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 favorable circumstances, a star's diameter equal to that which the sun presents 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 surprises 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 nebulæ, 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 system, 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 telescopic harvest was nearly gathered, and another instrument of progress was looked for. Arago thought he had found it in the discovery of Malus, to which he made brilliant additions, and earnestly endeavored 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.