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22 THEORETICAL BIOLOCxY 



been moved ; of this we can convince ourselves by covering 

 the eye that has remained still. 



If we imagine the retinae to be superimposed on which 

 an image is to be thrown, and let one retina be shifted laterally 

 by its eye-muscles, the image no longer falls on identical 

 retinal spots, and must appear double, because now there 

 are two identical places everywhere. The movement that 

 appears when this happens is not perceived by the retina 

 that has shifted, for on it the two kinds of direction-signs 

 balance one another ; it is perceived by the stationary eye, 

 which has not followed the impulse. The displacement of 

 the stationary retina relatively to the one that has been 

 moved is not balanced by inner direction-signs, and conse- 

 quently must appear as movement in the outside world. 



That outer direction -signs should appear even when the 

 image has not been moved shows that these also are in- 

 dependent central qualities like the inner direction-signs, 

 which in all points they essentially resemble. Similar direc- 

 - tion-signs (inner and outer) appear sometimes in association 

 with muscular actions, sometimes with local signs, and either 

 combine together or else cancel one another. 



Hence the remarkable fact that, though when we look 

 around us, the images of objects on the retina continually 

 stimulate fresh local signs, we do not always notice this, 

 and the objects do not leave their places : consequently, 

 they acquire a position in space which appears to be in- 

 dependent of us. If the direction-signs did not balance one 

 another in this way, the world would lose all its repose ; in 

 place of its calm, there would be an intolerable dancing about 

 of objects, such as we experienced when we moved the eye- 

 ball with the finger. 



To sum up briefly : — We get knowledge of the direction- 

 sensations or direction-signs from two different sources. 

 Firstly, they may appear on appropriate excitation of the 



