made out. At Guajara it was an easy object twenty-five degrees from the zenith; and stars of the fourteenth magnitude were discernible. Now, according to the usual estimate, a step downward from one magnitude to another means a decrease of luster in the proportion of two to five. A star of the fourteenth order of brightness sends us accordingly only one thirty-ninth as much light as an average one of the tenth order. So that, in Professor Smyth's judgment, the grasp of his instrument was virtually multiplied thirty-nine times by getting rid of the lowest quarter of the atmosphere.[1] In other words (since light falls off in intensity as the square of the distance of its source increases), the range of vision was more than sextupled, further depths of space being penetrated to an extent probably to be measured by thousands of billions of miles!
This vast augmentation of telescopic compass was due as much to the increased tranquillity as to the increased transparency of the air. The stars hardly seemed to twinkle at all. Their rays, instead of being broken and scattered by continual changes of refractive power in the atmospheric layers through which their path lay, traveled with relatively little disturbance, and thus produced a far more vivid and concentrated impression upon the eye. Their images in the telescope, with a magnifying power of one hundred and fifty, showed no longer the "amorphous figures" seen at Edinburgh, but such minute, sharply-defined disks as gladden the eyes of an astronomer, and seem, in Professor Smyth's phrase, to "provoke" (as the "cocked-hat" appearance surely baffles) "the application of a wire-micrometer" for purposes of measurement.[2]
The luster of the milky way and zodiacal light at this elevated station was indescribable, and Jupiter shone with extraordinary splendor. Nevertheless, not even the most fugitive glimpse of any of his satellites was to be had without optical aid.[3] This was possibly attributable to the prevalent "dust-haze," which must have caused a diffusion of light in the neighborhood of the planet more than sufficient to blot from sight such faint objects. The same cause completely neutralized the darkening of the sky usually attendant upon ascents into the more ethereal regions, and surrounded the sun with an intense glare of reflected light. For reasons presently to be explained, this circumstance alone would render the Peak of Teneriffe wholly unfit to be the site of a modern observatory.
- ↑ The height of the mercury at Guajara is 21·7 to 22 inches.
- ↑ "Philosophical Transactions," vol. cxlviii, p. 477.
- ↑ We are told that three American observers in the Rocky Mountains, belonging to the Eclipse Expedition of 1878, easily saw Jupiter's satellites night after night with the naked eye. That their discernment is possible even under comparatively disadvantageous circumstances is rendered certain by the well-authenticated instance (related by Humboldt, "Cosmos," vol. iii, p. 66, Otte's translation) of a tailor named Schön, who died at Breslau in 1837. This man habitually perceived the first and third, but never could see the second or fourth, Jovian moons.
