212 PSYCHOLOGY. wafer or a black cross, the subject, although he denies that he sees it when he looks straight at it, nevertheless gets a
- negative after-image ' of it when he looks away again,
showing that the optical impression of it has been received. Moreover reflection shows that such a subject must dis- tinguish the object from others like it in order to be blind to it. Make him blind to one person in the room, set all the persons in a row, and tell him to count them. He will count all but that one. But how can he tell ivhich one not to count without recognizing "^ho he is ? In like manner, make a stroke on paper or blackboard, and tell him it is not there, and he will see nothing but the clean paper or board. Next (he not looking) surround the original stroke with other strokes exactly like it, and ask him what he sees. He will point out one by one all the new strokes, and omit the original one every time, no matter how numerous the new strokes may be, or in what order they are arranged. Similarly, if the original single stroke to which he is blind be doubled by a prism of some sixteen degrees placed before one of his eyes (both being kept open), he will say that he now sees one stroke, and point in the direc- tion in which the image seen through the prism lies, ignor- ing still the original stroke. Obviously, then, he is not blind to the kind of stroke in the least. He is blind only to one individual stroke of that kind in a particular position on the board or paper — that is to a particular complex object ; and, paradoxical as it may seem to say so, he must distinguish it with great ac- curacy from others like it, in order to remain blind to it when the others are brought near. He discriminates it, as a preliminary to not seeing it at all. Again, when by a prism before one eye a previously in- visible line has been made visible to that eye, and the other eye is thereupon closed or screened, its closure makes no difference ; the line still remains visible. But if then the prism be removed, the line will disappear even to the eye which a moment ago saw it, and both eyes will revert to their original blind state. We have, then, to deal in these cases neither with a blind- ness of the eye itself, nor with a mere failure to notice, but
