MOUNTAIN OBSERVATORIES. 393 



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 mag- 

 nitude to another means a decrease of luster in the proportion of two 

 to five. A star of the fourteenth order of brightness sends us accord- 

 ingly 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.* In other words (since light 

 falls off in intensity as the square of the distance of its source in- 

 creases), 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 be- 

 ing 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 Pro- 

 fessor Smyth's phrase, to "provoke" (as the "cocked-hat" appearance 

 surely baffles) " the application of a wire-micrometer " for purposes of 

 measurement.! 



The luster of the milky way and zodiacal light at this elevated sta- 

 tion was indescribable, and Jupiter shone with extraordinary splendor. 

 Nevertheless, not even the most fugitive glimpse of any of his satel- 

 lites was to be had without optical aid. J 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 re- 

 flected 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-Y to 22 inches. 



f " Philosophical Transactions," vol. cxlviii, p. 4'7'7. 



X 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 Schon, who died at Breslau 

 in 1837. This man habitually perceived the first and third, but never could see the sec- 

 ond or fourth, Jovian moons. 



