As we have seen, this new examination of the question of the habitability of other worlds by thinking beings presents a new interest, and the author has been able to escape the usual error of scientific writers, which consists in supposing that the first condition of habitability of another world is that it resembles the Earth. That is always the reasoning of the fish, which would affirm without an absolutely logical conviction irrefutable to himself that it is impossible to live outside water. But it seems to us that our conception of the universe can be even more vast and elevated than that of the learned German astronomer. As regards the planetary systems differing from ours, we are no longer obliged to fall back upon suppositions. We know already with certainty that our Sun is no exception, as some theoreticians maintained even quite recently. The discovery is of considerable interest. It is surely a rather exceptional situation that a sidereal system consisting of a central sun and one or more bodies gravitating round it should have its system just in our line of vision so that the revolution of the bodies which compose it should bring dark bodies between us and the star and produce a more or less complete eclipse. Since, on the other hand, such eclipses would be our only means of proving the existence of these unknown planets (except perturbations as in the case of Sirius and Procyon), it seems that it would have been absolutely daring to hope for such a circumstance to discover solar systems different from ours. Yet this exceptional case occurs in various parts of the sky. Thus, for example, the variable star Algol owes its variation of brightness, which reduces it from the second to the fourth magnitude at intervals of 69 hours, to the interposition of a body between it and the Earth, and celestial mechanics has already been able to determine with precision the orbit of this body, its dimensions and its mass, and even the ellipticity of the Algol sun. Thus we have here a system of which we know the sun and one enormous planet whose revolution takes place in 69 hours with a very high velocity as measured in the spectroscope, a planet still self-luminous, although less luminous than its sun, as the Earth was long ago, but a planet in the course of cooling and approximating to the state of Jupiter. The star Delta Cephei is in the same case: it is an eclipsing variable with a period of 129 hours, and its eclipsing planet also revolves in the plane containing our line of vision. The star U Ophinchi shows a similar system, and observation has revealed several others. If therefore chance has brought it about that a certain number of different solar systems should have been revealed to terrestrial observation by presenting side views, that is evidence of the existence of innumerable solar systems disseminated through the depths of space, and we are no longer reduced to mere conjectures.