6 COLOUR VISION 



in greatest quantity, so that the dominant colour of the light reaching 

 the eye is red ; but it is not pure red. Most blue substances, such as 

 copper salts, allow the blue rays to pass, but also some of the green 

 and violet, though few of the red and yellow. Yellow substances allow 

 much red and green to pass as well as the yellow, but little blue and 

 violet. The true composition of the transmitted light can only be 

 determined with the spectroscope. 



The case of pigments is similar. Each speck of powder is a small 

 transparent body which absorbs certain rays of light. When light falls 

 upon such a powder a small portion is reflected from the upper surface ; 

 this is white. The remainder passes deeper and is reflected from deeper 

 layers. The deeper it passes the greater is the absorption and the more 

 intense the colour. Hence a coarse powder appears more intensely 

 coloured than one which is finely divided. Reflection varies with the 

 number of surfaces, not with the thickness of the particles. The larger 

 the latter the deeper the light must penetrate for the same number of 

 surfaces to be met as when the particles are smaller. The absorption 

 is therefore greater in a coarse than in a fine powder. The reflection 

 at the surfaces is diminished when the intervals between the particles 

 are filled with a fluid of refractive index nearer their own than that of 

 air. Hence powders are generally whiter when dry than when mixed 

 with water or oil. 



The amount of absorption of light by a transparent body can be 

 measured and expressed in the form of a coefficient. If a spectrum 

 is viewed through an orange glass very little red, orange and yellow, 

 but much green and all the blue are absorbed, as shown by dark bands 

 in the regions of absorption. In this case the coefficient of absorption 

 increases as the blue is approached. By knowing the coefficients of 

 absorption of different media the effects of combining them in various 

 ways can be calculated. By empirical experiments colour screens or 

 filters can be made which transmit certain portions only of the spectrum, 

 and in some cases approximately monochromatic light can be obtained 

 in this manner. These filters are much used for photographic purposes. 

 The characters of the absorption by Jena glass filters and by various 

 fluid media are described in Tigerstedt's Handbuch der pkysiologischen 

 Metliodih^, which also gives an excellent resume of the methods which 

 have been employed for the investigation of colour vision. 



No pigments accurately represent spectral colours, for the reflected 



1 Bd. III. Abt. 2, Sinnespkijsiologie ii pp. 47 and 52. 



