OF THE RETINA TO LIGHT AND COLOUR. 
175 
11. Extinction of Colour from Spots of different Diameters. 
Following on the extinction of light came the investigation of the loss of colour 
from areas of varying angular aperture. In Part HI. of “ Colour Photometry,” a 
method was described of estimating the point where all colour disappeared. In that 
paper no reference was made to the size of the area under examination, it being 
greater than the 4° diameter, and all apertures greater than this, as we have seen, 
behave alike. In this investigation a somewhat difterent method was adopted. Two 
apertures were placed side by side and a very feeble white light from the arc light 
Fio-. 8. 
was caused to illuminate one aperture, while the colour under examination tilled the 
other. The two were darkened together and the point of diminution where they 
perfectly matched in tint was taken as the point at which the colour of the latter 
vanished. It was veiy necessary to make the apertures of equal area and equally 
bright, as, if not, the measurements became more difficult. A large number of 
different rays were examined, with the centre of the retina, for colour persistency in 
this way, but the following will suffice to show that the colour extinction does not 
follow that of the light extinction when regard is had to the sizes of the apertures 
