4 THE COLOUKS OF ANIMALS 



a polished surface like that of a mirror, does not 

 cause whiteness : it renders the surface itself invisible, 

 but produces images of surrounding objects. A white 

 appearance is produced by irregular reflection, which 

 causes the light to be scattered or reflected in all 

 directions. To produce such a result there must 

 be an immense number of surfaces in an immense 

 number of different directions. If a coloured sub- 

 stance be reduced to powder of various degrees of 

 fineness, the colour will diminish in intensity, and 

 the whiteness will increase, according to the fineness 

 of the powder ; this is because the number of re- 

 flecting surfaces is increased, while the thickness of 

 the grains is diminished. This will be clear from the 

 following consideration. When a beam of light falls on 

 a sheet of glass, a known fraction (about 4 per cent.) 

 of the light is reflected back from the first surface : the 

 larger portion, however, enters the glass, and, after 

 suffering a certain amount of absorption, reaches the 

 second surface and is again partially reflected. If the 

 glass be powdered, the number of surfaces will be so im- 

 mensely increased that all the light will be reflected 

 by a small thickness of the powder. The light 

 reflected from the second surface of each grain of 

 coloured glass will still be coloured by absorption, but 

 not sufficiently to produce any visible results, when 

 the thickness of the grain is very small. 



Eeflection is the immediate cause of whiteness, and 

 the amount of reflection is due to the difference 



