Some of the ancient philosophers, including Democritus, formed just conceptions as to the real nature of this appearance. Though they lacked instruments wherewith to observe the stars forming it, they yet saw them with the eye of reason. But very vague and incorrect notions prevailed in early times, when superstition was rife, as to many celestial phenomena. Some of the ancient poets and learned men refer to the Galaxy as the path by which heroes ascended to heaven. Thus we read in Ovid:—
“A way there is in heaven’s extended plain, Which when the skies are clear is seen below, And mortals, by the name of Milky, know; The ground-work is of stars, through which the road Lies open to great Jupiter’s abode.”
_Scintillation of the Stars._—The rapid variations of light known as the “twinkling” of the stars received notice from many ancient observers, including Aristotle, Ptolemy, and others, and they severally endeavoured to account for it, but not in a manner altogether satisfactory. At low altitudes bright stars exhibit this twinkling or scintillation in a striking degree, but it is much less perceptible in stars placed at considerable elevations. Sirius, the brightest star in the sky, is a noted twinkler. His excessive lustre and invariably low position are conditions eminently favourable to induce this effect. But the planets seldom exhibit scintillation in a very marked degree. The light of Jupiter and Saturn is steady, even when these planets are close to the horizon. Mercury, however, twinkles most obviously, and Venus and Mars, when low down, are often similarly affected, especially in stormy weather when the air is much disturbed. Hooke, in 1667, concluded that the scintillation was due “to irregular refractions of the light of the stars by differently heated layers of atmosphere.” M. Arago said it arose “from the peculiar properties possessed by the constituent rays of light, of moving with different velocities through the strata of the atmosphere, and of producing what are called interferences.” More recently, M. Montigney has conducted some interesting researches into this subject, and he believes “that not only is twinkling caused, to a great extent, by the deviations of portions of a star’s light altogether away from us by variable layers of atmosphere, but it is also affected, both in frequency and in the colours displayed, by the nature of the light emitted by the individual star.” The planets are little subject to scintillation, as they present disks of sensible size, and thus are enabled to neutralize the effect of atmospheric interferences. It is curious, however, that the steadiness of telescopic images does not appear to be much improved at high altitudes, and that the phenomenon of scintillation still operates powerfully as observed from mountainous stations. In February 1888, Dr. Pernter, of the Vienna Academy of Sciences, found “that the scintillation of Sirius was actually greater at the top of Sonnblick, 10,000 feet high, than it was at the base of the mountain, and he formed the opinion that scintillation has its origin in the _upper_ strata of the atmosphere and not in the lower as usually assumed.” It would appear from this that lofty situations do not possess all the advantages claimed for them in regard to the employment of large telescopes.