248 



THE POPULAR EDUCATOR. 



RECREATIVE SCIENCE. IV. 



THE REFLECTION OF LIGHT, AND DECEPTIONS WITH 

 PLANE AND CONCAVE MIERORS. I. 



A PENCIL or ray of light will continue in a straight path through 

 a medium of the same density, but may be acted upon in four 

 ways if it passes into another portion of the same medium of a 

 different density, or into any other gaseous, liquid, or solid body. 

 When light is affected in one or other of these ways, it is said 

 to be reflected, 



refracted, polar- jr 



ised, or absorbed. 

 The reflection of 

 light from the va- 

 rious objects pro- 

 duced by nature 

 or art, enables us 

 to see and enjoy 

 the works of God 

 and man, which 

 must otherwise 

 have been invisi- 

 ble, or seen only 

 in outline, if they 

 did not'one and all 

 possess the pro- 

 perty of throwing 

 off more or less of 

 the raya which 

 fall upon them 

 from luminous 

 bodies. The in- 

 fluence of the 

 light in promoting 

 the well-being of 

 all is undoubted. 

 Dr. Forbes Win- 

 slow says, " Where light is not permitted to permeate, there are 

 found, in the highest state of manifestation, bodily deformities, 

 intellectual deterioration, crime, disease, early, and often sudden 

 death." A material, as well as a moral and mental, etiolation 

 or blanching occurs when the vital stimulus of light is with- 

 drawn. The reflection of 

 light into the dwellings of 

 the poor, where the direct 

 rays of the sun cannot 

 reach them, has been in- 

 sisted on over and over 

 again by philanthropists 

 and philosophers. The 

 late Sir D. Brewster, in 

 one of his addresses to 

 the Eoyal Society of Edin- 

 burgh, thought it a mat- 

 ter of sufficient import- 

 ance to state how light 

 might be reflected into 

 rooms situated in narrow 

 Janes. He says : " If, in 

 a very narrow street or 

 lane, we look out of a 

 window with the eye in 

 the same plane as the 

 outer face of the wall in 

 which the window is 

 placed, we shall see the 

 whole of the sky by which 

 the apartment can be il- 

 luminated. If we now withdraw the eye inwards, we shall gra- 

 dually lose sight of the sky till it wholly disappears, which may 

 take place when the eye is only six or eight inches from its first 

 position. In such a case the apartment is illuminated only by 

 the light reflected from the opposite wall, or the sides of the stones 

 which form the window ; because if the glass of the window is 

 BIX or eight inches within the wall, as it generally is, not a ray 

 of light can fall upon it. If we now remove our window, and 

 substitute another, in which all tlie panes are roughly ground on 

 the outside, and flush with the outer wall, the light from the 



whole of the visible sky, and from the remotest part of the 

 opposite wall, will be introduced into the apartment, reflected 

 from the innumerable faces or facets which the rough grinding 

 of the glass has produced. The whole window will appear as 

 if the sky were beyond it, and from every point of this luminous 

 surface light will radiate into all parts of the room." " Further," 

 he says, " the opposite sides of the street or lane should be kept 

 whitewashed with lime ; and, for the same reason, the ceilings 

 and walls of the apartments should be as white as possible, and 



all the furniture 

 of the lightest 

 colours. Having 

 seen such effects 

 produced by im- 

 perfect means, we 

 feel as if we had 

 introduced our 

 poorworkmen and 

 needle- women 

 from a dungeon 

 into a summer- 

 house, where the 

 aged can read 

 their Bibles; 

 where the in- 

 mates can see 

 each other, and 

 carry on their 

 work in society 

 and comfort." 

 The reflection of 

 light is governed 

 by laws that ad- 

 mit of easy de- 

 monstration. 



1st. The inci- 

 dent and reflected 



rays are always found in the same plane or direction which is 

 perpendicular to the surface reflecting the light. 



A pencil or ray of light can always be obtained in the 

 daytime by making a small hole in the shutter of a darkened 

 room, and at night by putting a sheet-iron or copper chimney 



over a burning argan<3 

 oil or gas lamp. The 

 chimney must have a 

 small hole punctured in 

 it, or a series of holes of 

 different sizes may be 

 bored, and each kept 

 closed, with a circular 

 flap crossed at the upper 

 edge by a wire properly- 

 riveted; a glass chimney 

 may be coated with tin- 

 foil, in which holes of the 

 required size can be made 

 and pasted up with slips 

 of foil when not required. 

 The experimental results 

 are seen so much better 

 when the spectator con- 

 fines his attention only to 

 the ray with which he is 

 operating. 



In Fig. 1 the rays of 

 light, i" K (made parallel 



3. by a small double convex 



lens), called the incident 



rays, and proceeding from the hole in the chimney, are falling on 

 the slanting mirror A B ; and the rays B B", the reflected rays, are 

 proved to be in the same plane or direction, because they strike 

 or impinge on ths chimney at E", and therefore in a perpendicular 

 plane corresponding with the upright chimney. 



2nd. The angle of incidence is always equal to the angle of 

 reflection ; thus, in Fig. 1, if a line perpendicular to the surface 

 of the mirror, A B, be raised at E and drawn to P, then the angle 

 p B i" is equal to the angle P B B", or, in other words, the incident 

 and reflected rays form equal angles. By moving the mirror 





