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FIEST CLASS OP OBJECTS FOE THE SUBJECT. 71
object also. Now, it should be well observed, that in this
process it is not the outer side of one retina which corre
sponds to the outer side of the other, and the inner to the
inner of each, but the right side of one retina which corre
sponds to the right side of the other, and so forth ;
so that
this symmetrical correspondence must not be taken in a
physiological, but in a geometrical sense. Numerous and
very clear illustrations of this process, and of all the
phenomena which are connected with it, are to be found in
Robert Smith s
"Optics," and partly also in Kastner s
German translation (1755). I only give one (fig. 2), which,
properly speaking, represents a special case, mentioned
further on, but which may also
whole,
serve to illustrate the
if we leave the point E out of question. Ac
cording to this illustration, we invariably direct both eyes
equally towards the object, in order that the symmetrically
corresponding places on both retinas may catch the rays
projected from the same points. Now, when we move our
eyes upwards and downwards, to the sides, and in all
directions, the point in the object which first impinged
upon the central point of each retina, strikes a different
place every time, but in all cases one which, in each eye,
corresponds to the place bearing the same name in the
other eye. In examining (perlustrare) an object, we let our
eyes glide backwards and forwards over it, in order to
into contact with the
bring each" point of it successively
centre of the retina, which sees most : distinctly we feel it
all over with our eyes.
It is therefore obvious that seeing
singly with two eyes is in fact the same process as feeling
a body with ten fingers, each of which receives a different
impression,
each moreover in a different direction: the
totality of these impressions being nevertheless recognised
by the Understanding as proceeding from one object, whose
shape and size it accordingly apprehends and constructs in
Space. This is why it is possible for a blind man to become