LAW AND NATURE OF REFRACTION.

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In glass and air, in glass and water, in water and air, or in any two transparent media, it is always the same for the same media. This might at first seem no more than saying that the change of direction produced by one piece of glass, or one glass of water, was just the same as that produced by any other piece of glass, or glass of water, of the same size; and this would not excite much wonder or admiration. But it

means more than this: it means that if, at any angle at which light falls, the line a is, or, or of the line b, that it will have the same ratio at any other angle, whether greater or smaller. If by depressing A c I shorten a by one-half, the ray c B would be raised so as to shorten B b one-half also.

Some thoughtful reader may say that when A c is coincident with a c b, the line of junction, then a will be 0, while B b will still be a real distance; also that when the emergent ray c B is coincident with a c b, then B b will be 0, while a will be a real

distance. But the law is for light passing from one medium to another, not for light continuing in the same medium.

(11.) Nature of Refraction.-Just as reflection is bending a ray of light, heat, or sound more or less back again towards the starting-point, so refraction is bending such ray more or less away from its direction. Roughly speaking, the distinction between reflection and refraction is, that in refraction the ray is affected by passing through a body, while in reflection it is affected by the surface only. The name refraction suggests the theory that light is broken up into its constituents.

SUMMARY.

Light, heat, or sound, passing through any body is affected more or less in direction, velocity, and wave-length. This is called refraction.

Page 307.

White light, when refracted by prismatic bodies, is decomposed into its constituent coloured rays.

Page 308.

Or it is so altered in its velocity and wave-length as to give the impression of colour.

Page 311. Light passing through bodies having parallel surfaces is only affected as to its direction, and not as to its colour. Page 312. Some prismatic bodies disperse light more than others-i.e., produce a longer spectrum.

Page 313.

The amount of refraction is affected by changes of density or of temperature.

Page 314.

An eye-glass, a microscope, a telescope, are examples of refrac

tive apparatus.

Page 314.

Total reflection is an example of refraction.

Page 315.

Refraction is governed by fixed laws.

Page 316.

Refraction is so called because it is supposed to break up light.

Page 317.

TRANSMISSION.

(1.) Introduction.-We are so familiar with the passage of heat, light, or sound through air, and of light through glass, that these are often considered as matters of course. But they are as remarkable as any other phenomena, when really thought of. Why should light pass through glass, and not through wood or iron? When light does so pass through glass, what is really done? The source of light is on one side of the glass and our eyes on the other side; what effect does the glass produce on the rays of light? what effect do the rays of light produce on the glass?

If we assume the existence of an ether, then we must consider transmission to consist of the passage of the ether through the interstices of the glass; otherwise, we must assume the particles of glass themselves to vibrate, and thus transmit the light. In either case we must assume the glass to consist of particles arranged very regularly.

(2.) Examples of Transmission. Sound is transmitted through almost any substance. Heat passes through rock-salt, sulphur, glass, and ice, in solids; through bisulphide of carbon, olive-oil, sulphuric acid, and water, in liquids; through air, oxygen, nitrogen, and hydrogen, in gases. In these enumerations I have placed first the substance that most readily allows the passage of heat-i.e., rock-salt in solids, and bisulphide of carbon in liquids. All the four gases named appear to allow heat to pass through with equal completeness. Light passes through glass and horn in solids, water and many other liquids, and through gases, with great readiness.

We are so familiar with the transmission of light through glass, that we seldom stay to think how remarkable it is. We know that sound is conveyed by solids; but light is so much more delicate a vibration, so infinitely minute, that it is amazing to think that each pulsation is conveyed accurately and completely through

so solid a material as glass, a substance so compact that it keeps out every breath of air, every speck of dust.

Force is also transmitted in the form of electricity or magnetism; but not by the method of radiation, which we usually call transmission.

Heat, light, electricity, &c., are examples of the form in which force may be transmitted; so radiation, reflection, refraction, conduction, are examples of the method of transmission.

(3.) Results of Transmission.—The result of transmission is simply the transfer of force from one place to another. Radiation, reflection, refraction, are all examples of transmission; but the term transmission is used for the passage of heat, light, or sound, through a solid, liquid, or gaseous body, without reference to any change in character or direction, but only to amount. So that it simply means the transfer of so much force, either as light, heat, or sound, from one side of a given body to another.

(4.) Nature of Transmission.—The transmission of matter is always evidenced by the fact that it was here, and it is there; and it has been moved from one place to another. So the transmission of force is shown-it was here, and it is there. But just as a body that has changed its place must have passed through every point of the path along which it travelled, so transmission of a force implies a continuous medium of transmission. I cannot pull by means of a chain of which a single link is missing, nor push by a rod that is not continuous. Just so, a vibration cannot be transmitted across an absolute vacuum. Transmission of heat, light, or sound, is simply a continuous vibration: it may be the vibration of an ether, or of ordinary matter: the transmission may be between, or by means of, the particles of the familiar objects surrounding us; but it is the continuance of motion from particle to particle.

SUMMARY.

Force is transmitted from place to place in the same manner as matter.

Page 319. Light is transmitted through horn, glass, and air; heat through rock-salt, sulphur, glass, oil, water, and air; sound through most substances.

Page 319.

The result of transmission of heat, light, sound, is the transfer of force from one place to another.

Transmission of force is performed by the continuance of motion from particle to particle of the medium of transmission.

Page 320.

Page 320.

POLARISATION.

(1.) Introduction.-The light falling from a candle upon a looking-glass divides into two rays, each reflecting an image of the candle. One of these comes from the first surface, and one from the second-i.e., when the light falls on the glass, part of it is reflected and part refracted: one part is transmitted through the glass, and part back through the air; and the part passing through the glass falling on the mercury, is again divided, one part entering the mercury, the other being reflected. It depends upon the angle of incidence which of these reflected images shall be the more distinct. The smaller the angle between the ray and the surface, the more distinct the first image, and the less the second.

This is a simple example of one ray being divided into two, or rather of one ray producing two. Under certain circumstances these two rays, so produced from one, differ in very important points from each other. The questions to be answered are-1. What circumstances divide one ray into two? 2. What differences exist between these rays?

(2.) Causes of Polarisation.-(a) Division of Ray.-Iceland spar is transparent, so is tourmaline, so is glass; but the transparency of glass differs in a very important manner from that of spar or tourmaline. In well-made glass the particles are all at the same distance from each other; in spar or tourmaline these particles are more compressed in some directions than in others. The result is, that a single ray of light falling upon glass continues a single ray through the glass (not counting the reflected ray); while a single ray falling upon spar or tourmaline becomes a double ray-i.e., two rays passing through, besides the reflected ray from the surface. This arises probably from the compression of the spar or tourmaline being greater in some directions than in others. A rough parallel may be indicated thus: Suppose water to be running through one pipe into another, and that at one place the substance of the pipe is very thin. If such thin

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