VISIBILITY OF OBJECTS. 55 



lens, white lines on a black ground were seen at an angle 

 of \"-2; a spider's thread at 0"-6 ; and a fine glistening 

 wire at scarcely 0"2. This problem does not admit gen- 

 erally of a numerical solution, since it entirely depends on 

 the form of the objects, their illumination, their contrast with 

 the back-ground, and on the motion or rest, and the nature 

 of the atmospheric strata in which the observer is placed. 



During my visit at a charming country-seat belonging to 

 the Marquess de Selvalegre at Chillo, not far from Quito, 

 where the long-extended crests of the volcano of Pichincha 

 lay stretched before me at a horizontal distance, trigonomet- 

 rically determined at more than 90,000 feet, I was much 

 struck by the circumstance that the Indians who were stand- 

 ing near me distinguished the figure of my traveling com- 

 panion Bonpland (who was engaged in an expedition to the 

 volcano) as a white point moving on the black basaltic sides 

 of the rock, sooner than we could discover him with our tel- 

 escopes. The white moving image was soon detected with 

 the naked eye both by myself and by my friend the unfor- 

 tunate son of the marquess, Carlos Montufar, who subsequent- 

 ly 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, distinguish- 

 ed at a greater distance than black objects on a white ground. 

 The light was transmitted in serene weather through rar- 

 efied strata of air at an elevation 15,360 feet above the 

 level of the sea t> our station at Chillo, which was itself sit- 

 uated at an elevation of 8575 feet. The ascending distance 

 was 91,225 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 *4, 

 while at the lower station they were found, by accurate ob- 

 servation, to be 222 inches and 65 0, 7. Gauss's heliotrope 

 light, which has become so important an element in German 

 trigonometrical measurements, has been seen with the naked 

 eye reflected from the Brocken on Hohenhagen, at a distance 

 of about 227,000 feet, or more than 42 miles, being fre- 

 quently visible at points in which the apparent breadth of a 

 three-inch mirror was only 0" - 43. 



