26 THEORETICAL BIOLOGY 



in the retina is affected ; this is the yellow spot, in which 

 lie special nerve-endings, the cones. With this part of the 

 retina, which is at the same time the region of sharpest vision, 

 we " sweep " objects with a glance. 



When we read, the eye has to follow the guidance of the 

 lines of letters and figures ; we do no more than " glance." 

 Through the frequent repetition of the same sequence of 

 direction-signs, this sequence of symbols fixes itself firmly in 

 our memory like a melody, and enables us not only to recog- 

 nise the known letter when we see it again but also to create 

 an image of it in our mind. Finally, by copying the letter, 

 we establish the melody of the direction-sign so firmly that 

 the impulse-sequence for the muscles of the arm follows in 

 the manner that the melody prescribes ; then we are able to 

 write. In a previous experiment, we convinced ourselves 

 that the same impulse-sequence in our two arms, of which 

 one is the mirror-image of the other, gives rise to a reversal 

 of the direction the line takes. As a matter of fact, when 

 once we have learnt to write with the right hand, we can 

 straightway produce mirror- writing with the left — given, of 

 course, that the left hand is not by nature too clumsy. 



The same process that is gone through in learning letters 

 of the alphabet, occurs over and over again when we observe 

 objects. We " sweep " with the yellow spot the outlines of 

 objects over and over again until within ourselves a melody 

 of direction-signs is established. We use this melody in 

 order to recognise the objects again ; on the other hand, we 

 use it very seldom or very imperfectly for reproducing them 

 in our imagination as the melody of memory-signs. Thus 

 it comes about that we are but imperfectly informed as to 

 the number and affinities of the impulse-sequence ; we have 

 no means of recognising them directly, and we know them 

 only indirectly through the medium of the melody of the 

 direction-signs. 



