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variation; local perturbations; the influence of the Aurora, which affects all the three expressions of magnetical power ; the disturbance of the horary motion, simultaneously over thousands of miles, as from Kazan to Paris. In the mean time, the theory of magnetism improved as the facts came out. Its germ was the Cartesian vortices, suggested by the curvilinear forms of iron filings in the vicinity of magnetic poles. The subsequent mathematical discussion was conducted

upon the same principles as in the case of electricity. Electro Then came the Danish discovery of the relations of electrimagnetism.

city and magnetism, illustrated in England by rotatory motions, and in France adorned by the electrodynamic theory, embracing the action of currents and magnets, magnets and magnets, currents and currents. The generation of magnetism by electricity was after a little delay followed by its converse, the production of electricity by magnetism; and thermoelectric currents, arising from the unequal application or propagation of heat, were rendered serviceable in producing the

most sensitive of all thermometers. Of light and

The investigation of the nature and properties of light rivals optics.

in interest and value that of electricity. What is this agent, light, which clothes the earth with verdure, making animal life possible, extending man's intellectual sphere, bringing to his knowledge the forms and colours of things, and giving him information of the existence of countless myriads of worlds ? What is this light, which, in the midst of so many realities, presents him with so many delusive fictions, which rests the coloured bow against the cloud—the bow once said, when men transferred their own motives and actions to the Divinity, to

be the weapon of God? Optical The first ascertained optical fact was probably the propagadiscoveries.

tion of light in straight lines. The theory of perspective, on which the Alexandrian mathematicians voluminously wrote, implies as much; but, agreeably to the early methods of philosophy, which was inclined to make man the centre of all things, it was supposed that rays are emitted from the eye and proceed outwardly, not that they come from exterior objects and pass through the organ of vision interiorly. Even the

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great geometer Euclid treated the subject on that erroneous principle; an error corrected by the Arabians. In the mean time the law of reflection had been discovered ; that for refraction foiled Alhazen, and was reserved for a European. Among natural optical phenomena the form of the rainbow was accounted for, notwithstanding a general belief in its supernatural origin. Its colours, however, could not be ex- Colours and plained until exact ideas of refrangibility, dispersion, and the white light. composition of white light were attained. The reflecting telescope was invented; the recognized possibility of achromatism led to an improvement in the refractor. A little previously the progressive motion of light had been proved, first for reflected light by the eclipses of Jupiter's satellites, then for the direct light of the stars. A true theory of colours originated with the formation of the solar spectrum ; that beautiful experiment led to the discovery of irrationality of dispersion and the fixed lines. The phenomena of refraction in the case of Iceland spar were examined, and the law for the ordinary and extraordinary rays given. At the same time the polarization of light by double refraction was discovered. A century later it was followed by polarization by reflection and single refraction, depolarization, irised rings, bright and black crosses in crystals, and unannealed or compressed glass, the connection between optical phenomena and crystalline form, uniaxial crystals giving circular rings and biaxial oval ones, and circular and elliptical polarization.

The beautiful colours of soap-bubbles, at first mixed up with those of striated and dotted surfaces, were traced to their true condition-thickness. The determination of thickness of a film necessary to give a certain colour was the first instance of exceedingly minute measures beautifully executed. These soon became connected with fringes in shadows, and led to ascertaining the length of waves of light.

Meantime more correct ideas respecting vision were ob- Vision; the tained. Alhazen's explanation of the use of the retina and the eye. lens was adopted. This had been the first truly scientific investigation in physiology. The action of the eye was reduced to that of the camera-obscura described by Da Vinci, and the

functions of

struments.

368

The Undulatory Theory. old notion of rays issuing therefrom finally abandoned. It had held its ground through the deceptive illustration of the magic lantern. Of this instrument the name indicates the popular opinion of its nature. In the stories of necromancers and magicians of the time are to be found traces of applications to which it was insidiously devoted,—the raising of the dead, spectres skipping along the ground or dancing on the walls

and chimneys, pendulous images, apparitions in volumes of Optical in. smoke. These early instruments were the forerunners of

many beautiful inventions of later times,—the kaleidoscope, producing its forms of marvellous symmetry; the stereoscope, aided by photography, offering the very embodiment of external scenery; the achromatic and reflecting telescope, to which physical astronomy is so greatly indebted; and the achromatic microscope, now working a revolution in anatomy and phy

siology. Theundula In its theory optics has presented a striking contrast to tory theory. acoustics. Almost from the very beginning it was recognized

that sound is not a material substance emitted from the sounding body, but only undulations occurring in the air. For long, optics failed to reach an analogous conclusion. The advancement of the former science has been from the general principle down to the details, that of the latter from the details up to the general principle.

That light consists of undulations in an elastic medium was first inferred in 1664. Soon after, reflection, refraction, and double refraction were accounted for on that principle. The slow progress of this theory was doubtless owing to Newton's supremacy. He gave a demonstration in the second book of the 'Principia' (Prop. 42) that such motions must diverge into the unmoved spaces, and carried popular comprehension with him by such illustrations as that we hear sounds though a mountain interpose. It was thought that the undulatory theory was disposed of by such facts as the impossibility of seeing through a crooked pipe, though we can hear through it; or that we cannot look round a corner, though we can listen round one.

The present century finally established it through the dis

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covery of interference, the destruction of the emission theory being inevitable when it was shown that light, interfering under certain circumstances with light, may produce darkness, as sound added to sound may produce silence,-results arising from the action of undulating motion. The difficulties presented by polarization were not only removed, but that class of phenomena were actually made a strong support of the theory. The discovery that two pencils of oppositely polarized light would not interfere, led at once to the theory of transverse vibrations. Great mathematical ability was now required for the treatment of the subject, and the special consideration of many optical problems from this new point of view, as for example, determining the result of transverse vibrations coming into a medium of different density in different directions. As the theory of universal gravitation had formerly done, so now the undulatory theory began to display its power as a physical truth, enabling geometers to foresee results, and to precede the experimenter in conclusions. Among earlier results of the kind was the prediction that both the rays in the biaxial crystal topaz are extraordinary, and that circular polarization might be produced by reflection in a rhomb of glass. The phenomena of depolarization offered no special difficulty; and many new facts, as those of elliptic polarization and conical refraction, have since illustrated the power of the theory.

Light, then, is the result of ethereal undulations impinging The ether on the eye. There exists throughout the universe and among movements. the particles of all bodies an elastic medium, the ether. By reason of the repulsion of its own parts it is uniformly diffused in a vacuum. In the interior of refracting media it exists in a state of less elasticity compared with its density than in

Vibrations communicated to it in free space are propagated through such media by the ether in their interior. The parts of shining bodies vibrate as those of sounding ones, communicating their movement to the ether, and giving rise to waves in it. They produce in us the sensation of light. The slower the vibration, the longer the wave; the more frequent, the shorter. On wave-length colour depends. In all

2 B

VOL. II,

370

The Human Eye; Photography.

cases the vibrations are transverse. The undulatory movement
passes onward at the rate of 192,000 miles in a second. The
mean length of a wave of light is 0.0000219 of an iuch; an
extreme red wave is twice as long as an extreme violet one.
The yellow is intermediate. The vibrations which thus occasion
light are, at a mean, 555 in the billionth of a second. As with
the air, which is motionless when a sound passes through it,
the ether is motionless, though traversed by waves of light.
That which moves forward is no material substance, but only
a form, as the waves seen running along a shaken cord, or the
circles that rise and fall, and spread outwardly when a stone
is thrown into water. The wave-like form passes onward
to the outlying spaces, but the water does not rush forward.
And as we may have on the surface of that liquid waves the
height of which is insignificant, or those which, as sailors say,
are mountains high in storms at sea, their amplitude thus
differing, so in the midst of the ether difference of amplitude
is manifested to us by difference in the intensity or brilliancy
of light.

The human eye, exquisitely constructed as it is, is nevertheeye: its ca. pabilities. less an imperfect mechanism, being limited in its action. It

can only perceive waves of a definite length, as its fellow organ, the ear, can only distinguish a limited range of sounds. It can only take note of vibrations that are transverse, as the ear can only take note of those that are normal. In optics there are two distinct orders of facts,--the actual relations of light itself, and the physiological relations of our organ of vision, with all its limitations and imperfections. Light is altogether the creation of the mind. The ether is one thing, light is another, just as the air is one thing and sound another. The ether is not composed of the colours of light, any more

than the atmospheric air consists of musical notes. Chemical

To the chemical agency of light much attention has in reinfluences of light.

cent times been devoted. Already, in photography, it has furnished us an art which, though yet in its infancy, presents exquisite representations of scenery, past events, the countenances of our friends. In an almost magical way it evokes invisible impressions, and gives duration to fleeting shadows.

The human

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