VISIBILITY OF OBJECTS. 71 



copes. The white moving image was soon detected with 

 the naked eye both by myself and by my friend the unfortu^ 

 nate son of the Marques, Carlos Montufar, who subsequently 

 perished in the civil war. Bonpland was enveloped in a white 

 cotton mantle, the Poncho of the country; assuming the 

 breadth across the shoulders to vary from three to five feet, 

 according as the mantle clung to the figure or fluttered in the 

 breeze, and judging from the known distance, we found that 

 the angle at which the moving object could be distinctly seen, 

 varied from 7" to 12". White objects on a black ground are, 

 according to Hueck's repeated experiments, distinguished at a 

 greater distance than black objects on a white ground. The 

 light was transmitted in serene weather through rarefied strata 

 of air at an elevation 15360 feet above the level of the sea to 

 our station at Chillo, which was itself situated at an elevation 

 of 8575 feet. The ascending distance was 91225 feet, or 

 about 17-^ miles. The barometer and thermometer stood at 

 very different heights at both stations, being probably at the 

 upper one about 17'2 inches and 46 0- 4, while at the lower 

 station they were found, by accurate observation, to be 22*2 

 inches and 65- 7. Gauss's heliotrope light, which has become 

 so important an element in German trigonometrical measure- 

 ments, has been seen with the naked eye reflected from the 

 Brocken on Hohenhagen, at a distance of about 227000 feet, 

 or more than 42 miles ; being frequently visible at points 

 in which the apparent breadth of a three-inch mirror was 

 only 0"-43. 



The visibility of distant objects is modified by the absorp- 

 tion of the rays passing from the terrestrial object to the 

 naked eye at unequal distances, and through strata of air 

 more or less rarefied and more or less saturated with moisture ; 

 by the degree of intensity of the light diffused by the radiation 

 of the particles of air ; and by numerous meteorological pro- 

 cesses not yet fully explained. It appears from the old ex- 



