BY THE PRESIDENT. 
123 
of the retina appears to ns as white, and blue light appears darker. 
Hence it follows that the inequalities of the retina affect the colour 
as well as the distinctness of the image. We find also that the 
susceptibility of the retina to red rays diminishes towards its cir¬ 
cumference, so that red colours seem much darker when viewed 
indirectly. We recognise this when, after looking at a red 
geranium, we raise our eyes so that the image of the plant falls 
outside the yellow spot, for we find that we then see the leaves 
and their colour, but not the blossoms. 
The inequalities of the retina as to red rays are also illustrated 
in one of the toys of the nursery. Pour figures of mice are stitched 
with red wool on a blue or green ground, and as the child moves 
the piece of work up and down in front of her eyes, the mice dis¬ 
appear and reappear as their image falls on or outside the yellow spot. 
But to return to our subject. All the contradictory sensations 
of colour are reconciled not only by the constant movement of the 
eye, but also by our learning to judge from the colour-sensation of 
indirect vision that colour which we would, see if the same object 
were viewed by direct vision. Under most circumstances we form 
an opinion of the colour of an object by direct vision, i.e. by placing 
our eye in such a position that the image falls on the yellow spot. 
How we have already seen that the pigment of this yellow spot 
absorbs some of the rays, and that it thus affects the colour of 
an object we look at. We see then that there can be no exact 
correspondence between the rays of light and the colours they 
produce, and in truth colours are only subjective. This is of sub¬ 
ordinate importance. The primary use of colour is the facility 
with which it enables us to discern objects, and the eye duly 
performs this function inasmuch as similar light produces under 
like conditions a like sensation of colour. 
It is curious to observe that the yellow spot is less sensitive to 
weak light than the other parts of the retina. Thus stars of 
inferior magnitude become visible or more brilliant when viewed 
indirectly . This may be due to the yellow pigment weakening the 
blue rays. There is also an analogy between the retina and the 
skin. Those parts which are most discriminating in the sense of 
touch are the least tender because more constantly used. So the 
yellow spot is less sensitive to weak light than the other parts of 
the retina. 
I have already referred to the red-colour blindness of a great 
portion of the retina, and this condition is seen in an extreme form 
in the inability of many persons to distinguish green from red. 
The fact that colour blindness is nearly twice as prevalent among 
