28 COLOUR VISION 



starting place, red. This is a very important fact, for it can be proved 

 that with the help of the colours thus obtained, either pure or mixed 

 with each other or with black in various proportions, all known colours 

 and tints can be reproduced. 



We can map out the spectrum into its separate colours, using the 

 Fraunhofer lines as convenient fixed points, but as the colours pass 

 gradually into each other the limits are more or less arbitrary, 

 v. Helmholtz gives the following names to the different regions of the 

 spectrum : 



Speaking generally then, change of wave-length causes a change in 

 colour, or in the hue or tone of a colour. The tone changes most rapidly 

 on both sides of the yellow, most slowly near the ends of the spectrum. 

 For a certain distance at each end change of wave-length is no longer 

 accompanied by change of tone, at the red end beyond 655 /x/u, at the 

 violet end beyond 430 ixfi. 



Apart from the change in colour the most striking feature of the 

 spectrum is the difference in brightness or luminosity of different parts. 

 The brightest part is in the yellow at about the D line, the luminosity 

 diminishing continuously on both sides to the extreme ends. The 

 brightness varies with the intensity of the light, but if the intensity 

 is increased beyond a certain point the colours also change in tone. 

 The colours on each side of about 500 fx/n behave differently ; the red, 

 orange, yellow and green approximate to yellow, the blue-green, blue 

 and violet approximate to blue. 



Though the brightness of the colours increases with the intensity 

 of the light it does not follow the curve of energy of the spectrum. 

 Whereas the brightest part of the spectrum is in the yellow the curve of 

 energy rises continuously from the violet to the red end^. 



^ Nichols, Phys.Rev. xxi. 147, 1905; Knirmp, Physisch-ophthalmologische Grenzprobleme. 

 Leipzig, 1906, p. 4. 



