CUBVSD KEFT.ECTOBS.] 



UNDULATORY FORCES. LIGHT. 



43 



the same loss occurs ; and thus light reflected from 

 water or glass, undergoes considerable diminution in 

 its intensity. 



We cannot refrain from remarking on the value o 

 this circumstance, when viewed in connection with 

 natural objects. If all bodies reflected light with eqvia 

 power, the whole face of nature would present a mono- 

 tonous appearance. Those gradations of tint which form 

 the beauty of the landscape, and the variety of our 

 fields and gardens the fine effect which a stream o 

 water or river has in the distant prospect, would all be 

 absent. The various reflecting power of different soils, 

 is also of great importance both in a cosmical and agri- 

 cultural point of view. We here have another instance 

 of one law which is at once universal and beneficent in 

 its operations. 



The reflection of light exists on a grand scale in our 

 planetary system. In the centre we have the sun, the 

 source at once of heat and light. Around him are the 

 planets, receiving on, and reflecting from, their surface 

 his borrowed rays. Round the planets we have their 

 satellites, which, like our moon, act as so many enor- 

 mous reflectors, cheering, during the night season, the 

 side of the planet turned from the sun. Hence, again, 

 the phenomena of eclipses, which occur from the inter- 

 ference, by the eclipsing body, of the rays of solar 

 light, and induce its consequent non-reflection from the 

 eclipsed body. 



Two interesting optical toys the kaleidoscope, in- 

 vented by Sir David Brewster; and the Debusscope, 

 lately* invented by M. Debus are arranged on the prin- 

 ciple of reflection. In the kaleidoscope, plane mirrors 

 are employed, each of which reflect the image of an 

 object placed between them ; and thus an infinite variety 

 of beautiful and ever-changing forms may be produced. 

 In the Debusscope, any object placed between the mir- 

 rors is multiplied, so as to present a fourfold appear- 

 ance ; and it thus effect* a oleasing expansion of any 



REFLECTION FROM CURVED SURFACES. 



WB have hitherto referred only to the effects produced 

 by the reflection of light from plane or level surfaces ; 

 but we shall now examine the results obtained when 

 rays of light are incident on any polished body of a 

 curved form ; and also enter into various applications of 

 the principles we may investigate. 



.ans of curved reflectors, we are enabled to con- 

 centrate the rays of heat and li"ht to a very large ex- 

 tent; or, in other words, we collect rays passing in a 

 straight lino on to the mirror ; and by its peculiar form, 

 all those are brought together, and joined in one point, 

 called a focus. We may be better understood, if we 

 state, that the object of curved reflectors, is to produce, 

 at one point, the effects of all the rays of light, which 

 had previously only passed side by side, or at an angle 

 with each other. 



To illustrate our meaning, we refer to the annexed 

 diagram, Fig. 4, in which rays of light are passing 

 from any source in parallel lines, as from o, o, o, <tc., 

 to 6, 6, <tc. 



Fi. 4. 



Tf 



If a curved reflector be placed in any part, say c, 

 tween o and 6, the rays will not only be stopped in 

 their passage, but will also be diverted from the original 

 straight line in which they had previously travelled, 

 with the exception of the middle one, which coincides 

 with the axis, or line passing through the centre of the 

 mirror at c. Wo shall find, that aU the rays thus re- 

 flected are joined together at a point (/); and this is what 

 U termed the focal point, or focus of a curved reflector. 



In practice, this focus can easily be ascertained by 



holding some burning paper before the mirror, when the 

 junction of the reflected rays will be detected. 



It thus follows, that parallel rays of light, passing 

 from a distant source, may, by means of a curved mir- 

 ror, be brought to a focal point, and their combined 

 effect be concentrated thereon. 



If, however, the source of light be placed behind the 

 focus (/, Fig. 4), and nearer the mirror, the result will 

 be different. The rays of light will be still reflected, 

 but will pass from the luminous object to the curved 

 surface, and will then diverge in all directions. The 

 student will do well to remember the following epitome 

 of what we have stated, viz. : When rays of light from 

 a distant source impinge on a curved reflector, they are 

 converged to a focus ; but when the source of light is 

 placed behind the focus of, and near to, such a reflec- 

 tor, the rays are diverged by the reflecting surface. 



Before we enter into more minute details in reference 

 to curved surfaces, we shall give an illustration of the 

 use of the reflector, which will make plain the state- 

 ments we have above mentioned. An ordinary reflector, 

 such as is used in carriage-lamps, is an instance of the 

 curved surface we have described. If such a reflector be 

 exposed to the rays of the sun, as a distant source of 

 light, it will collect the rays, and bring them to a focal 

 point. If, however, a candle be placed behind the focal 

 point, then the rays of light will be reflected in all direc- 

 tions, after reaching the surface of the mirror. These 

 two modes of employing the same mirror, will thus 

 illustrate the difference between the convergence and 

 divergence of the rays of light ; and show that the two 

 reflective results are the reverse of each other in effect. 

 When the rays passing from the reflector are examined 

 beyond the focus, it U found that they have crossed each 

 other ; and if an object be placed near the focal point, 

 and observed some distance therefrom, it will be found 

 that an inverted view of it is obtained. We are hero 

 confining our attention to a concave mirror, or one in 

 which the reflecting surface is "hollowed out," or bent 

 inwards. We shall now examine more minutely into 

 the results obtained in employing this form of reflecting 

 surface, by varying the position of a source of light, or 

 of an object placed at different distances from its focus, 

 together with some of the practical applications wliich 

 result from the principles at which we shall arrive. 



We shall refer, in the following remarks, at first to re- 

 flectors of a spherical form j that is, such as would be pro- 

 duced by taking any portion of a globe of glass, or metal 

 in a polished state. The best type of this class of reflec- 

 tor is a common watch-glass, whoso outside surface has 

 been covered with quicksilver, so as to present an inter- 

 nal or concave reflecting surface. In practice, reflectors 

 of polished metal, of various sizes, are employed; of 

 which, however, we shall speak more fully hereafter. 



The law with respect to the reflection of light from 

 plane surfaces, also holds good in reference to curved 

 ones ; the angle of incidence being always equal to the 

 angle of reflection. Indeed, a curved surface may be 

 regarded as made up of an infinite number of plane sur- 

 faces ; and by reflection from each of these planes, to 

 one point, of the rays of light impinging on them, the 

 reflection of curved surfaces is in reality effected. 



The first case of reflection from a spherical concave 

 mirror which we shall deal with, is that in which the 

 rays of light arrive at its surface in parallel lines. This 

 occurs only from objects at a great distance from the 

 eflector as the sun, moon, and stars. In Fig. 6, we 

 lave an illustration of the effects which result on parallel 

 rays arriving on a concave mirror. 



rig. t. 



