300 OPTICAL PROJECTION 



shown by the prism to be generally of a most composite 

 character. 



Another experiment may be made. Go up to within a 

 yard or two of the screen, and hold up a large piece of coloured 

 glass in the rays. It will be seen that in some rays it casts a 

 perfectly black shadow it is opaque to light of that colour 

 but in other rays it is more or less transparent. 



With more colouring matter, absorption increases, and it 

 is instructive to prepare a few cells of wedge shape, or wider 

 one end than the other, and fill them with coloured solutions. 

 Placed as in fig. 166, it will be found that often the apparent 

 colour changes when a thicker cell is used ; a colour only 

 slightly obstructed by a thin layer, being completely obstructed 

 by a thicker layer. These experiments may be varied ad 

 libitum. 



That the surf ace -colours of bodies are largely transparent 

 colours, or remainders left after other colours have been 

 absorbed by a layer of the surface molecules, is shown 

 similarly, by holding large flowers with good masses of colour 

 in the rays of the spectrum ; or the coloured prints sold with 

 floral magazines will answer very well. Each colour, in some 

 rays, will appear bright and natural ; in others dull grey, and 

 in others black. The monochromatic lights mentioned further 

 on have the same effect. 



171. Transmitted and Reflected Colours. Surface colours 

 are, however, not all due to transparency, and hence there is 

 sometimes a great difference between the colours a substance 

 reflects and transmits. This is specially true of metallic and 

 semi-metallic bodies, and may be shown by a film of gold-leaf 

 mounted as a slide. It appears green by transmitted light. 

 Deflect the lantern parallel with the screen, condense all the 

 light from the condensers through the open nozzle on its 

 surface, at an angle of 45, l and focus the reflecting surface 

 with the loose lens, and it is yellow. The aniline dyes used for 



1 Exactly as the soap-film, fig. 179, p. 827. 



