In his investigations on colour blindness, Dr. Houstoun produced two identical patches of colour on a screen; one of these patches was gradually altered until a difference in tint could just be observed. These two patches were at a distance of 8 mms. apart on the screen, and Dr. Houstoun suggested that if these patches were juxtaposed the observer's colour sensitivity might be improved. The following communication records the results of investigations carried out under that suggested condition.