106 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
In the case of observers of class I., the light just before it is extinguished at the 
fovea gives the sensation of colour. This is particularly well marked in the green, 
where the faintest light observable with central fixation appears of a dull but very 
saturated green. In the case of observers of class II., on the other hand, as long as 
the stimulus light is red they can distinguish the red colour when the light is visible 
at all, but throughout the rest of the SDectrum the light loses colour a considerable 
time before it is extinguished. These class II. observers have an achromatic interval 
throughout the whole of the spectrum except perhaps in the red. 
It may be remarked, that when AY. attempted to determine his foveal threshold 
values without using the arrangements which have been described above, but 
employing a continuous illumination which was gradually decreased in intensity, his 
