182 SECTIONAL ADDRESSES 
when discrimination of ‘red’ is present the sensation of yellow is 
aroused by a region of the spectrum which separates that giving a red 
sensation from that which gives another colour, namely, green. 
The deficiency is always characterised by a spreading out of the 
portion of the spectrum which gives rise to a sensation of yellow until, 
in severe cases, the whole of the spectrum from 4900 A. to the extreme 
‘red’ end is distinguished only by characters such as brightness or 
decrease in blueness. 
