36 THEORETICAL BIOLOGY 



head. If, on the contrary, we see a bit of blue sky through 

 a small window placed high up, the blue surface runs vertically 

 downwards parallel to the window-frame. 



The size of objects is just as dependent on how much of 

 the whole field is visible. To illustrate this, we must retrace 

 our steps a little. 



In normal vision, what is presented to the eye is not all 

 equally definite, for when the lens is focussed for distance, 

 the outlines of objects close at hand are vague, and vice versa. 

 If we focus our eyes first on the nearest objects, and then, 

 little by little, on the more distant, the direction-signs that 

 appear in the process give us the sensation of gliding into the 

 depths, until the muscles are relaxed. As this happens, we 

 perceive in succession object-signs which serve as indications 

 of distance. But other special signs of distance come in, as 

 Leonardo da Vinci long ago pointed out. For instance, the 

 gradually increasing admixture of white with all the other 

 colours serves as a criterion of distance. 



When the muscles are quite relaxed, then, thanks to the 

 distance-signs, objects very far off appear to lie not ail in 

 one plane but placed behind one another. 



The eye, when it is looking, always tries to penetrate into 

 the furthest distance until the limit of all object-signs is 

 reached ; and it stops there only because it must. This 

 uttermost, objectless region which surrounds the whole visible 

 world, is never the horizon, but always lies beyond that. On 

 a dark night, the sky may be the limit ; but by day that very 

 seldom happens, for as soon as the firmament appears as a 

 definite covering, we seek behind it the region without objects. 



This region devoid of objects comes most clearly to our 

 consciousness when we look at the starry sky on a dark night ; 

 then it spreads out immediately behind the stars as the final 

 and invisible. 



It is not empty space, for even empty space is filled with 



