OF THE RETINA TO LIGHT AND COLOUR. 
195 
intensity of the radiation which will just fail to produce the sensation of 
light dejjends (within limits) on the size of the image. 
2. That the smallest diameter of the image and not its area determines the 
necessary reduction in intensity. 
3. That the reduction in the intensity of the light of an image falling excentric- 
ally, which will just fail to produce the sensation of light, follows the same 
general law as if the image were received centrally. 
4. That the visual brightnesses of illumination of a small and a large aperture 
when illuminated with the same light differ, and that such visual bright¬ 
nesses are connected by a simple law. 
5. That the reduction in the brightness of an image just sufficient to extinguish 
the sensation of colour, varies with the size of the image and follows a 
definite law, which, however, differs from that for the extinction of liglit. 
6. That all fields for colours will have the same boundary when the intensity of 
the coloured ray is properly adjusted. 
7. That there is a simple connection between the intensity of a colour and the 
extent of field. 
8. That the colour fields depend on the size of the object viewed, and that the 
dependence appears to follow a simple empiric law. 
9. That the retina is most sensitive to light at its central part and the sensitive¬ 
ness diminishes towards the periphery. 
These results as they stand do not seem to confirm either one of the two 
main theories of colour vision. The existence of a colour field at all is difficult to 
explain on the Young theory, and the fact that a colour field for red can be 
obtained with bright illumination although the disappearance of this colour and light 
takes place almost together at the centre of the retina, is not easily accounted for on 
Hering’s theory. It appears as if light were the fundamental sensation caused by 
the main vibration generally, whilst colour is as it were an overtone to which the 
receiving nerves are less susceptible than to light, the further away they are situated 
from the centre, and may be due to the form of vibration. 
In closing this paper I shoi.dd be doing an injustice if I did not place on record the 
great assistance I have had during the whole of these investigations, which have 
extended over three years, from my assistant, Mr. Walter Bradfield ; with every new 
step I took he made himself thoroughly acquainted, and every series of measures I made 
myself he repeated with his own eyes. There is nothing stated as being fairly proved 
which has not been confirmed by him. Measurements of the kind recorded above are 
by no means as simple as they look on paper, but those given are the results of 
hundreds of observations, repetition being an absolute necessity to avoid false 
deductions. 
2 c 2 
