M.-P— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 67 



eye 1 two of Jupiter's satellites, when close together, as one; 

 and at one mountain station (3,200 feet elevation) all the 

 members of our party witnessed a similar phenomenon. 

 Very curiously, that evening was slightly hazy, and very 

 faint stars were not visible ; but the atmosphere was remark- 

 ably steady. Under atmospheric condition of supreme 

 quietness we have placed eleven stars in the Pleiades. 



The visibility of Jupiter's satellites by the naked eye has 

 generally been received with much doubt. Humboldt, in 

 his Cosmos, has related a well authenticated case, but we 

 can come to our own observers for confirmation. At the 

 time of the total solar eclipse of August, 1878, one of the 

 United States Naval Observatory observers in Colorado saw 

 with the naked eye three satellites of Jupiter separately — 

 two very distinctly and one less so. The atmosphere was 

 supremely serene, and the station (Idaho Springs) 7,548 

 feet above the sea. Two other persons, not observers, also 

 witnessed the phenomenon. In 1874 * ne Professor and his 

 colleagues had doubted my seeing even two satellites close 

 together as one. 



Conditions of Steadiness and Unsteadiness of 

 the Atmosphere. 



•Under similar atmospheric conditions the limbs of the 

 Sun, Moon and planets are sharply defined and remarkably 

 steady; the Sun-spots are beautifully distinct and their 

 changes of form readily followed; the irregularities of the 

 outline of the Moon are unmistakable; the spurious discs of 

 the stars are encircled by delicate diffraction rings, and 

 the overlapping spurious discs of very close double stars 

 present new forms. Under conditions of supreme steadi- 

 ness of the atmosphere, we believe that an observer with 

 keen eyesight should see, with unassisted vision, the larger 

 satellites of Jupiter, when favorably located, and from ten 

 to twelve stars in the Pleiades. Either is a severe and 

 unique test of steadiness and eyesight. In these periods of 

 great steadiness the observer aches for finer micrometers, 

 higher power, and more stable instruments. 



1 Except with spectacles for short-sightedness. 



