According to Arago, scintillation is due to the difference of velocity of the various colored rays traversing the unequally warm, unequally dense, unequally humid atmospheric strata. Thus, in tropical regions, where the atmospheric strata are more homogeneous, scintillation is rarely observed in stars the elevation of which above the horizon is more than 15°, or the sixth of the distance of the horizon from the zenith. “This circumstance,” says Humboldt, “gives to the celestial vault of these countries a particularly calm and soft character.” Another specific character of the stars is that their diameters are without appreciable dimensions. To the naked eye, this distinction would be insufficient, since, the moon and the sun excepted, the most considerable planets have not sensible diameters. But, while the magnifying power of optical instruments shows us the principal planets under the form of clearly defined disks, the most powerful glasses only show a star as a luminous point. The distance which separates us from these bodies is so great that there is nothing to astonish us in such a result. Wollaston affirms that the apparent diameter of the most brilliant star in the heavens, Sirius, is not more than the fiftieth part of a second of an arc. But let us hasten to say that this result still leaves a good margin as to the real dimensions of the star, since, at the distance of Sirius, an apparent diameter would represent a real diameter of 11,000,000 miles; that is, twelve times the diameter of our sun. Let us add, lastly, that the absence of appreciable dimensions does not suffice to distinguish absolutely the stars from the planets, since a certain number of the latter, as we have before seen, appear in telescopes only as simple luminous points. Let us come, then, to a permanent specific characteristic, the knowledge of which will always prevent us from confounding a star with one of the known or unknown bodies which form part of our solar group. This characteristic is as follows: The stars, properly so called, preserve among themselves—nearly enough for our present purpose—the same relative distances. They form, then, on the celestial vault apparent groups, the configuration of which is nearly invariable. Centuries must elapse to show a change of form, unless we employ extremely delicate measures. A planet, on the contrary, moves rapidly across these groups, to such a degree that, in the interval of a night, or at most of a few nights, this displacement is very perceptible; hence the old denomination of _fixed stars_, in opposition to the _wandering_ ones, or planets.