30 THEORETICAL BIOLOGY 



optical local signs furnishes us with but one plane, which Is 

 composed of surfaces, mutually influencing one another as to 

 colour, form and luminosity. Strange to say, Helmholtz, 

 while describing as laws the rules that influence luminosity 

 and colour, relegated to the realm of optical illusion aU the 

 rules he investigated concerning the influencing of shape and 

 size. 



Thus it has come about that such phenomena are still 

 treated as " curiosities," and in this province we are without 

 the groundwork for a theory of symphony. And yet it is 

 possible to recognise at the first glance general rules which 

 serve to unify and simplify the impression we receive of the 

 whole. When we look at an object, the eye is guided most 

 easily along the two chief directions of space ; every tailor 

 knows that clothes with longitudinal stripes make the wearer 

 look slighter, and that transverse stripes, on the other hand, 

 make him look broader. The eye attempts to render parallel 

 all lines that do not deviate too far. The branching of leafless 

 trees seen against the sky takes on the simplest possible 

 pattern. 



This compensating activity of the eye can be observed 

 most clearly and interestingly by filling up the blind spot in 

 the retina. Hold at arm's length in front of the right eye a 

 silver-knobbed walking-stick, keeping the left eye shut ; then, 

 holding the knob level, move it towards the right : you will 

 find that, if the eye is kept still, the knob disappears com- 

 pletely at one point. In its place appear the lines, shadows or 

 pattern of what is the background for the time being. When 

 the blind spot is fiUed up, the optical imagination always 

 completes the surfaces only, and never the object. 



Though in the twilight our imagination transforms trees and 

 bushes, or in the day-time the shapes of clouds, into the most 

 surprising objects, yet the optical imagination is exceedingly 

 limited in its activity. Nothing new ever appears in the 



