But these adherents of inherent imperfectibility instance the fact of vanished and variable stars, as well as those that have suddenly appeared, and, after brief periods of intense brilliancy, as suddenly disappeared, to show that there are mighty disturbances in the sidereal heavens which entirely negative the idea of "conservation" as a geognostic law. But the phenomena of variable stars, with all their apparent irregularity of motion and fluctuations in luminosity, are now being traced to definite and well-determined laws of motion, if not of light, while the theory of extinguished and disappearing stars belongs exclusive to the age of Tycho Brahe. Where there is one self-luminious body (or sun) in the interstellary spaces, there are probably not less than forty non-luminous or dark cosmical bodies revolving about their respective centres of light and heat, as the attending planets revolve about the common centre of gravity in our own system. And this is especially true of that vast and fathomless star-stratum, called the Milky-way, in which most of these peculiar phenomena occur, with the exception of the variable stars only. That stars should vary in their intensity of light by the probable transits of these dark cosmical bodies across their discs, is no matter of wonder or astonishment: on the contrary, it is surprising that these sidereal phenomena do not occur with much greater frequency. This would inevitably be the case if the planes of revolution, in the case of these non-luminous bodies about their central orbs, were coincident with the lines of vision from our own planet--a circumstance by no means improbable from the vastness of the sidereal heavens and the innumerable hosts of stars marching therein. Besides, these periodical variations may be accounted for in part--especially in the case of double stars--from their apparent rather than real change of place in the heavens. For if our sun-system is travelling towards a point in the constellation Hercules at the rate of 194 thousand miles an hour (the rapidity of Arcturus' flight), it is impossible to determine, in the present state of astronomical knowledge, whether the apparent change of place in any star is real or merely optical. But, in the case of double stars, each is travelling (independently of its other motions) about the common centre of gravity obtaining in its own system, and these relative movements may account for the greater or less intensity of light as the two stars, viewed as one, present a greater or less area of luminosity in their united surfaces.