488 



PHYSIOLOGY 



saturated than the mixture of green and violet. In describing the complementary 

 pairs of colours, we have mentioned that if the spectral colours are placed in a closed 

 ring, complementary pairs are found to be opposite to one another. If now the 

 three fundamental colours are placed at equal intervals round the ring, we may 

 regard white as occupying the centre, because it is equidistant from the three fundamen- 

 tals, and at the same time lies on the diameter between the various colours and their 

 complementaries. If the other spectral colours are arranged in position relatively 

 to the three fundamentals, they form a figure that in shape resembles a triangle more 

 closely than it does a ring. This is due to the facts already mentioned (1) with regard 

 to the exact matching of the spectral colours between red and green, by mixtures of 

 those two fundamental colours; (2) with regard to the approximate matching of the 

 region between green and violet by the mixtures of those colours, and (3) with regard to 

 the exact matching of mauves and purples by mixtures of red and violet. The colour 

 triangle which is shown in figure 243 therefore has a purely experimental basis, and 

 has no association whatever with theories of vision. 



GftJLEJ* 

 100 



SOOO 

 BLUE GRE.&N 



KED 800 

 TO 7500 



FIG. 243. Colour triangle. 



The Mack line shows the shape of the curve along which the different rays of 

 the spectrum fall for white to occupy the central position. 



THE OPTICAL PROPERTIES OF MATTER 



Since matter is permeated by the ether, we should expect matter to be transparent 

 to light. We find however that all matter absorbs light to a greater or less extent ; 

 even substances that are called transparent, like glass and water, absorb strongly when 

 in sumciemt thickness. Beside substances which may be classed as transparent 

 or opaque, there is a large class of bodies which reflect light. When the body presents 

 a smooth surface to the light rays, the reflected ray forms a compact bundle, and the 

 surface is therefore said to reflect light. If on the other hand the surface presented 

 to the light rays is rough, the light bundle is, split up into a number of separate units 

 which scatter diffusely in every direction. Such a surface is therefore said to diffuse or 

 scatter light. If light is incident on matter, there are thus four different processes that 

 may occur, viz. the light may be partially absorbed, it may be partially transmitted, 

 it may be partially reflected and lastly it may be partially scattered. In the great 

 majority of cases, all these processes take place to a certain extent, and are found to affect 

 the (lilTm-nt p.uts of tin- speetnim dilTnvntly. For example, while the colours of short 

 wavelength are absorbed, those .,f lon^ wavelength may be almost completely reflected. 



(A polished copjx-r surface is found to have the-, proper! ics.) Another example \\oiild 

 be a .substance \\liidi \\hile alisorliin^ colours of long wavelength, scatters almost 



