NATURAL PHILOSOPHY. 145 



dark, is quickened by the actinic force, and retarded and often stopped 

 by the luminous power. 2. Lignijlcation. The decomposition of 

 carbonic acid by the plant is due to some excitement of luminous 

 power, and is slopped by the actinic force. 3. Formation of Chloro- 

 phyll. Due entirely to the luminous rays. 4. Flowering and Fruit- 

 ing. Dependent upon the action of the thermic or parathermic rays 

 of the spectrum, as distinguished from both the luminous and actinic 

 forces. 5. Motion of Plants. Bending to the blue light, and reced- 

 ing from the red, proving the excitement of actinic force. London 

 Athenaum, April. 



THE VELOCITY OF LIGHT PROVED BY ACTUAL EXPERIMENT. 



IT is well known that the proof of the enormous velocity of light, 

 amounting to 192,000 miles per second, has hitherto been derived only 

 from the observations and calculations of astronomers and geometri- 

 cians, and that this velocity has never been demonstrated by any ex- 

 periment. In 1675 Roemer first announced the extraordinary velocity 

 of light, which he had derived from observations on the satellites of 

 Jupiter, and in 1728 Bradley was led to the same result by studying 

 the phenomena known as " the aberration of light." Since the same 

 result was thus arrived at in two totally different ways, there could be 

 no doubt of the fact ; but still scientific men have long desired to ren- 

 der it more evident by actual experiment. This has at last been ac- 

 complished T>y a French savcm, M. Hippolyte Fizeau, from whose 

 communication to the French Academy, on July 23d, we make the fol- 

 lowing extracts. "I have succeeded in demonstrating the velocity of 

 light by a method which seems to me to furnish a new means of study- 

 ing with precision this important phenomenon ; this method is founded 

 on these principles. When a disk turns in its plane with great rapidity 

 around its centre of figure, it is possible to estimate the time occupied 

 by a point in the circumference in describing a very small angu- 

 lar space, a thousandth of the circumference for example. If the 

 rapidity of rotation is great enough, this time is very short, being for 

 ten or a hundred revolutions per second only one ten-thousandth 

 or one hundred-thousandth of a second. If the circumference of 

 the disk is divided, like a toothed wheel, into equal intervals, alter- 

 nately open and closed, the time occupied by the passage of each of 

 these intervals through the same point of space will be the same small 

 fractions. During so short periods the light passes over quite limited 

 spaces, being 31 kilometres (19.5 miles) for the first fraction, and 3 

 kilometres (2 miles) for the second. If a ray of light which has 

 passed through one of the divisions of the wheel is reflected from a 

 mirror placed at a certain distance, and returns to the same point, the 

 time occupied in the propagation of this ray must necessarily inter- 

 vene, and the ray at its return will pass through an open space in the 

 wheel, or will be stopped by a closed one, according to the rapidity of 

 the motion of the wheel and the distance from which the light is re- 

 flected. 



* A system of two telescopes directed towards each other, so that the 



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