22 OLD FLIES IN NEW DRESSES 



The word " tone " expresses variations 

 of wave-lengths within a named colour, and 

 "brightness" is used to indicate the in- 

 tensity of the sensation produced upon the 

 retina. 



The enormous difficulty of working out 

 into a monochrome the shade- values of a 

 collection of colours, with several tones 

 and shades of brightness in each of the vari- 

 ously coloured parts of the object we wish 

 to imitate, can be imagined on considering 

 these facts only ; but there are more facts 

 which lead me to believe that to do this is 

 not only difficult, but impossible. 



Two theories have been propounded to 

 explain the sensation of colour produced 

 upon the retina. 



The Young-Helmholtz " theory teaches 

 that there are three primary sensations- 

 red, green, and violet. Other colours are 

 a mixture of these sensations ; white is pro- 

 duced when all three sensations are excited 

 together, and black ""is an absence of 

 sensation, 



Hering's theory is that there are six 



primary sensations arranged in three pairs 



white and black, red and green, and 



