LECTURE  XIII

§ I. CO.YIBINATION OF SIMILAR STEREOSCOPIC IMAGES. § II. IDEA¬
TIONAL Change in the Combination of Different Images.
§ III. Reflection and Lustre ; Theory of Lustre. § IV.
Phenomena of Suppression in Binocular Vision.

§1

THE fusion of the two retinal images to a single idea is
_^ only a particular instance of a general law of the forma¬
tion of ideas. In the visual idea which comes from the two
eyes we do not discover any trace of the perception of one eye
as distinct from that of the other ; but we blend them at once
into a single and indissoluble perception. In this sense it is
true that the two eyes constitute only a single organ of vision.
That they are really like two independent observers, regarding
things from two different points of view, and that we become
acquainted with the properties of objects only by combining the
result of these observations, are facts which we do not remark;
there is given in consciousness simply the result of this combi¬
nation. That is to say, it is not until the two perceptions of
binocular vision have fused that we have an idea ; or, in other
words, the fact that the two eyes unite to perform a common
function is a necessary result of the nature of the mental pro¬
cesses of association.

This connection of the separate elements of an association to
a single idea, which is inevitably conditioned by the nature of
the mind, is also furthered by the laws of external perception.
Our external perceptions are of such a character, that they can
only be referred to an object corresponding to the idea which
has been formed of it. So that we can raise the further ques¬
tion : how does the ideational activity behave in the presence
of impressions which cannot be referred to one and the same