78 



Prof. F. Allen on Four Transition Points in the 



by various spectral colours. These curves for colour-blind- 

 ness, therefore, constitute strong additional evidence for the 

 existence of three primary colour-sensations — red, green, and 

 violet. 



Fig-. 22. 



•40/x 



•50 -55 -60 



Wa\/e Length. 



65 



•70 



According to this theory seven general classes of colour- 

 blindness are possible : three classes when any one colour- 

 sensation is absent or defective : three classes when any two 

 sensations are absent or defective; and one class when all 

 three sensations are absent or defective. The seven types of 



