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ous colors. The presence of oxygen is necessary, according to Wiesner, in order that the rays of light may effect the decomposition of chlorophyll, and this author has made an interesting investigation into the action of different portions of the spectrum. He concludes that all the chemical changes caused in chlorophyll cells by the rays of light (namely, their development, decomposition, and assimilation of other substances) take place most rapidly in the brightest portion of the spectrum; and that, while all portions of the visible spectrum have the power of inducing these changes, the mechanical effects of light upon plants are to be especially ascribed to the rays of high refrangibility. – Poggendorff Annalen, CLII., 503.
EXPERIMENTS ON THE VELOCITY OF LIGHT. While astronomy has been busy with the problem of the sun's distance, physics has contributed an independent solution to this question. M. Cornu, of the French Academy of Sciences, has repeated the celebrated experiments on the velocity of light which were proposed by Foucault and Fizeau some twenty-five years ago. He has himself improved the method of Fizeau, and his experiments were exhaustively conducted and have been perfectly successful.
M. Cornu chose for his two stations the Observatory of Paris and the tower of Montlhéry, whose distance apart (about fifteen miles) is very precisely known. The beam of light passing from the observatory fell upon a toothed wheel revolving no less than 1600 times per second; a portion of the beam, escaping through the interval between two teeth, passed on to a reflector at Montlhéry, and returned thence to the revolving wheel. If the rotation of the wheel during the time required for the light to travel to and fro over twice the distance between the two stations interposes a tooth in the path of the returning ray, an extinction of the luminous impression occurs, and from the known velocity of rotation of the wheel, and from the known distance of the stations, the velocity of light can be had. By revolving the wheel at different rates this extinction can be made to occur at the fourth, fifth, sixth, etc., tooth. A great accordance characterizes Cornu's results, and high importance attaches to this delicate research. All the observations were made at
night by the aid of a Drummond light, except one which was made by sunlight. Exceptionally still weather was chosen for each experiment. From these observations there results a velocity for light in vacuo of 300,400 kilometers per second (186,700 English miles), with a probable error of less than Toto of the whole amount. The solar parallax is directly deducible from Cornu's velocity of light in two ways. Thus, combining it with Delambre's value for the equation of light, we find a solar parallax of 8.88", while Bradley's value of the aberration of light gives a parallax of 8.88", and Struve's value of the aberration constant gives 8.80". M. Cornu in his elegant memoir gives a summary of the values of the sun's parallax as deduced by various methods. The harmony of the results is marvelous when we consider with how minute a quantity we are dealing. The eight values that may be thus deduced range between 8.80" and 8.88"; and it is possible that the transit of Venus may not give a much better determination.
The measurement of the chemical intensity of the solar light has not yet become a subject of regular meteorological observation, because of the want of a proper instrument. This want is now partly supplied by a method proposed by Roscoe, who proposes to effect the measurement by the blackening of a paper saturated with chloride of silver; or, rather, by means of the time required in order that the exposure to the light may bring about a given intensity of shade. A uniformly prepared paper is placed in the apparatus during the previous night, and is, by a mechanical arrangement, hourly exposed during a given interval to the sunlight. In order to estimate correctly the intensity of the solar action, Roscoe arranges the apparatus so that the paper shall, cach hour, many times in quick succession, be exposed to the light for from two to thirty seconds. We thus have, at each hour of the day, a complete series of small spots of various tints, and have only to seek that tint which corresponds to the normal to know at once the number of seconds of exposure corresponding to the strength of the sunlight at that time. A series of observations made during the months of May,
June, and July have given very accordant results in the hands of Roscoe.-19 C, VII., 202.
NEW METHOD OF MEASURING THE VELOCITY OF LIGIIT.
A simple and possibly accurate method of measuring tlie velocity of light is suggested by Mr. Burgue. A disk, turning very rapidly about its axis, is at each turn to be illuminated by an intermittent and instantaneous light. A single dark radial line on the disk will seem at rest, like the disk itself. Now withdraw the source of light to a distance, and the time the light takes to reach and illuminate the disk will become greater, and the position of the radial line will appear displaced to a new position, forming with its previous position a certain small angle, which will measure the time of the light's passage over a given distance.—1 A, II., 262.
GREAT FRENCH LIGHT-HOTSE AT LA HAVE. The great French light-house at La Have, near Havre, is said to be the most magnificent establishment of its kind in the world. The electric light was first used at this place in 1863. The electricity is furnished by magneto-electric machines, and the simple uncondensed beam of light is equal to 4000 candles, and it seems to be the testimony of navigators that this electric light is always seen, even in clear weather, before the oil lamps nearer them. Its range of visibility is also correspondingly greater, the radius of the circle lighted up being from five to ten miles greater than that of first-class oil lamps. The difference between the two is, in fact, similar to the comparison of a candle and a gas-light. At some distance there is also a notable difference in their aspects, the electric light appearing white and brilliant, the other red and smoky. The superiority of the former is still more manifest during foggy weather, since at such times, cven before perceiving the electric light, its presence is marked by the illumination of the thick atmosphere surrounding it, and its range thus increased. This is an important advantage of the electric light, and may be of great practical utility, as bas, in reality, frequently been the case. While its brilliancy gives it this superiority, its inferior power of penetration diminishes the range of the electric
light in foggy weather, and more notably as the fog thick
In general, it is found that if the electric light has an intensity two and a half times greater than the oil-light, it will penetrate fog as well as the latter. The expense of the powerful electric light at La Have is about one seventh greater than that of the decidedly inferior oil-lights. General testimony seems to be in favor of the introduction of the magneto-electric light at all important stations, and both the English and French are extending its application. Elliots European Light-house System, p. 248.
THE ROMAN PHAROS IN DOVER CASTLE.
There is still standing within the walls of the castle at Dover, England, an old Roman pharos. The antiquity of this light-house, which has probably not been used as such since the Norman conquest, no doubt exceeds that of any light-house in Great Britain, it having been built, as is supposed, about A.D. 44. Upon it burned for many centuries great fires of wood or coal, the modern system of lamps and reflectors having superseded coal fires during the last century. This pharos, like the one at Boulogne, is built of bricks in color and shape like those found elsewhere in the Roman structures of Great Britain. They are of a light red color, about fourteen inches long, and not more than an inch and a half thick. The mortar joints are of nearly the same thickness. The preservation of this famous relic is doubtless due to the fact that some centuries ago the tower was turned into a belfry, and was surrounded by walls of stone. The latter are now nearly destroyed by time, and the old remaining work is again exposed.-Elliot's European Lighthouse System, p. 73.
THE POWER OF THE ELECTRIC LIGIIT.
The most powerful artificial light at present in existence is that employed for the great light-house at Souter Point, on the coast of England, near the mouth of the Tyne.
On both banks of this river there is an immense number of manufactories of all kinds, the smoke from which, under the influence of the west wind, seriously obstructs the approaches from the sea. Fogs at this part of the coast are also frequent; and the problem of light-house illumination
required that such light should be secured as would penetrate through any slight fog or haze, it having been generally acknowledged that not even the sunlight itself can penetrate an ordinary dense fog. The electric light established at Souter Point after condensation is equal in power to 800,000 standard candles, being eight times as powerful as the best American fixed lights. The electric spark passes between slender pencils of carbon, which are themselves consumed at the rate of about one inch per hour. The electric current is generated by two of Professor Holmes's patent rotary magneto-electric machines, driven by steam-engines of six horsepower. The number of revolutions made by each machine is 400 per minute, and 12,800 sparks pass per minute when both machines are at work. These sparks are, of course, formed so rapidly that the eye does not separate them, and the result is an intense beam of light, so dazzling that the eye of a person within the lantern can not rest upon them for an instant without intense pain. As observed from a distance of several miles, this light is so bright as to cast a well-defined shadow upon the deck of a vessel. --- Elliot's European Light-house System, p. 120.
ELECTRIC LIGHT FOR LOCOMOTIVES. A series of satisfactory experiments has lately been made in Russia in regard to lighting railway tracks from the locomotives by means of the electric light. The track on one occasion, with a battery of forty-eight cells, was brilliantly illuminated 492 yards ahead.-23 A, April 9, 1875, 467.
THE BLACK-BULB-IN-VACUUM THERMOMETER. As is well known, the black-bulb-vacuum thermometers employed for observing the solar radiation give very discordant results, even in the hands of the best observers, and the origin of this has recently been studied by Mr. Hicks, of London, who states that in his opinion the discordances are in a great measure due to the imperfect vacuum that exists within the inclosing bulb. Having made a large number of thermometers with special care, in which the vacuum has been reduced to the lowest attainable limit, Mr. Hicks finds that it is possible with proper care to always construct instruments that shall be perfectly comparable with each