Humboldt, when at Chillo, near Quito, where the crests of the volcano of Pichincha lay at a horizontal distance of 90,000 feet, was much struck by the circumstance that the Indians standing near distinguished the figure of Bonpland (then on an expedition to the volcano), as a white point moving on the black basaltic sides of the rock, sooner than Humboldt could discover him with a telescope. Bonpland was enveloped in a white cotton poncho: 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, 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, distinguished at a greater distance than black objects on a white ground. Gauss’s heliotrope light has been seen with the naked eye reflected from the Brocken on Hobenhagen at a distance of about 227,000 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 SMALLEST BRIGHT BODIES. Ehrenberg has found from experiments on the dust of diamonds, that a diamond superficies of 1/100th of a line in diameter presents a much more vivid light to the naked eye than one of quicksilver of the same diameter. On pressing small globules of quicksilver on a glass micrometer, he easily obtained smaller globules of the 1/100th to the 1/2000th of a line in diameter. In the sunshine he could only discern the reflection of light, and the existence of such globules as were 1/300th of a line in diameter, with the naked eye. Smaller ones did not affect his eye; but he remarked that the actual bright part of the globule did not amount to more than 1/900th of a line in diameter. Spider threads of 1/2000th in diameter were still discernible from their lustre. Ehrenberg concludes that there are in organic bodies magnitudes capable of direct proof which are in diameter 1/100000 of a line; and others, that can be indirectly proved, which may be less than a six-millionth part of a Parisian line in diameter. VELOCITY OF LIGHT. It is scarcely possible so to strain the imagination as to conceive the Velocity with which Light travels. “What mere assertion will make any man believe,” asks Sir John Herschel, “that in one second of time, in one beat of the pendulum of a clock, a ray of light travels over 192,000 miles; and would therefore perform the tour of the world in about the same time that it requires to wink with our eyelids, and in much less time than a swift runner occupies in taking a single stride?” Were a cannon-ball shot directly towards the sun, and were it to maintain its full speed, it would be twenty years in reaching it; and yet light travels through this space in seven or eight minutes.