586 LIGHT Time of passage of light f resi 1 stars. (such as Mira Ceti) owe their rapid periodical changes of brightness to eclipses, and if different homogeneous rays travel with different velocities in free space, it is evident ^at suc ^ stars wou ^ s ^ ow a g ra dual change of colour as they wax, and an opposite change as they wane. Nothing of the kind has as yet been observed, though it has been carefully sought for. Hence we have every reason to con clude that, in free space, all kinds of light have the same velocity. It will be seen later that dispersion has been accounted for by the different velocities of light of different wave-lengths in the same refracting medium, this being a consequence of the ultimate grained structure of ordinary matter, which is on a scale not incomparably smaller than the average wave-length. Behaviour of Light at the Common Surface of Two Homogeneous Media. Effect at When a ray of light/moving in one homogeneous medium, a surface falls on the bounding surface of another homogeneous of separ- me( jium, it is in general divided into several parts, which between P ursue different courses. These parts are respectively two () reflected ; (/?) refracted (singly or doubly) ; (y) scat- media. tered ; (8) absorbed. In the first two categories the result is two or three rays of light pursuing definite paths according to laws presently to be given. The fraction of the incident light which is reflected is in general greater as the angle of incidence is greater. In one important class of cases the reflexion is total. But at direct incidence the reflected portion is much greater for some bodies, such as mercury, than for others, such as water or glass. In bodies which give no scattering, the refracted portion of a ray consists of all the non-re flected portion, and therefore usually diminishes as the angle of incidence increases. In the third category the common surface of the two media becomes illuminated, and behaves as if it were itself a source of light, sending rays in all directions. It may be objected to this, that in many cases the rays are scattered while penetrating the second medium. But in such cases the second medium cannot be called homogeneous. This will come up for discussion when we treat of absorption and colours. In the fourth category the light ceases for an instant to exist as light ; but its energy may either become heat in the absorbing body, or it may again be given out by the absorbing body in the form of light, but of a degraded character. This is called fluorescence, or phosphorescence, according as the phenomenon is practically instantaneous or lasts for a measurable time. In category (a) the light is sent back into the first medium ; in (/3) it penetrates into the second ; in (y) it goes, in general, mainly to the first ; in (8) it is shared by both. Visi- It is by scattered light that non-luminous objects are, bility of in general, made visible. Contrast, for instance, the effects non- when a ray of sunlight in a dark room falls upon a piece !iect U3 ^ P^ sne d silver, and when it falls on a piece of chalk. Unless there be dust or scratches on the silver you cannot see it, because no light is given from it to surrounding bodies except in one definite direction, into which (practi cally) the whols ray of sunlight is diverted. But the chalk sends light to all surrounding bodies from which any part of its illuminated side can be seen ; and there is no special direction in which it sends a much more powerful ray than in others. It is probable that if we could, with sufficient closeness, examine the surface of the chalk, we should find its behaviour to be of the nature of reflexion, but reflexion due to little mirrors inclined in all conceivable aspects, and at all conceivable angles, to the incident light. Thus scattering may be looked upon as ultimately due to reflexion. "When the sea is perfectly calm, we see in it one intolerably bright image of the sun only. But when it is continuously covered with slight ripples, the definite image is broken up, and we have a large surface of the water shin ing by what is virtually scattered light, though it is really made up of parts each of which is as truly reflected as it was when the surface was flat. We have spoken above of the behaviour of light at the Gradiu common surface of two media. Now we do not by this phrase necessarily mean two media different in their; chemical composition. We mean merely media optically ^ed ii different. Thus water with steam above it, and in very to au- epecial cases layers of water or air of different temperatures, other. give surfaces of separation at which reflexion and refraction may and do take place. But, except in such special cases, we rarely have an abrupt change, such as is necessary for reflexion, between two portions of the same substance in the same molecular state. In general the transition is gradual ; and special mathematical methods must be applied for the purpose of tracing the behaviour of the ray, which is now really travelling in a non-homogeneous medium. REFLEXION OF LIGHT. If light be reflected from a Reflexi plane surface bounding two dissimilar isotropic media, the of incident and reflected rays are in one plane ivith, and are equally inclined (on opposite sides) to, the perpendicular to the reflecting surface at the point of incidence. This is sometimes stated in the form The angles of incidence and of reflexion are equal to one another, and in one plane. The best experimental proof of the truth of this statement is deduced from the use of a reflecting surface of mercury in observations with the mural circle. The graduation of such an instrument is the most perfect that human skill can accomplish, and no one has ever been able to find by it the slightest exception to the pre ceding statement. The principle of Hadley s quadrant, and of the sextant as now used (an invention of Newton s), is founded on this fact. If a plane mirror on which a ray falls be turned through any angle about an axis perpendicular to the plane of reflexion, the reflected ray is turned through twice that angle. This is an immediate consequence of the above law. For, if the plane be turned through any angle 6, the perpendicular to it is turned through the same angle. Hence the angle between the incident ray and the perpen dicular is increased or diminished by 9, and therefore that between the incident and reflected rays (which is double of this) is altered by 20. A plane mirror is now exten sively used for the purpose of indicating, by the change of direction of a reflected ray, the motion of a portion of an instrument to which the mirror is attached. Thus the magnetometers of Gauss, the tuning-forks of Lissajoux, and the electrometers and galvanometers of Sir W. Thom son are all furnished with mirrors. The law of reflexion is also the basis of the goniometer, for the measurement of the angles of crystals and prisms. It follows from this law that, if a ray pass from one Mini- point to another, after any number of reflexions at fixed mum surfaces, the length of its whole path from one point to ^l?* 11 the other is the least possible subject to the condition that it shall meet each of the reflecting surfaces. For the point in a given plane the sum of whose distances from two given points (on the same side of the plane) is the least possible is that to which, if lines be drawn from the points, they are in one plane with the normal (or perpendicular) to the given plane and make equal angles with it. And, as the same is true of each separate reflexion, it is true for the whole course of the ray, since for any one of the reflecting surfaces may be substituted its tangent plane at the point of incidence.
