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.
The result given in the Annuaire for 1842 for the velocity of light in a second is 77,000 leagues, which corresponds to 215,834 miles; while that obtained at the Pulkowa Observatory is 189,746 miles. William Richardson gives as the result of the passage of light from the sun to the earth 8´ 19″·28, from which we obtain a velocity of 215,392 miles in a second.—Memoirs of the Astronomical Society, vol. iv.
In other words, light travels a distance equal to eight times the circumference of the earth between two beats of a clock. This is a prodigious velocity; but the measure of it is very certain.—Professor Airy.
The navigator who has measured the earth’s circuit by his hourly progress, or the astronomer who has paced a degree of the meridian, can alone form a clear idea of velocity, when we tell him that light moves through a space equal to the circumference of the earth in the eighth part of a second—in the twinkling of an eye.
Could an observer, placed in the centre of the earth, see this moving light, as it describes the earth’s circumference, it would appear a luminous ring; that is, the impression of the light at the commencement of its journey would continue on the retina till the light had completed its circuit. Nay, since the impression of light continues longer than the fourth part of a second, two luminous rings would be seen, provided the light made two rounds of the earth, and in paths not coincident.
APPARATUS FOR THE MEASUREMENT OF LIGHT.
Humboldt enumerates the following different methods adopted for the Measurement of Light: a comparison of the shadows of artificial lights, differing in numbers and distance; diaphragms; plane-glasses of different thickness and colour; artificial stars formed by reflection on glass spheres; the juxtaposition of two seven-feet telescopes, separated by a distance which the observer could pass in about a second; reflecting instruments in which two stars can be simultaneously seen and compared, when the telescope has been so adjusted that the star gives two images of like intensity; an apparatus having (in front of the object-glass) a mirror and diaphragms, whose rotation is measured on a ring; telescopes with divided object-glasses, on either half of which the stellar light is received through a prism; astrometers, in which a prism reflects the image of the moon or Jupiter, and concentrates it through a lens at different distances into a star more or less bright.—Cosmos, vol. iii.