concerning the solar system ? The methods 

 in use did not yet permit this. 



The most delicate measurement of parallaxes has proved that the 

 star nearest to us is two hundred thousand times as far off as we are 

 from the sun. We should, then, need a telescope magnifying more 

 than two hundred thousand times to show us, under the most favor- 

 able circumstances, a star's diameter equal to that which the sun pre- 

 sents to the naked eye. Such a power is a hundred times greater than 

 the strongest powers that can be used. We are, therefore, obliged to 

 stay within the limits of our system, and proceed by analogy when we 

 desire to go out of it. The analogies, it is true, were very powerful 

 means even with Copernicus and Galileo, but with Kirchhoff and 

 Huggins they were destined to acquire very shortly an irresistible 

 force. Nature habitually reserves for the assiduous and sagacious 

 observer sui'prises that exceed his hopes. So, while the study of the 

 stars, considered as individual worlds, still remained beyond our reach, 

 a great observer discovered facts of a very general bearing. 



This leads us to a second phase of the period of telescopes a phase 

 that was characterized by the observations of the great Herschel. 

 Herschel changed the form of the instrument, and adopted one that 

 was more adaptable to the realization of the great powers he sought 

 to obtain. Then, by his magnificent studies of the nebulae, and by his 

 discovery of multiple stars revolving around one another, he laid the 

 foundations of the theory of worlds with multiple centers. This was 

 a new conception, which did not proceed from that of the solar sys- 

 tem, but was much more general. The problem was thus resolved in its 

 extreme terms ; but between these extremes yawned an immense gap. 



The gap has not yet been filled. We can not directly study those 

 worlds which each star forms with the planets of its suite, but a new 

 method of investigation has come forward to shed unexpected light 

 on the unanswered questions. 



The first period of physical astronomy was opened with the modest 

 telescope of Galileo, and closed with the large telescopes of Herschel. 

 As early as the beginning of this century, when the astronomer of 

 Slough had finished his review of the sky, it was felt that the tele- 

 scopic harvest was nearly gathered, and another instrument of prog- 

 ress was looked for. Arago thought he had found it in the discovery 

 of Malus, to which he made brilliant additions, and earnestly endeav- 

 ored to found a new branch of physical astronomy on polarization. 

 He was not successful. His discoveries ceased after a few beautiful 

 applications of his method. At this time the polariscope process is 

 only employed to determine whether light is reflected or emitted. 



