COMMON THINGS COLOUR. 



case of an object illuminated by another, as in fig. 3, than in that 

 of a luminary, as in fig. 2. 



This effect is easily explained. According to what has been 

 already stated, each point of the surface of the illuminated object 

 A B is a focus of a pencil of rays of light having the colour pecu- 

 liar to such point. Thus, each portion of the pencil of rays which 

 radiates from the point B, and has for its base the area of the 

 aperture c, will pass through the aperture, and will continue its 

 rectilinear course until it arrives at the point b upon the screen, 

 where it will produce an illuminated point corresponding in colour 

 to the point B. 



In the same manner, the pencil diverging from A, and passing 

 through the aperture c, will produce an illuminated point on the 

 screen at a, corresponding in colour to the point A. 



Each intermediate point of the object will produce a corre- 

 sponding illuminated point on the screen. It is evident, therefore, 

 that a series of illuminated points corresponding in arrangement 

 and colour to those of the object will be formed upon the screen 

 between a and b, their position, however, being inverted, the 

 points which are highest in the object will be lowest in the 

 picture. 



These effects may be witnessed in an interesting manner in any 

 room which is exposed to a public thoroughfare frequented by 

 moving objects. Let the window-shutters be closed and the 

 interstices stopped so as to exclude all light except that which 

 enters through any small hole in them, and if no hole be found in 

 the shutters sufficiently small, a piece of paper or card may be 

 pasted over any convenient aperture, and a hole of the required 

 magnitude pierced in it. Coloured inverted images of all the 

 objects passing before the window will thus be depicted on a screen 

 conveniently placed. They will be exhibited on the opposite wall 

 of the room ; but unless the wall be white, the colours will not 

 be distinctly perceptible. The smaller the hole admitting the 

 light is, the more distinct but the less bright the pictures will be. 

 As the hole is enlarged the brightness increases, but the distinct- 

 ness diminishes. The want of distinctness arises from the spots 

 of light on the screen, produced by each point of the object over- 

 laying each other, so as to produce a confused effect. 



10. Surfaces differ from each other in the proportion of light 

 which they reflect and absorb. In general, the lighter the colour, 

 other things being the same, the more light will be reflected and 

 the less absorbed, and the darker the colour the less will be re- 

 flected and the more absorbed ; but even the most intense black 

 reflects some light. A surface of black velvet, or one blackened 

 with lamp-black, are among the darkest known, yet each of these 

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