76 THEORETICAL BIOLOGY 



acquires a new direction are fixed concretely by the angles 

 or edges of a spatial figure. 



MARK-SIGNS 



By employing the method of determination of threshold, 

 and developing the same principle, we arrive at the concept of 

 the mark-sign. We divide up the whole colour-band between 

 two turning-points into tiny segments lying alongside one 

 another, and make them so small that at least two adjacent 

 parts, considered by themselves, are indistinguishable by the 

 eye. Now let us magnify the individual parts until every 

 two adjacent ones become just distinguishable from one 

 another. Then the number of individual parts gives the 

 number of mark-signs that the colour-band holds for us. 

 ^ Interpreted in this way, the mark-sign means the alteration 

 in the content that is just perceptible to the attention. 



The number of mark-signs for colour increases with the 

 skill of the individual observer in distinguishing colours ; it 

 gives us a clue to the amount of colour in his appearance- 

 world. From careful investigation, we know that the world 

 of the colour-blind is ever so much poorer than our own. 

 While the person with normal sight can make, by means of 

 certain artifices, some representation of the colourless world, 

 the colour-blind is quite incapable of imagining the coloured 

 world of the normal eye. Just as little can the unmusical 

 man conjure up the world of melody in which lives the man 

 who has a musical sense. 



We can distinguish two kinds of mark-signs — those for 

 qualitative differences and those for differences of intensity. 

 The former are always associated with a definite quality. 

 They are represented by the angles in the spatial forms. The 

 majority of qualities, however, may appear in different grades 

 of intensity ; these are not bound to definite sensations of 



