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vision is regarded as resulting from the waves of ether striking the optic nerve, just as the phenomena of sound result from waves of air impinging on the auditory nerve.
When a ray of light falls on the surface of a body, it is said to be incident on that surface, and is called the incident ray, the point where it falls being termed the point of incidence. If the body be opaque, and have its surface polished, the ray is thrown back, or reflected, and after leaving the point of incidence, is termed the reflected ray. If the body be transparent the ray
enters it; but on doing so it is bent out of its course, or refracted, and is then called the refracted ray. On emerging from the opposite surface it is, if the surfaces be parallel, again bent at an equal angle, and its subsequent path is a straight line parallel to its original direction. It frequently happens that only a portion of the incident light is reflected, while another part is transmitted and refracted. If a line be drawn perpendicular to the surface from the point of incidence, it will make an angle with the incident ray, if that ray falls obliquely on the surface, and another angle on the opposite side with the reflected ray: the former is called the angle of incidence, and the latter the angle of reflexion. If the same perpendicular be produced within the body, it will form an angle with the refracted ray, termed the angle of refraction. The incident ray, the reflected ray, and the refracted ray, all lie in the same plane; and the angle of incidence is equal to the angle of reflexion. If one person stands before a mirror a little to the right side, and another occupies a similar position on the left, each will see the image of the other, but not of himself, which experimentally confirms the latter part of the above law.
The intensity of light falling on any surface is inversely as the square of the distance of that surface from the luminous source. Hence, a candle at one foot distance will give 4 times as much light as at 2 feet, and 9 times as much as when 3 feet distant; or, what is the same thing, one candle at one foot distance will give a light equal to 4 at 2 feet, or to 9 at 3 feet distance.
Some bodies neither reflect nor transmit light, but absorb it ; all such bodies are black, and black velvet is the most perfect absorbent known. Black is not a colour of itself, but the absence of all colour.
White light, as that of the sun, is compounded of three primary colours, blue, yellow, and red. If a ray of light entering a dark room through a small hole in the window. shutter be allowed to fall on a triangular glass prism, it will be decomposed into its elementary colours. A white sheet suspended at some distance behind the prism will exhibit the colours with great brilliancy. Besides the primary colours, blue, yellow, and red, part of the space will be covered with the secondary, or complementary colours, violet, green, and orange. This image is called the solar spectrum, the violet being at the upper extremity of the spectrum, and the red at the lower.
Light travels with a velocity of 190,000 miles in a second, and occupies about 8 minutes in passing from the sun to the earth. Of this velocity we can form no adequate conception, for a cannon ball, in traversing the same space, would occupy nearly 8 years. Notwithstanding this prodigious velocity, the rays of light fall harmless on the delicate organism of the eye, and from this we may infer the extreme rarity of the substance producing the sensation of vision; for had it the density of common air, it would strike the body of a moderate-sized man with a force of nearly two hundred thousand tons.
The law of the intensity of light is the same as that of radiant heat, and the rays of light are reflected, refracted, and transmitted in a similar manner to those of heat. From these, and other points of similarity, there is a general tendency among scientific men to refer the phenomena of light and heat to modified operations of the same cause.
LESSON XLVIII.- WEDNESDAY.
HYMN BEFORE SUNRISE IN THE VALE OF CHAMOUNI.
Hast thou a charm to stay the morning star
Rave ceaselessly; but thou, most awful form,
Didst vanish from my thought; entranced in prayer
Yet, like some sweet beguiling melody,
Into the mighty vision passing—there,
Awake, my soul! not only passive praise
Voice of sweet song! Awake, iny heart, awake! Green vales and icy cliffs, all join my hymn.
Thou, first and chief, sole sovran of the vale !
all night by troops of stars,
Who filled thy countenance with rosy light ?
And you, ye five wild torrents fiercely glad ! Who called you
forth from night and utter death, From dark and icy caverns called you
forth, Down those precipitous, black, jagged rocks,
For ever shattered and the same for ever?
And who commanded-and the silence came“Here let the billows stiffen and have rest ?”
Ye ice-falls ! ye that from the mountain's brow Adown enormous ravines slope amainTorrents, methinks, that heard a mighty voice, And stopped at once amid their maddest plunge! Motionless torrents! silent cataracts ! Who made you glorious as the gates of heav'n, Beneath the keen full moon ? Who bade the sun Clothe you with rainbows? Who, with living flowers Of loveliest blue, spread garments at your feet ?God !-let the torrents, like a shout of nations, Answer; and let the ice-plains echo, God! God! sing, ye meadow streams, with gladsome voice ; Ye pine groves, with your soft and soul-like sounds! And they, too, have a voice, yon piles of snow, And in their perilous fall shall thunder.-God! Ye living flowers that skirt the eternal frost! Ye wild goats sporting round the eagle's nest! Ye eagles, playmates of the mountain storm! Ye lightnings, the dread arrows of the clouds !
Ye signs and wonders of the elements ! Utter forth God, and fill the hills with praise !
Thou, too, hoar mount! with thy sky-pointing peaks Oft from whose feet the avalanche, unheard, Shoots downward, glittering through the pure serene Into the depth of clouds that veil thy breastThou, too, again, stupendous mountain ! thou, That as I raise my head, awhile bow'd low In adoration, upward from thy base Slow-travelling with dim eyes suffused with tears, Solemnly seemest, like a vapoury cloud, To rise before me-rise, O ever rise, Rise, like a cloud of incense, from the earth! Thou kingly spirit, throned among the hills,
Thou dread ambassador from earth to heaven,
And tell the stars, and tell yon rising sun,
LESSON XLIX.-THURSDAY. ENGLISH HISTORY—THE REVOLUTION OF 1688. The revolution of 1688 deserves more the attention of a philosopher from its indirect influence on the progress of human opinion, than from its immediate effects on the government of England. In the first view, it is perhaps difficult to estimate the magnitude of its effects. It sancti. fied, as we have seen, the general principles of freedom. It gave the first example in civilized modern Europe of a government which reconciled a semblance of political, and a large portion of civil liberty, with stability and peace. But, above all, Europe owes to it the inestimable blessing of an asylum for freedom of thought. Hence, England became the preceptress of the world in philosophy and freedom. Hence arose the school of sages, who unshackled and emancipated the human mind; from among whom issued the Lockes, the Rousseaus, the Turgots, and the Franklins, the immortal band of preceptors and benefactors of mankind. They silently operated a grand moral revolution, which was in due time to ameliorate tho social order. They had tyrants to dethrone more formidable than kings, and from whom kings held their power. They wrested the sceptre from supersti. tion, and dragged prejudice in triumph. They destroyed the arsenal whence despotism had borrowed her thunders and her chains. These grand enterprises of philosophic heroism must have preceded the reforms of civil government. The Colossus of tyranny was undermined, and a pebble overthrew it. From this progress of opinion arose the American revolution, and from this, most unquestionably, the delivery of France. Nothing, therefore, could be more natural than that those who, without blind bigotry for the forms, had a rational reverence for the principles of our ancestors, should rejoice in a revolution, where these principles, which England had so long suffered to repose in impotent abstraction, were called