UNDULATORY FORCES. LIGHT. 



[REFLECTION. 



axis. Of course, in ordinary circumstances, all the 

 spokes seem to bo lost to sight when the wheel goes 

 rapidly round. If, however, a flash of electric light is 

 made to illuminate them, at the instant the light ia pro- 

 duced, each spoke at once becomes visible, because they 

 pass through space in a much longer period of time 

 than does the light which has fallen on them. In pass- 

 ing through a railway tunnel, a red-hot cinder produces 

 a similar optical effect. Previous to the production of 

 its light, the divisions of the sides or bricks of the 

 tunnel would be undistinguishable ; but at the instant 

 of the fall of the cinder, every part is immediately ob- 

 served. Now these effects are due partly to the rapid 

 flight of light, and also to the fact, that the eye cannot 

 appreciate or separate rays of light whoso interval of 

 appearance is less than the eighth of a second of time. 

 Hence all the rays passing from the sides of the tunnel 

 seem to be merged together. When, however, only one 

 powerful ray reaches the eye, it at once produces an im- 

 pression on the retina ; and each surface from wliich it 

 has been reflected becomes instantly visible. This fact 

 may be easily verified by turning round rapidly a piece 

 of red-hot wood, by means of a string, in a dark room. 

 A ring of light will thereby be formed. If, however, 

 an assistant discharges a highly charged leyden jar, so 

 that the light of the spark may fall on the rotating wood, 

 it will at that instant appear stationary, for reasons we 

 have just assigned. 



Light, like all radial or central forces, diminishes in its 

 intensity from the source of emission, at a rate equal to 

 the square of the distance. We refer to our remarks on 

 this subject,* in which we fully entered into this law, both 

 in reference to heat and light. 



Having thus pointed out some general facts respect- 

 ing the nature of light, we now proceed to investigate 

 some of those laws to which it is subject in its pas- 

 sage from the surface, or through the mass of various 

 bodies, and the changes which it thereby undergoes. 



All natural substances may be divided into three 

 classes, so far as the passage of light through them is con- 

 cerned. Those which prevent the passage of light, or stop 

 the undulations of luminous ether, aro called opaque. 

 Some permit freely of its passage, such as glass, crystal, 

 <tc. ; and these are termed transparent substances. Others 

 only permit the partial transmission of light, as in the 

 case of ground glass, milk, and other substances ; and 

 these have accordingly been termed translucent. 



When light passes through a body, it undergoes vari- 

 ous changes, such as single refraction, double refraction, 

 and polarisation. When a ray of light falls on some 

 substances, such as polished metals, it is reflected ; but 

 if it falls at certain angles, it may also be polarised, 

 as we shall more fully point out when we deal with 

 polarised light. 



We shall treat on the phenomena of reflection, refrac- 

 tion, and polarisation, under separate heads; because 

 each subject has so many matters of both scientific and 

 general interest attached to it. In our progress, we 

 shall include special applications of the various laws we 

 have to examine, including the use and construction of 

 various optical instruments. We shall also add some 

 remarks on the eye and its structure, which will illus- 

 trate the application of many of the laws of the reflection 

 and refraction of light. 



THE REFLECTION OF LIGHT, OR CATOPTRICS. 



IH our chapter on heat, we stated that that force, as 

 well as light, could be reflected from a polished surface ; 

 and this may be of any form ; but the shape of the 

 reflective surface has various effects on a ray of either 

 force. The term Catoptrics has been applied to those 

 investigations which are connected with the laws of the 

 reflection of light; and to this subject we now direct 

 attention. 



If a ray of light fall on a plane or level surface, such 

 as a piece of glass or polished metal, it is reflected at an 

 angle which has a definite relation to that by which it 

 Sec ante, p. 11. 



arrived at the reflecting surface. The angle by which 

 the ray arrives at the reflector, is called the angle of ni- 

 cUUnce ; and that by which it leaves the reflecting sur- 

 face, is termed the angle of reflection. 



Those terms will be better understood by a reference 

 to the annexed diagram. 



In Fig. 2, a being the source of light from whirh a 

 ray passes to 6 (the reflecting surface), then the angle 

 c, a, d, is called the angle of incidence ; c, e, boing the line 

 by which the reflected ray travels to the eye of the 

 observer, then the angle c, e, d, will be the angle of re- 

 flection. 



Now the law of reflection from plane surfaces is, " that 

 the angle of incidence and the angle of reflection aro 

 equal to each other ;" so that by being acquainted with 

 either of these angles, the other is known also. 



Some interesting applications of this law are often 

 employed in daily life. By means of properly arranged 

 plane reflectors, light may be conveyed to remote 

 places ; and thus one source may be made available for 

 a great variety of purposes. Rooms remote from day- 

 light are often illuminated by such means. Coal aid 

 other mines have been lighted by reflectors, so arranged, 

 that one large source of light as, for instance, the 

 electric light may be made to illuminate distant pas- 

 sages ; and thus all danger from the ignition of lire- 

 damp is entirely avoided. The whole of one end or 

 length of a street may be observed from an apartment, 

 by a person sitting inside it, if a mirror is placed at an 

 angle of 45 on the outside of the window. A very in- 

 genious contrivance was suggested for employing simi- 

 lar means in warfare. It was proposed to place outside 

 the wall of a battery, an arrangement of mirrors, by 

 which a view of a distant fort might be constantly ob- 

 tained, without danger of the observer being "picked 

 off" by the enemy's riflemen. This plan is illustrated 

 in our next engraving; and, we believe, was employed 

 during the Crimean war. 



Fig. 3. 



In Fig. 3, a, b, c, d, represent four mirrors, of 

 wliich a receives the view of the fort. This view is 

 successively reflected over the wall (e), and observed by 

 the spectator at /. . Thus, any person placed at /, 

 under protection of the wall, will be enabled to observe 

 all that is going on at a distance from him, and yet be 

 unseen by the enemy. 



The amount of light wliich is reflected from any sur- 

 face, is not equal to that which impinges on it. This is 

 owing to the absorption of some of the rays. Wo may 

 hero remark, that light, like heat, is absorbed in different 

 proportions by various substances. We thus observe 

 very little light reflected from a body of a black colour ; 

 and as no metallic body can be perfectly polished, so, 

 from this cause, some of the rays of light are lost before 

 reflection. But even from the most polished surface 



