34 THEORETICAL BIOLOGY 



One of our greatest observers, the sculptor Hildebrandt, 

 advanced the theory that in all works of art, in painting as 

 well as in sculpture, the foremost plane should be the chief 

 plane, starting from which the eye must travel into the dis- 

 tance and the depths ; for in looking at Nature it behaves in 

 the same way. 



The physiological cause of this kind of vision is to be 

 found in the structure of the accommodating apparatus of our 

 eye. The lens of the eye is focussed for near vision by the 

 active pull of the lens muscles. Distance sight is brought 

 about by relaxation of the muscles, whereby the lens 

 apparatus, in virtue of its elasticity, returns automatically 

 to the position of rest. Both in contraction and in relaxation 

 of the muscles, direction-signs appear, which are brought into 

 relation with the corresponding plane of the apparatus that 

 orientates us in space. 



During active focussing of the lens on nearer and ever 

 nearer objects, whole groups of direction-signs reach our 

 consciousness together, and, as it were, in jerks : whereas 

 when we focus slowly from near to far objects, the individual 

 direction-signs appear one after the other in a uniform series ; 

 and as this is done without effort, it leaves behind it an 

 harmonious impression. 



This is spatial vision. Looking at solid bodies begins only 

 near at hand, when the two eyes noticeably converge and 

 begin to observe objects from two sides. That it really is the 

 convergent movement of the eyes which directly produces our 

 " plastic vision," we can prove if we look through a modern 

 stereoscopic telescope. This also enables us to look at objects 

 from two sides, but we do not get plastic vision ; instead of 

 that, the object becomes broken up into a number of planes 

 lying one behind the other. In this case, all the conditions are 

 present that accompany normal plastic vision, but the con- 

 vergent movement of the eyes is lacking. 



