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. _Star-Disks._—The stars as observed in telescopes are shorn of the false rays apparent to the naked eye, and they are seen with small spurious disks. That the disks are spurious is evident from the fact that the larger the telescope employed, the smaller the star-disks become. And moreover, when a star is occulted by the Moon, it disappears instantaneously. There is no gradual diminution of lustre; the star vanishes with great suddenness. Bright stars, like Aldebaran or Regulus, have been watched up to the Moon’s limb, and observers have been sometimes startled at the abruptness with which they were blotted out. An appreciable disk could not be withdrawn in this instantaneous manner; it would exhibit a perceptible decadence as the Moon increasingly overlapped it, but no such appearance is observed. On the occasion of the occultation of Jupiter on Aug. 7, 1889, the planet’s diameter was 41″·4, and the disappearance occupied 85 seconds. Now had Aldebaran or Regulus a real disk of only 1″ it would prevent their sudden extinctions, and their disappearances would be spread over perceptible though short intervals of time[50]. But there is every reason to conclude that the actual disks are to be represented by a small fraction of 1″, so that the largest instrument and the highest powers fail to reveal it. In this connection, Mr. Gore, in his ‘Scenery of the Heavens,’ p. 152, says:—“Let us take the case of α Centauri, which is, as far as is known at present, the nearest fixed star to the Earth. The distance of this star is about 25 billions of miles. From comparisons made between its light and the Moon, it has been found that its intrinsic brilliancy must be about four times that of the Sun. Supposing its greater lustre is due to its greater size—a not improbable supposition—it would subtend, if placed at the Sun’s distance, an angle twice as great, or about 1°, and hence we find that the angle subtended at its distance of 25 billions of miles would be about 1/76th of a second of arc, which the most powerful telescope yet constructed would be incapable of showing as a visible disk.”