PORTION OF THE COSMOS. VISUAL POWER. 53 



tion of the rays proceeding from the terrestrial object, and 

 arriving at the unassisted eye at different distances, through 

 strata of air more or less dense, and more or less charged 

 with aqueous vapour ; by the intensity of the diffused light 

 radiated from the particles of air ; and by many still im- 

 perfectly explained meteorological circumstances. According 

 to old experiments of the accurate Bouguer, a difference of 

 1-6 Oth in the intensity of the light is necessary for visibility. 

 We see, as he expresses it, only in a negative manner, hill 

 and mountain summits which radiate but little light, and 

 detach themselves as dark masses against the sky. We see 

 them only by the difference of the thickness of the atmos- 

 pheric strata, which extend in the one instance to the 

 object, and in the other to the extremest horizon. On the 

 other hand, bright or shining objects, as snowy mountains, 

 white limestone rocks, and cones covered with pumice, are 

 seen in a positive manner. The distance at which high 

 mountain summits can be recognised at sea is not without 

 interest in navigation, when exact astronomical determinations 

 of the ship's place are wanting. I have treated this subject 

 in detail in another place, ( 108 ) when discussing the distance 

 at which the Peak of Teneriffe may be visible. 



The power of seeing stars with the naked eye in the day- 

 time from the shafts of mines, or on \ry lofty mountains, 

 has been a subject of examination with me from early youth. 

 I was aware ( 109 ) that Aristotle had affirmed that stars could 

 sometimes be seen from vaults and reservoirs, as well as through 

 tubes. Pliny, also, mentions this, and notices at the same 

 time the circumstance that stars can be clearly distinguished 

 in the day-time during solar eclipses. In the course of my 

 professional engagements as a mining engineer, I for some 



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