(- te 4) 
Amongst the most important is undoubtedly individual experi- 
ence. In the early period of life of which we have no memory, 
the mind is learning to supplement some of the deficiencies of 
the eyes by associating with their impressions the impressions 
derived from other senses, notably that of touch: objects are 
touched and it is found that they are not flat, and it is also 
found that certain distinctions of light and shade go with the 
not-flatness, and so there gets to be established a mental 
association between certain delicacies of light and shade and 
not-flatness, and when this is thoroughly established we, 
being unaware of the association, and perceiving the shade 
delicacies by the eye, infer that the eye itself sees the non- 
flatness. That this is the case is proved by well-known facts, 
which it is unnecessary to trouble you with, as they are to be 
found in many physiological text-books. Individual experi- 
ence and the association of tactile impressions with the 
ocular impressions are amongst the important means by 
which the defects of picture-vision are remedied. Another 
indispensable aid to our sight is the mobility of the eyes: 
this is wonderfully perfect, and as the eyes are spherical in 
form and the muscles are attached at several points, a very 
beautiful capacity of moving the eye is the result; and the 
extreme and perfect mobility of our eye is one of the chief 
factors in its utility. 
Another very important feature in our vision is that we 
have two eyes and combine them on one object: the result is 
that we see single though our eyes really see double, the two 
pictures being slightly different; and by the unconscious in- 
ferences made from the slight differences between the two 
pictures we obtain some valuable properties of material 
objects which we think the eye directly perceives. The very 
important property of estimation of size of an object is also 
in us a mental and not an ocular power, the size of an object 
as pictured on the retina bearing but little proportion to its 
real magnitude. There are also other deficiencies in the 
nature of picture-vision to which I need not allude. But I 
will ask you to bear in mind the fact that the insect is 
destitute of the all-important secondary means I have alluded 
to, for rendering picture-vision adequate. Its life is but 
