SCINTILLATION, a twinkling of the stars; a familiar phenomenon to all who have directed their attention to the firma ment above us. Under ordinary atmos pheric conditions this flickering is pos sessed only by the so-called fixed stars. A planet shines steadily and by this mark can readily be picked out. When near the horizon, however, planets have been observed to scintillate slightly; while stars at low altitudes invariably twinkle more vigorously than stars overhead. This at once points to the atmosphere as an important factor, since the phenom enon is more pronounced when the light has to traverse a greater depth of air. Again, when viewed through sufficiently large telescopes stars cease twinkling al together. The action of the telescope is to concentrate on the eye a much larger pencil of rays than could naturally enter it. Instead of one slender ray the eye receives the integral effect of a great number of rays, whose individual features are lost in the general average. In the case of a planet, again, the rays which fall on the retina converge from all parts of a disk of sensible size; and in the in tegral effect of this pencil the individual features of the component rays are lost.
But a star is so far distant as to be vir tually a point of light. In this case we have an excessively slender ray infinitely narrow compared even to the small pen cil of light that comes to us from a planet. The vicissitudes of refraction which a star ray experiences in passing through the infinitely irregular varia tions of density, temperature, and humid ity in our atmosphere characterize its integral effect on our retina, and the re sult is twinkling. It is possible indeed by separating the images of a star pro duced in the two eyes to observe two dif ferent scintillations at one and the same time. Scintillation may thus be said to depend on three factors: (1) The vast distance even of the nearest stars re ducing the largest of them to mere points of light. (2) The ever-changing varia bleness in condition of the atmosphere through which the light must come to us. (3) The smallness of aperture of our eye, which receives an almost ideal single ray of light.