194 
CAPTAIN ABNEY ON THE SENSITIVENESS 
Xliey show that with au incroasG of double tne luminosity the field is extended about 
17° on the temporal side and about. 13 on the nasal. .A-s the increase of dimensions 
in these two directions of the colour fields for the same increase in lumiuosity is 
considerably less, it is evident that there is no exact connection between the 
luminosity of wdiite light at the different parts of the retina and the luminosity of 
a colour-ray when colour is extinguished. The sensitiveness of the peripheral 
portions of the retina to light and colour is therefore different, as it was shown to be 
when the centre of the retina was under consideration. 
Fig. 19. 
A good many experiments have been carried out regarding the persistence of 
coloured images and the rate of perception, but these have indicated that the subject 
is one which should be treated of in a separate communication. 
It may here be reiterated that the sensitiveness of the eye varies considerably at 
times, which may be due in all probability to the state of health of the observer. 
Aluch of the difficulty experienced in these observations has arisen from this variation. 
As the eye becomes practised to observation, however, the liability to variation very 
largely disappears, and at the present time readings made by my assistant and nnself 
are very fairly comparable at all times. AX hether, when the observations have ceased 
for some time and are then renewed, there will be a relapse, it is hard to say. 
It will be seen that scarcely any reference has been made to the work of other 
observers. It has not been thought advisable to do so for various reasons, the 
principal one being that in the experiments described spectrum colours have been 
employed. The results obtained with these last cannot be comparable with those 
obtained with the use of impure colours. 
20. General Summary. 
Tlie results of these investigations may be summarized as follows : 
1. That where an image is received on the centre of the retina, the reduction in 
