=Single Vision by the two Retinæ.=--We hear single with two ears, and smell single with two nostrils, and we also see single with two eyes. The difference is that we also _can_ see double under certain conditions, whereas under no conditions can we hear or smell double. The main conditions of single vision can be simply expressed. In the first place, impressions on the two foveæ always appear in the same place. By no artifice can they be made to appear alongside of each other. The result is that one object, casting its images on the foveæ of the two converging eyeballs will necessarily always appear as what it is, namely, one object. Furthermore, if the eyeballs, instead of converging, are kept parallel, and two similar objects, one in front of each, cast their respective images on the foveæ, the two will also appear as one, or (in common parlance) 'their images will fuse.' To verify this, let the reader stare fixedly before him as if through the paper at infinite distance, with the black spots in Fig. 8 in front of his respective eyes. He will then see the two black spots swim together, as it were, and combine into one, which appears situated between their original two positions and as if opposite the root of his nose. This combined spot is the result of the spots opposite both eyes being seen in the same place. But in addition to the combined spot, each eye sees also the spot opposite the _other_ eye. To the right eye this appears to the left of the combined spot, to the left eye it appears to the right of it; so that what is seen is _three_ spots, of which the middle one is seen by both eyes, and is flanked by two others, each seen by one. That such are the facts can be tested by interposing some small opaque object so as to cut off the vision of either of the spots in the figure from the _other_ eye. A vertical partition in the median plane, going from the paper to the nose, will effectually confine each eye's vision to the spot in front of it, and then the single combined spot will be all that appears.[11] [Illustration: FIG. 8.] If, instead of two identical spots, we use two different figures, or two differently colored spots, as objects for the two foveæ to look at, they still are seen in the _same place_; but since they cannot appear as a single object, they appear there _alternately_ displacing each other from the view. This is the phenomenon called _retinal rivalry_.