SPACE 35 



Soon, however, we learn to forget this deficiency, for other 

 features come to our aid, which enable us to see objects as 

 spatial magnitudes and not as surfaces, and this without our 

 eyes converging. We may sum up these features — penum- 

 brae, shadows, etc. — as object-signs. How great is the effect 

 of object-signs can be shown by a well-known physiological 

 experiment ; on any coin it is possible to make the relief 

 appear as a concavity, and vice versa, if, unknown to the 

 observer, we reverse, by means of a mirror, the way the light 

 falls. 



Moreover, in looking through the stereoscope, we must 

 first acquire plastic vision by supplying the lack of conver- 

 gent movement with object-signs. 



In such cases we have to do only with the symphony of 

 space-magnitudes : and here we must draw attention to a 

 further peculiarity of the human eye which has far-reaching 

 consequences. In the distribution of the nervous areas for 

 the local signs, our retina shows a clear division into an upper 

 and a lower half. The half that we chiefly use, since our 

 sight is essentially concerned with objects on the ground, 

 shows a greater wealth in local signs than the half we use for 

 looking at the sky. That is why still, inland lakes, in which 

 the surrounding trees are clearly reflected, produce such a 

 fairy-like impression ; in the reflection, the trees appear 

 richer in detail and therefore higher and the sky seems further 

 off, because the images of these things are perceived by a 

 greater number of local signs than in direct vision.' 



The peculiar shape of the firmament, which rises steeply 

 from the horizon and then flattens out into the form of a 

 watch-glass, depends on the same thing. 



In the blue and cloudless sky, devoid of all the object- 

 signs that would make us aware of the curve, we see, if we 

 look at it through our hollowed hand, a level blue surface, 

 always running parallel to the main direction-plane of our 



