82 



THE VISUAL PROCESS 



tive transmission. An opaque colored paper or cloth performs the former, 

 a translucent colored glass or liquid performs both. A colored object is 

 colored, instead of gray, because it absorbs some wavelengths and reflects 

 or transmits others. The latter being the ones which reach the eye, they 

 determine the color of the object. If the object is specially illuminated 

 only by wavelengths which it can absorb, it can reflect none of them and 

 will then appear black. An object which in sunlight appears black must, 



g yor 



Fig. 29 — The physical and psychological spectra. 



a, the visible spectrum as formed by a prism. 



V- violet; b- blue; g- green; y- yellow; o- orange; r- red. 



b, the psychological color circle. Red and violet intergrade through purple; diametrically 

 opposite hues are complementaries, and make white when mixed in correa amounts. 



c, the linear spectrum formed by a lens. The distance from the focus of violet to that of 

 red (greatly exaggerated in the diagram) is the 'linear chromatic aberration' of the lens. 



then, be one which absorbs all wavelengths, just as white objects, to ap- 

 pear white, must reflect all. Of course no object absorbs or reflects all of 

 the light striking it. Whether it reflects all wavelengths equally, or some 

 more^han others, it reflects only a certain percentage of the light energy. 

 This percentage is the object's reflection coefficient or 'albedo'. 



No object can reflect only a single wavelength, and hence no object 

 Y" can have a pure color. To obtain pure colors, we must select them from a 



