Influence of the Solar Rays on the Growth of Plants.— It has been already stated that the solar beam consists of three different principles, viz., light, heat, and actinism. Seeds will not germinate under the influence of light deprived of that principle upon which chemical change depends. Mr. Robert Hunt made some experiments with common cress and turnip seed placed upon moist earth, and slightly covered with sand. One-half of the seed-bed was screened from the rays by a blackened board, and the other freely exposed. Under the shaded half the germination was between two and three days in advance of the exposed portion.. This experiment was repeated, using a glass trough, containing a weak solution of bichromate of potash half an inch in thickness, over the illuminated portion. This solution admitted the permeation of 87 parts of the luminous, or light rays, 92 of the calorific, or heat rays, and 27 of the chemical, or actinic rays; the object being to ascertain if any greater retardation was produced by the luminous rays, from which the chemical principle was to a considerable extent removed, than by the pure solar beam, which he regarded as a compound of 100 parts of each-light, heat, and actinism. The result was that the seed under the bichromate of potash solution, or yellow medium, did not germinate until five days after the seeds in the dark part of the bed.

Germination of Seeds entirely prevented. Upon substituting a solution of sulphate of chromium and potash, which admitted the permeation of 85 parts of light, 92 parts of heat, and only 7 of actinism, the germination was entirely prevented in four experiments; and in the fifth, ten days after the seeds in the dark had germinated, half-a-dozen seeds of cress showed symptoms of germination, These experiments

were many times repeated, and always with similar results. We have thus satisfactory evidence that the solar beam, deprived of the principle or power of chemical action, arrests the development of the plant by preventing the vitality of the germ from manifesting itself.

The Origin of Vitality in Seeds.—In another series of experiments on common cress, mignonette, ten-week stocks, and minor convolvulus, when from 93 to 95 parts of actinism, 48 parts of heat, and only 25 parts of light passed through, it was found in every instance that the seeds influenced by the chemical or actinic rays germinated in one-half the time which the seeds placed in the dark required. It is evident, therefore, that the spring of vitality in seeds is due to some power or principle of solar origin, distinct from the light-and-heat principle.

How the Vital Power is exerted.-The manner in which this power is exerted in seed beneath the surface of the soil is not clear at present; we know not if it is a mere disturbance of something already diffused through matter, or an emanation from the sun; all we are enabled to declare is, that the germination of seeds is more rapid under the influence of the actinic rays, separated from the luminous ones, than it is under the influence of the combined solar radiations, or in the dark.

The Development of Roots from Plant Cuttings.—In the practice of planting cuttings, the use of blue glass screens is highly advantageous, as the light and heat rays are partly absorbed, whilst the actinic rays pass through and accelerate the development of roots.

Light Rays essential to the Formation of Woody Fibre. -Dr. Daubeny in England, and Dr. Gardner in

America, made numerous experiments, which show that the decomposition of carbonic acid increases with the increase of light rays, that it is more rapid under the influence of the yellow ray than any other, and that the largest quantity of woody matter is found in those plants which have had the largest amount of light. The author of this treatise, who travelled extensively over the United States and Canada, where the settlers, in clearing the primeval forests for cultivation, generally cut or hew the trees at about three feet above the surface of the ground, found, in thousands of the stumps he examined, that the radius of the stump from the core southward was invariably longer than the radius from the core northward, and, further, that the thickness of each ring of the stump was greater towards the south than towards the north. This clearly proves that the direct influence of the solar beam, or the directly combined influence of light, heat, and actinism, produces more woody fibre than is produced by diffused light.

The Value of Seeds speedily determined by Blue Glass. -The commercial value of seeds depends upon the extent that the vital principle is active in them. For instance, if one hundred seeds of any sort be sown, and the whole germinate, the seed will be of the highest current value; but if ninety only germinate, its value is about ten per cent. less; if eighty, then its value falls about twenty per cent. Messrs. Lawson, of Edinburgh, extensive seed merchants, found that seeds planted in a case, the sides and cover of which were formed of blue glass, germinated in from two to five days, whereas the same kind of seeds sown in a hotbed usually took from eight to fourteen days to germinate.

Scorching prevented at the Royal Gardens at Kew.

The late Sir Wm. Hooker found that the solar beams after their passage through white glass at the conservatory at Kew scorched the foliage of the plants. This scorching was prevented by the substitution of a peagreen glass stained with oxide of copper for the white glass. The pea-green glass admits the passage of sufficient rays to promote healthy vegetation, but obstructs the passage of the rays which produce the scorching.

The Knowledge of Optics useful to the Engineer, Architect, &c.-By the laws of optics combined with a knowledge of practical geometry, the engineer and architect are enabled to draw a representation of a building, or any other object, in perspective, or as it would appear to the eye in any given position. The projection of shades and shadows on geometrical elevations and sections of buildings, or other objects, is also dependent on the same principles.

The Laws of Light applied to the laying of Submarine Telegraphic Cables.-When the Atlantic cable was first laid, the currents of electricity became so feeble as to render the motion of the electric needle imperceptible to the eye, the operators not knowing whether any current was really passing along the cable or not. To test this a very small lamp of the lightest possible material was constructed with a narrow slit in it to

permit the light to pass through. This lamp was placed on the electric needle with the slit facing a screen placed several feet from it. The light from the lamp passing through the slit was projected upon the screen as a bright oblong figure; the smallest motion of the needle being imparted to the lamp, caused the oblong bright spot on the screen to move over a large space, which readily indicated whether feeble

currents were passing along the cable, or had entirely ceased.

The Illumination of Lighthouses.—Of the many useful applications of the laws of light there are few that surpass in importance their application to the illumination of lighthouses for the guidance and protection of mariners.

Up to the year 1780 the means of illuminating lighthouses throughout the world generally consisted of wood or coal fires or tallow candles. At the period mentioned, Argand lamps and paraboloidal reflectors were first used in the lighthouse at Corduan, under the directions of the distinguished French philosopher, Borda. This method was found to have been a decided improvement upon the fires and tallow candles. The Trinity House of London sent a deputation to France to inquire into the system carried out under Borda, who reported favourably of it. In 1787 the old castle at Kinnaird-head, Scotland, was lighted by means of paraboloidal reflectors of silvered copper and Argand lamps; and in 1807, the tallow candles used at that time in the Eddystone Lighthouse, one of the most dangerous points on the coast of Britain, were obliged to hide their diminished heads by the far more brilliant and effective light of the Argand burners and paraboloidal reflectors. Soon afterwards the system became general throughout the United Kingdom.

The Fresnelian System of illuminating Lighthouses.Many unsuccessful attempts had been made between the middle of the eighteenth century and the early part of the nineteenth to illuminate lighthouses by means of lenses. It is to the genius of M. Augustin Fresnel the world is indebted for the excellent system which he invented in 1819, and successfully carried out in practice three or four years afterwards.