COMMON THINGS COLOUR. 



It is the same with artificial lights. A lamp placed in a room 

 illuminates directly all those objects accessible to its rays. These 

 objects reflect irregularly the light incident upon them, and illu- 

 minate thus more faintly others which are removed from the 

 direct influence of the lamp, and thus, these again reflecting the 

 light, illuminate a third series still more faintly; and so on. 

 When it is desired to diffuse uniformly by reflection the light 

 which radiates from a luminary, the object is often more effectu- 

 ally attained by means of an unpolished opaque reflector than 

 by a polished one. White paper or card answers this purpose 

 very effectually. Shades formed into conical surfaces placed 

 over lamps are thus found to diffuse by reflection the light 

 in particular directions, as in the case of billiard-tables or 

 dinner-tables, where a uniformly diffused light is required. A 

 polished reflector, in a like case, is found to diffuse light much 

 more unequally. 



In case of white paper or card, each point becomes a centre of 

 radiation, and a general and uniform illumination is the conse- 

 quence. The light obtained by reflection in such cases is always 

 augmented by rendering the reflector perfectly opaque ; for if it 

 be in any degree transparent, as is sometimes the case with paper 

 shades put over lamps, the light which passes through them is 

 necessarily subtracted from that which is reflected. 



15. We have stated that the colour of objects is that of the 

 light which they reflect. It may then be asked how it happens 

 that objects illuminated by the white light of the sun are not 

 all white instead of having the infinitely various tints of colour 

 by which they are characterised. The answer is, that the white 

 light of the sun itself is a composition of all these various hues ; 

 that some objects reflect only the component lights of particular 

 tints, and others those of other tints ; that, in fact, the solar 

 light falling on an object is generally decomposed, a part of it 

 being absorbed by, or transmitted through, the object, and a 

 part only reflected, and the object appears to have the colour 

 peculiar to this latter part. 



16. That solar light is actually a compound of lights of various 

 tints was established by Newton by means of a memorable arid 

 beautiful experiment. 



Let a ray of solar light be admitted through a small hole, P 

 (fig. 5.) in a screen or partition s T, all other light being excluded 

 from the space into which the pencil enters. If a white screen 

 x z be placed parallel to s T, and at a distance from it of about 

 12 feet, a circular spot of light nearly equal in diameter to the 

 hole will appear upon it at P', the point where the direction of 

 the pencil meets the screen. Now let a glass prism, such as is 

 66 



